research papers for food processing

Food preservation techniques and nanotechnology for increased shelf life of fruits, vegetables, beverages and spices: a review

• Published: 09 November 2020
• Volume 19 , pages 1715–1735, ( 2021 )

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• Adithya Sridhar 1 ,
• Muthamilselvi Ponnuchamy 1 ,
• Ponnusamy Senthil Kumar   ORCID: orcid.org/0000-0001-9389-5541 2 &
• Ashish Kapoor 1  

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Food wastage is a major issue impacting public health, the environment and the economy in the context of rising population and decreasing natural resources. Wastage occurs at all stages from harvesting to the consumer, calling for advanced techniques of food preservation. Wastage is mainly due to presence of moisture and microbial organisms present in food. Microbes can be killed or deactivated, and cross-contamination by microbes such as the coronavirus disease 2019 (COVID-19) should be avoided. Moisture removal may not be feasible in all cases. Preservation methods include thermal, electrical, chemical and radiation techniques. Here, we review the advanced food preservation techniques, with focus on fruits, vegetables, beverages and spices. We emphasize electrothermal, freezing and pulse electric field methods because they allow both pathogen reduction and improvement of nutritional and physicochemical properties. Ultrasound technology and ozone treatment are suitable to preserve heat sensitive foods. Finally, nanotechnology in food preservation is discussed.

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Introduction

Food is vital for human survival and development. A recent review shows that food transmission of the coronavirus disease 2019 (COVID-19) is overlooked (Han et al. 2020 ). Food can be consumed in raw or processed form to obtain energy and sustain growth. Food wastage has become a major issue worldwide in the recent times. A considerable amount of food gets wasted at various stages of the food production and consumption chain. According to the report of Rethink Food Waste Through Economics and Data (ReFED), the data in Fig.  1 show the food wastage distribution for various types of food materials (ReFED 2016 ). Globally, due to inefficient supply chains, rising population and climate change, a large number of people are deprived of food on regular basis (Leisner 2020 ). Griffin et al. ( 2009 ) showed a detailed study about the waste generation of different food communities. Out of the food waste generated, 20% comprised production waste, 1% of processing waste, 19% of distribution and 60% of consumer generated waste. The major reasons for wastage were due to shrinkage of food while cooking, manufacturing issues, supply chain barriers, high consumer standards, changing climatic conditions, soil runoffs and policy constraints (Bräutigam et al. 2014 ; Silvennoinen et al. 2014 ; Filimonau and De Coteau 2019 ; Gomez-Zavaglia et al. 2020 ).

Food wastage for different food materials based on weight percentage. The demand for variety and abundance as well as inefficient storage conditions increases the amount of overall food wastage. Fruits and vegetables are among the least expensive and fastest spoiling foods followed by milk and dairy products. Data from ReFED ( 2016 )

A recent analysis conducted in Finland in 2019 found more than 50% of the food waste is from households (Filimonau and De Coteau 2019 ). The decision between ‘best before’ or ‘use by’ was a tough call to take in determining shelf life of product for the customers.

However, with the increase in population, consumers demand food that is fresh, healthy and nutritious. Although enough food is produced every day to feed the world, the technology and food produced fails to reach those in need. Thus, food wastage has become a key challenge to in all food processing sectors.

Any kind of food when harvested begins to show spoilage responses. One of the sustainable solutions to counter the food wastage issues is food preservation. The idea of food preservation was introduced in the ancient times when our ancestors were finding ways to keep the food fresh and edible. Concepts like sun drying, salting and pasteurization were introduced depending on climatic and seasonal factors. Preservation enabled humans to form communities, stopped them from killing animals and brought about a leisure attitude keeping food for additional time.

Rapid industrialization and advent of lean methods paved the way for processes like thermal treatment, canning and freezing which gave a better shelf life extension by controlling the pathogens. However, food safety and security became a major concern due to the growing population and increasing consumer standards and demands providing healthy and nutritious food (Saravanan et al. 2020 ). Thus, the concept of preserving food grew rapidly with an aim to provide food to all. The goal of food preservation is to inhibit any biochemical reactions and to restrict entry of bacteria or fungi. The technique allows minimization of wastage with improved shelf life extension. Some of the popular conventional preservation techniques like heating, drying and freezing have been implemented in large industries (Pereira et al. 2018 ; Białkowska et al. 2020 ; Said 2020 ). However, it has been found that there are certain disadvantages in heat treatment and freezing methods such as food shrinkage, texture and nutrient loss and organic properties leading to a huge overall loss in the food product (Jayasena et al. 2015 ).

In the recent years, chemical and microbiological treatments have been carried out with additives, coatings and various polyphenolic plant extracts thus posing an effective solution to food preservation. There is a lack of research in bridging the gap between the food wastage and food preservation techniques. This review investigates the upcoming food preservation technologies which are likely to play a dominant role in the food preservation industry. Current trends and advancements in preservation techniques and their applications to foods including fruits, vegetables, liquid foods and spices are the key aspects discussed here. The review covers a wide range of changes brought in conventional technologies and current technologies in the above fields. Special focus is also given to nanotechnology with its application in foods, agriculture and packaging sectors. The data have been collected after an extensive literature search over the subject surveyed for the last 15 years taking into account the challenges faced in industry during preservation. This work could be a perfect platform for understanding the advancements in food preservation techniques and its relevance to industry. The advent of nanotechnology in research and a combination of various advanced technologies as discussed in the literature (Butnaru et al. 2019 ; Nile et al. 2020 ; Rech et al. 2020 ; Tsironi et al. 2020 ) as well as in this manuscript could be the “go-to” technologies in the future. Thus, positive steps need to be taken to narrow down on the enhancements of these technologies for having a sustainable and cost-effective lifestyle.

Prevalent food preservation technologies

Thermal treatment.

Heat or thermal treatment is considered as one of the novel techniques for food preservation. For many years, the technique is well proven in various food sectors: from bakery and dairy to fruits and vegetables (Wurlitzer et al. 2019 ; Gharibi et al. 2020 ; Prieto-Santiago et al. 2020 ; Christiansen et al. 2020 ). The process generally involves heating of foods at a temperature between 75 and 90 °C or higher with a holding time of 25–30 s. Study on preservation enhancement of apple juice beverage by pasteurization and thermal treatment of maize showed a great impact on the flavor, digestibility, glycemic index, aroma, color and sensory attributes (Charles-Rodríguez et al. 2007 ; Zou et al. 2020 ). A recent report also highlighted five different types of rice when undergoing hydrothermal treatment showing results in par with respect to the quality of market rice (Bhattacharyya and Pal 2020 ).

The heating of foods reduces the pathogens. However, extensive research has also concluded nutrient losses, energy wastages, flavor changes and reduction in the food matrix (Roselló-Soto et al. 2018 ). A study conducted on light and dark honey showed changes in physicochemical characteristics, antioxidant activities and nutrient variations post-treatment (Nayik and Nanda 2016 ; Zarei et al. 2019 ). Liquid foods, juices and beverages too have a negative impact causing gelatinization and browning reactions (Codina-Torrella et al. 2017 ; de Souza et al. 2020 ). Over the years, constant investigation has been done on optimization studies of heat on exposure of food to improve its shelf life. Adjustments and slight modification to former technologies have recently contributed to significant advances with a combination of electrical and thermal methods. Different processes like electroplasmolysis, ohmic heating, and microwave heating of foods have created a dramatic impact in the food industry advancements. Table 1 shows the advanced electrothermal treatment techniques applied to different foods.

Cooling and freezing of products have been extensively applied for preservation of leafy vegetables, spices and milk products to maintain the sensorial attributes and nutrition qualities. Extensively used freezing techniques involve air blast, cryogenic, direct contact and immersion freezing, while advanced techniques involve high pressure freezing, ultrasound assisted freezing, electromagnetic disturbance freezing and dehydration freezing (Cheng et al. 2017 ; Barbosa de Lima et al. 2020 ). Cooling and freezing process mainly relies on the process of heat transfer. During cooling, there is a transfer of heat energy from the food and packaged container to the surrounding environment leading to an agreement of cooling. Thus, thermal conductivity and thermal diffusivity greatly affect the cooling or freezing rate. During the recent years, the storage technique has gained significant interest with the start of ready-to-eat foods catering to the needs of the consumer. The foods with their appropriate packaging material and cool temperature will always inhibit entry of microorganisms as well as maintain food safety. Although cooling and freezing are effective in their own terms, cooling time, uneven speed of ice crystal formation, storage expenses and specialized environments are concerning issues. In order to understand and overcome these challenges, technological tools like three-dimensional mathematical models and computational fluid dynamics models were evaluated to understand the heat transfer and fluid flow patterns with various food formulations thus showing an approach to minimize the issue (Zhu et al. 2019a , b ; Barbosa de Lima et al. 2020 ; Brandão et al. 2020 ; Stebel et al. 2020 ). Table 2 shows a description of the various advanced freezing techniques applied to different foods.

Ultrasound treatment involves use of high intensity and frequency sound waves which are passed into food materials. The efficient technology is chosen due to its simplicity in the equipment usage and being low cost as compared to other advanced instruments. The versatility of ultrasound is shown in its application in different fields ranging from medicine, healthcare to food industry (Dai and Mumper 2010 ).

Figure  2 illustrates a representation of different types of sonicators used for powdered and liquid foods. The process deals with ultrasonic radiation passing through the target solution. This action causes a disturbance in the solid particles in the solution leading to particles breaking and diffusing into the solvent (Cares et al. 2010 ). It should be noted that the intensity of the technique should be kept constant. This is because as intensity increases, intramolecular forces break the particle–particle bonding resulting in solvent penetrating between the molecules, a phenomenon termed as cavitation (Fu et al. 2020 ; Khan et al. 2020 ). Further enhancement of ultrasound extraction is dependent on factors like improved penetration, cell disruption, better swelling capacity and enhanced capillary effect (Huang et al 2020 ; Xu et al 2007 ). Table 3 shows the types of ultrasound technologies available which have created paths for efficiency improvements.

Types of ultrasound treatments: bath sonicator and probe sonicator. The treatment works on the principle of cavitation in which there is an energy transfer among food particles leading to bubble formation and collapsing. The technique requires minimal power providing more efficiency than traditional drying methods. It is used for treating various powdered or liquid foods

Ultrasound is slowly paving way into two most thriving sectors in the food industry, namely wine making and dairy production. Figure  3 shows the thermosonication process widely used in processing of milk and wine.

Thermosonication processing generally used for treating milk and wine samples for improving the shelf life. The treatment can prove to be cost-effective with reduced processing temperature due to the use of sonication as compared to conventional heat treatment or addition of synthetic preservatives

Milk is generally pasteurized in various industries to prevent spoilage and kill the microorganisms present. The utilization of a low-frequency ultrasound or combination of thermosonication (to 11.1 s) or manothermosonication could enhance the safety, quality and functional properties of product by 5 log times (Bermúdez-Aguirre et al. 2009 ; Deshpande and Walsh 2020 ; Gammoh et al. 2020 ). Low-frequency ultrasound alone has also played a significant role in improving the textural and homogenization effects of yoghurt, cheese and skimmed milk (Yang et al. 2020 ). With a shorter time interval, and thermosonication-applied (20 kHz, 480 W, 55 °C) production was improved to 40% and also had a positive impact on its organoleptic properties (Tribst et al. 2020 ).

Production of wine fermentation and alcoholic drinks always faces an issue in tackling microorganisms or yeast. Conventional methods generally involve use of chemical preservatives like sulfur oxide to prevent spoilage or thermal pasteurization followed by filtration to get the pure beverage. A recent study reported significant reduction of about 85–90% lactic acid bacteria with high power ultrasound at 24 kHz for 20 min for treatment of wine (Luo et al. 2012 ; Gracin et al. 2016 ). However, careful handling should be carried out in order to maintain the flavor and texture (Izquierdo-Cañas et al. 2020 ; Xiong et al. 2020 ).

Ultrasound studies have also found applications in isolation of bioactive compounds and processing pastes and juices in many fruits and vegetables. Recently, the technique was used to find the total phenolic content in spices like saffron (Teng et al. 2019 ; Azam et al. 2020 ; Yildiz et al. 2020 ). Table 4 shows the application of ultrasound technologies for various food crops. Thus, it can be concluded that ultrasound is a more sustainable technique than other traditional drying treatments.

• Ozone treatment

With the growing demands of consumer slowly moving towards healthy meals and sustainable lifestyle, the demand for organic foods have increased rapidly. Consumers need a functional food that is free from additives, preservatives with a decent shelf life span. Thus, the concept of ozone treatment technology has risen in recent years. The reason for choosing ozone is due to its diverse properties and quick disintegration.

In simple words, ozone is an allotrope of oxygen. The molecule is formed when oxygen splits into a single oxygen or nascent oxygen in the presence of light or ultraviolent radiation. Ozone formation is described by chemical equations as mentioned below (Eqs. 1 and 2 ) (Brodowska et al. 2018 ).

The compound quickly decomposes into oxygen molecule and possess a high oxidation potential (2.07 V) making it a good antimicrobial and antiviral agent (Fisher et al. 2000 ; Nakamura et al. 2017 ) as compared to chemical preservatives like chlorine (1.35 V), hydrogen peroxide (1.78 V) and hypochlorous acid (1.79 V) (Pandiselvam et al. 2019 ; Afsah-Hejri et al. 2020 ). Apart from this, ozone removes the necessity to store harmful chemicals as the gas can be made instantly. The energy required is also minimal as compared to thermal treatment giving more importance to the shelf life (Pandiselvam et al. 2019 ).

Over the recent years, ozone has been listed by the Food and Drug Administration (FDA) as a generally recognized as safe (GRAS) solvent. This has led to a demanding choice in food processing and preservation sectors to ensure safety and standards in products. When in comparison with chlorine, its degradation leaves negligible residue when treated with solid foods or beverages. The technology in combination with ultrasound was also shown to enhance the bacterial safety without any damage in cabbages (Mamadou et al. 2019 ). Consumer grade ozone was recently proven effective in disinfecting plastic boxes for storage (Dennis et al. 2020 ).

Table 5 shows the effect of ozone treatment on pesticide degradation in various fruits and vegetables production. The effect of ozone treatment depends on the type of pesticide and food material, environmental conditions, time interval and the strength of pesticide. When horticulture crops were compared, tomato and lettuce had the best pesticide removal efficiency while apple and chili were the least. It was seen that the type of food matrix and structure also play a key role in preventing the growth of pathogens. Ozone can thus be considered as an advanced emerging method for multiple sectors due to its feasibility, easiness and less time consumption.

• Pulse electric field

Pulse electric field technology is an advanced pre drying treatment involving shorter residence time for treatment of foods. The method was widely recognized due to its continuous operation and low requirement of electric fields (1–5 kV/cm). The method could be considered as a substitute for thermal drying and could enhance the food drying as it requires a very low temperature of 40 °C for functioning (Barba et al. 2015 ; Wiktor et al. 2016 ). Figure  4 shows the representative diagram of the process involved in treatment of liquid foods and paste using pulse electric field.

Application of pulse electric field generally used for treating liquid foods and pastes. The technique is a nonthermal food preservation method involving usage of pulses of electricity into the food material. The treatment gives high quality food with almost no change in texture or quality thus maintaining the original taste of food

The methodology of pulse electric field involves placing the food (fruit, vegetable, milk or any juices) between two electrodes after which a pulse is applied with high voltage (50 kV/cm) for short time intervals. The principle is a combination of electroporation and electropermeabilization (Barba et al. 2015 ). The electric field breaks the cell membrane matrix of the food thus enhancing the nutritive qualities, safety and increasing shelf life. The factors affecting pulse electric field involve field strength, pulse width, frequency, treatment time, polarity and temperature used (Odriozola-Serrano et al. 2013 ; Wiktor et al. 2016 ).

Over the years, demand for pulse electric field has grown drastically in all food sector areas. It can be used for destruction of bacteria (E coli) in milk. The treated milk was found to be high in quality and possessed an increased shelf life. A recent investigation was also carried out on watermelon and citrus juices which showed changes in physicochemical and antimicrobial properties (Aghajanzadeh and Ziaiifar 2018 ; Bhattacharjee et al. 2019 ). Table 6 summarizes the outcomes of application of pulse electric field treatment on various food materials.

Nanotechnology for food preservation

Nanotechnology has become a huge breakthrough with great potential to promote sustainability. It integrates branches of applied sciences such as physics, biology, food technology, environmental engineering, medicine and materials processing. In simple terms, nanotechnology involves any material or nanoparticle having one or more dimensions to the order 100 nm or less (Auffan et al. 2009 ; He et al. 2019 ). The technology is preferred as they possess different properties like slow release action, target specific nature, precise action on active sites and high surface area (Joshi et al. 2019 ). The reason for the success of nanotechnology is due to its promising results, no pollutant release, energy efficient and less space requirements. Apart from these success factors, nanotechnology has also shown versatile applications in terms of safety, toxicity and risk assessment in areas of agriculture, food and environment (Kaphle et al. 2018 ). Figure  5 shows the different avenues of nanotechnology development in the food sector.

Applications for nanotechnology in agriculture, food processing and packaging. Nanotechnology has gained a lot of interest with versatile applications and unique properties enabling efficient processes and quality products. The use of nanomaterials, nanosensors, precision agriculture and advanced packaging can play a promising role in improving the food sector

Nanomaterials are broadly classified into two types, namely organic and inorganic, depending on their nature and functionalities (Table 7 ).

Nanotechnology has been regarded as a promising tool for growing the economy in near future as well as maintaining the plant growth and nutritional qualities of the food commodity. Use of nanofertilizers and precision farming has posed several benefits in weed control and decrease in chemical pesticide thus enhancing shelf life. Growing use of nanotechnology in agro-food system industry may even pose as a solution to solve challenges in food security and agriculture (Yata et al. 2018 ; Ghouri et al. 2020 ). The three primary avenues where the technology could grow include food processing, agriculture and packaging.

Nanotechnology in food processing

The concept of nanotechnology has paved the way in processing and formulation of colorants, sensors, flavors, additives, preservatives and food supplements (nanoencapsulation and nanoemulsion) in both animal and plant based products (He et al. 2019 ). The diversity of nanotechnology in various fields has led to introduction of nanosensors in food processing industries. Nanomaterials have shown several electrochemical and optical properties in different sauces, beverages, oils and juices. Table 8 shows the different nanomaterials used as sensors in food industry.

Distinctive characteristics have shown great qualities in the area of food processing as ingredients and supplements. Oxide chemicals such as magnesium oxide and silicon dioxide can act as a food flavor, food color and a baking agent. The use of titanium dioxide has also been certified as an additive in gums, sauces and cakes (Weir et al. 2012 ). Additionally, copper oxide, iron oxide and zinc oxide have been categorized as GRAS materials by European Food Safety Authority (EFSA) for animal and plant products (He et al. 2019 ).

Nanotechnology in agriculture

The use of nanotechnology in agriculture and the concept of precision agriculture has gained a lot of interest in the recent years. The main goal of agriculture is to reduce the volume of chemicals, minimize nutrient losses and increase the overall performance of crops. Although chemical fertilizers are added for increasing the crop yields, it pollutes and harms the soil, water, food and environment (Riah et al. 2014 ). Precision agriculture is one of the green ways to tackle this issue. It is a system based on artificial intelligence that understands crop quality, soil quality and detects weed controls generally through drones. The area has recently gained interest in nutritional management and various optical properties to address food wastage and to feed the growing population (Duhan et al. 2017 ). Majority of plant species (cereal grains like wheat, rice, barley, tobacco, soybean, rye) follow the biophysical process of photosynthetically active radiation and electron transport. These targets have been identified to improve photosynthesis activity.

There has been many discussions and investigation on the concept of plant nanobionics and photosynthesis. Plant nanobionics deals with appropriate insertion of nanoparticles into the chloroplast of the plant cell for improving the plant productivity. It has been proven that titanium dioxide nanoparticles (nTiO 2 ) have become the “go-to” nanoparticles for efficient photosynthesis process (Hong et al. 2005 ; Gao et al. 2006 , 2008 ). The application of nTiO 2 with spinach and tomato leaves under mild heat stress improved the overall photosynthesis process showing significant improvement in the transpiration and conductance rates (Gao et al. 2008 ; Qi et al. 2013 ).

Nanomaterials like silver ions, polymeric compounds and gold nanoparticles are also being investigated for use in pesticides. Usage of gold and silver nanoparticles has also had a positive effect to restrict the pest and improve plant growth (Ndlovu et al. 2020 ). Studies have also investigated on sulfur-based nanoparticle (35 nm) for organic farming which prevent fungal growth from apple tomatoes and grapes (Joshi et al. 2019 ).

Nanotechnology in food packaging

Many fresh fruits and vegetables are sensitive to oxygen, water permeability and ethylene leading to deterioration of food quality (Gaikwad et al. 2018 , 2020 ). Thus, food packaging plays a critical role in addressing this issue. Nanoparticles and polymer-based composites have proven to be the best solutions (Auffan et al. 2009 ; Joshi et al. 2019 ). The application of a natural polymer or a biopolymer and coating it on the food surface has recently shown promise in preserving foods (Luo et al. 2020 ). Table 9 shows the different applications of nanomaterials used in food packaging. Although the application of nanomaterials in smart packaging is in its early stage, rapid advancements have been carried out through the years as it offers safe and sustainable approach (Rai et al. 2019 ).

The usage of chitosan and chitosan-based additives and films has been recently explored with multiple functionalities with positive outcomes. Chitosan-based films, in general, possess antioxidant, antimicrobial and antifungal properties making it a good replacement for synthetic chemicals (Yuan et al. 2016 ; Yousuf et al. 2018 ). The use of chitosan-based derivatives offer a promising solution towards maintaining the shelf life of foods without disturbing its sensorial properties (Kulawik et al. 2020 ). A recent study proved that chitosan-based matrices can also be used for clarification, preservation and encapsulation of different beverages (alcoholic, non-alcoholic as well as dairy based), fruit juices, tea and coffee (Morin-Crini et al. 2019 ). Apart from this, nanocomposites (combination of different nanomaterials) have shown efficient thermal and barrier properties at a low cost. Researchers evaluated the concept of the nanocomposites membranes and concluded that it decreased the water permeability in foods by a value of 46 (Jose et al. 2014 ). An increase in corrosion resistance was evaluated with use of clay and epoxy composites (Gabr et al. 2015 ).

Edible coatings with nanomaterials have also shown increasing potential towards food storage of fruits and vegetables. These coatings hold useful while transportation from factory to retailers and also maintain the nutritional qualities without causing any physical damage. Edible coatings are generally prepared from fats, proteins and polysaccharides which have been shown to block gases. Nanoclays and nanolaminates have also shown promising results to improve their barrier properties to gases for efficient food packaging (Echegoyen et al. 2016 ). Nanolaminates involve layer-by-layer deposition of a special coating where the charged surface is applied on food. The application of carbon nanotubes as nanofillers in gelatin films has also been successfully demonstrated (Rai et al. 2019 ). The biofilms are found to have improved tensile strength, mechanical, thermal and antimicrobial properties (Jamróz et al. 2020 ; Zubair and Ullah 2020 ). Thus, nanomaterials have emerged as an integral part while addressing nanotechnology in food packaging.

With tons of foods being wasted every single day, food preservation has been the need of the hour for extending the shelf life to help feed millions of people globally. Although plenty of advanced technologies have been introduced, major strides need to be taken to have a sustainable food system. Availability, access and proper utilization of food should be well balanced in order to understand the value of food security. It is important to maintain a correct and precise balance of technology with respect to design and cost effectiveness. Constant investigation is also being carried out in the area of finding more natural preservatives with excellent antioxidant and antimicrobial properties as they are safe to consume and eliminate processed food. The concept of hurdle technology, which combines multiple techniques to measure different variables like temperature, water activity, pH, moisture content and enzyme activities has also been explored to meet the consumer demands for an efficient food system. Another growing solution is in the area of nanotechnology in foods which has been discussed in this article. However, research on different nanomaterials, its toxicity, its safety to consumers and genetic factors is still under debates and discussions. The concept of bioencapsulation and nanoencapsulation in food supplements and drug developments is also growing at a fast pace keeping in mind the health and environmental effects. Further work needs to be done in data visualization and artificial intelligence, internet of things and machine learning. This would help changing the food and agricultural industry in the area of functional foods and crops through digitalization.

Abbreviations

Rethink Food Waste Through Economics and Data

Gallic acid equivalent

Total phenolic content

Food and Drug Administration

Generally recognized as safe

European Food Safety Authority

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The authors are thankful to Sivaraman Prabhakar for insightful discussions.

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Sridhar, A., Ponnuchamy, M., Kumar, P.S. et al. Food preservation techniques and nanotechnology for increased shelf life of fruits, vegetables, beverages and spices: a review. Environ Chem Lett 19 , 1715–1735 (2021). https://doi.org/10.1007/s10311-020-01126-2

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Modern Processing of Indian Millets: A Perspective on Changes in Nutritional Properties

N. a. nanje gowda.

1 Department of Food Technology, Faculty of Life and Allied Health Sciences, Ramaiah University of Applied Sciences, Bangalore 560054, India; [email protected] (Y.B.); moc.liamg@dellaparten (B.P.N.); [email protected] (C.G.)

Kaliramesh Siliveru

2 Department of Grain Science and Industry, Kansas State University, Manhattan, KS 66506, USA

P. V. Vara Prasad

3 Department of Agronomy, Kansas State University, Manhattan, KS 66506, USA; ude.usk@arav

Yogita Bhatt

B. p. netravati, chennappa gurikar, associated data.

Not applicable.

Globally, billions of people are experiencing food insecurity and malnutrition. The United Nations has set a global target to end hunger by 2030, but we are far from reaching it. Over the decade, climate change, population growth and economic slowdown have impacted food security. Many countries are facing the challenge of both undernutrition and over nutrition. Thus, there is a need to transform the food system to achieve food and nutrition security. One of the ways to reach closer to our goal is to provide an affordable healthy and nutritious diet to all. Millets, the nutri-cereals, have the potential to play a crucial role in the fight against food insecurity and malnutrition. Nutri-cereals are an abundant source of essential macro- and micronutrients, carbohydrates, protein, dietary fiber, lipids, and phytochemicals. The nutrient content and digestibility of millets are significantly influenced by the processing techniques. This review article highlights the nutritional characteristics and processing of Indian millets, viz. foxtail, kodo, proso, little, and pearl millets. It also envisages the effect of traditional and modern processing techniques on millet’s nutritional properties. An extensive literature review was conducted using the research and review articles related to processing techniques of millets such as fermentation, germination, dehulling, extrusion, cooking, puffing, popping, malting, milling, etc. Germination and fermentation showed a positive improvement in the overall nutritional characteristics of millets, whereas excessive dehulling, polishing, and milling resulted in reduction of the dietary fiber and micronutrients. Understanding the changes happening in the nutrient value of millets due to processing can help the food industry, researchers, and consumers select a suitable processing technique to optimize the nutrient value, increase the bioavailability of nutrients, and help combat food and nutrition security.

1. Introduction

Millets are termed as “yesterday’s coarse grains and today’s nutri-cereals.” Millets are considered to be “future crops” as they are resistant to most of the pests and diseases and adapt well to the harsh environment of the arid and semi-arid regions of Asia and Africa [ 1 ]. Millets are small-seeded grains, the most common and important for food being sorghum ( Sorghum bicolor L.), pearl millet ( Pennisetum glaucum ), finger millet ( Eleusine carocana ), teff ( Eragrostis tef ), proso millet ( Panicum miliaceum ), kodo millet ( Paspalum scrobiculatum ), foxtail millet ( Setaria italica ), little millet ( Panicum sumatrense ) and fonio ( Digitaris exilis ) [ 1 ]. After decades of negligence, nutri-cereals are making a strong comeback in the Indian cereal’s production segment. India dominates the global production of millets with a total share of about 40.62% and an estimated production of about 10.91 million tonnes during 2018–2019 [ 2 ]. Although India ranks first in nutri-rich millet production and second in rice and pulses across the globe, it also—unfortunately—ranks second in child malnutrition incidences. India is home to more than one-third of the world’s malnourished children [ 3 ]. By contrast, the country has also become a hub for diabetic and overweight populace, putting the country under a double burden of malnutrition [ 4 ]. The majority of millets are three to five times more nutritious than most cereals (rice, Oryza sativa ; wheat, Triticum aestivum ; maize, Zea mays ) in terms of vitamins, fiber, proteins, and minerals (calcium and iron) and are gluten-free; hence, they are known as “superfoods” [ 2 ]. The nutri-rich millets are the viable solution to reduce the rising incidences of malnutrition and metabolic disorders and can enhance the nutrition and food security of the country.

Millets are a highly nutritious crop and contain considerable amounts of vitamins and minerals. Millets are a good source of energy, dietary fiber, slowly digestible starch, and resistant starch, and thus provide sustained release of glucose and thereby satiety [ 5 , 6 ]. Compared to cereals, millets are a good source of protein- and sulphur-containing amino acids (methionine and cysteine) and have a better fatty acid profile [ 5 , 7 ]. However, millets contain a limited amount of lysine and tryptophan, which varies with the cultivar. Millets are rich in vitamin E and vitamin B and in minerals such as calcium, phosphorus, magnesium, manganese, potassium, and iron [ 1 , 8 ]. The abundant nutrients of millets provide multiple benefits such as reducing the incidence of cancer [ 9 , 10 ], obesity and diabetes [ 11 ], cardiovascular diseases [ 12 , 13 ], gastrointestinal problems [ 14 ], migraine, and asthma [ 1 , 15 ]. Consumption of millets helps manage hyperglycemia due to their lente carbohydrate and high dietary fiber content, thus making millets a perfect food for the diabetic populace [ 3 , 15 ]. Therefore, millets play an important role in the modern diet as a potential source of essential nutrients, especially in underdeveloped and developing countries [ 16 ]. Although millets have a diversified and high food value, their consumption, especially by the Indian populace, has not reached a significant level due to various factors, depicted in Figure 1 . Recently, these grains have been slowly fueling the start-up revolution to improve nutri-rich food availability and create employment.

Millets: health benefits, production, and challenges in India. Data taken from various issues [ 17 ].

Millets are usually processed before consumption to remove the inedible portions, extend the shelf life, and improve nutritional and sensory properties. Primary processing techniques such as dehulling, soaking, germination, roasting, drying, polishing and milling (size reduction) are followed to make millets fit for consumption. At the same time, modern or secondary processing methods such as fermenting, parboiling, cooking, puffing, popping, malting, baking, flaking, extrusion, etc., are used to develop millet-based value-added processed food products [ 8 ]. Although these processing techniques aim to enhance the digestibility and nutrient bioavailability, a significant amount of nutrients are lost during subsequent processing [ 18 ]. This review article aims to provide an overview of the effect of processing techniques on the nutritional properties of important Indian millets, viz. pearl millet, proso millet, kodo millet, foxtail millet, and little millet.

2. Methodology

Review was conducted based on the methodology reported earlier with slight modification [ 19 ]. The current topic was selected based on a literature survey to identify the gap between the available literature resources pertaining to the effect of processing treatment on specific nutrient components of millet with respect to the Indian scenario. The objective of the review was to evaluate the millet processing treatments in order to identify the appropriate processing treatment for maximum retention of nutrients. The review includes peer-reviewed research articles published in the English language after the year 2016. The articles exclusive to dehulling, fermenting, germination, parboiling, cooking, puffing, popping, malting, and extrusion millet processing were included. The literature review was carried out using databases such as PubMed and Google Scholar as search engines. The common search terms used were millets processing, millet nutrition, dehulling, nutri-cereals processing, value addition to millets, fermenting, germination, parboiling, cooking, puffing, popping, malting, extrusion of millets, etc.

3. Nutritional Characteristic of Selected Indian Millets

3.1. nutritional profile of millets.

The nutritional content of food is an important factor in the maintenance of a human body’s metabolism and wellness. The nutritional content is critical for developing and maximizing the human genetic potential. Millet’s nutrition is comparable to major staple cereals (rice, wheat, and maize), since they are an abundant source of carbohydrates, protein, dietary fiber, micronutrients, vitamins and phytochemicals. Millets provide energy ranging from 320–370 kcal per 100 g of consumption ( Table 1 ). Millets have a larger proportion of non-starchy polysaccharides and dietary fiber compared to staple cereals and comprise 65–75% carbohydrates. Millets with high dietary fiber provide multiple health benefits such as improving gastrointestinal health, blood lipid profile, and blood glucose clearance. Millets with minimal gluten and low glycemic index are healthy options for celiac disorder and diabetes [ 20 ]. Millets are also rich in health-promoting phytochemicals such as phytosterols, polyphenols, phytocyanins, lignins, and phyto-oestrogens. These phytochemicals act as antioxidants, immunological modulators, and detoxifying agents, preventing age-related degenerative illnesses such as cardiovascular diseases, type-2 diabetes, and cancer [ 1 ]. A study [ 21 ] reported that millets contain about 50 different phenolic groups and their derivatives with potent antioxidant capacity, such as flavones, flavanols, flavononols, and ferulic acid. A significant amount of phenolic components, which are important antioxidants in millets, are found in bounded form in proso and finger millet and in free form in pearl millet [ 22 ]. Another study [ 23 ] reported that proso millet comprises various phytochemicals such as syringic acid, chlorogenic acid, ferulic acid, caffeic acid, and p-coumaric. It has also been reported that almost 65% of the phenolics are present in the bound fraction. The presence of these phytochemicals and important antioxidants indicates the potential benefits of millets to human health. A detailed summary of the nutritional profile of selected Indian millets is discussed below and highlighted in Table 1 .

Nutritional profile of millets in comparison with cereals (per 100 g).

Source: Indian Food Composition Tables and nutritive value of Indian foods [ 30 , 31 ] .

• Proso millet has a higher nutritional value when compared with staple cereals as it contains a higher concentration of minerals and dietary fiber ( Table 1 ). Proso millet is a rich source of vitamins and minerals such as iron (Fe), calcium (Ca), potassium (K), phosphorus (P), zinc (Zn), magnesium (Mg), vitamin B-complex, niacin, and folic acid. Proso millet contains essential amino acids in significantly higher quantities, except for lysine, the limiting amino acid. However, proso millet has an almost 51% higher essential amino acid index than wheat [ 24 ]. Moreover, the products prepared from proso millet exhibit a lower glycemic response than staple cereal-based products. A review reported that products prepared from proso millet show a significantly lower glycemic index (GI) compared to wheat- and maize-based products [ 25 ].
• Pearl millet shows an energy value comparable to the staple cereals. Pearl millet contains a lesser amount of carbohydrates than the staple cereals, and it mainly contains high amylose starch (20–22%), and the insoluble dietary fiber fraction helps in exhibiting a lower glycemic response. Pearl millet protein is gluten-free and contains a higher prolamin fraction, making it suitable for people with gluten sensitivity. The amino acid score in pearl millet is good; however, it is poor source of lysine, threonine, tryptophan, and other sulphur-containing amino acids [ 23 , 26 ]. Pearl millet is high in omega-3 fatty acids and also important nutritional fatty acids such as alpha-linolenic acid, eicosapentaenoic acid, and docosahexaenoic acid. It also contains other micronutrients such as Fe, Zn, copper (Cu), K, Mg, P, manganese (Mn), and B-vitamins [ 23 ].
• Kodo millet provides an energy value similar to the other millets and staple cereals. However, with the exception of finger millet, the protein content of kodo millet is lower than that of other selected millets and it provides gluten-free protein ( Table 1 ). Kodo millets contains high amounts of vitamins and minerals, especially B-complex vitamins, B6, niacin and folic acid, Fe, Ca, Mg, K, and Zn. Kodo millet is very easy to digest and thus can be beneficial for infant and geriatric product formulation.
• Foxtail millet has a greater nutritional value compared to major cereals such as wheat and rice due to its copious dietary fiber content, resistant starch, vitamins, minerals, and essential amino acids, except for lysine and methionine, but it is richer than most cereals. Among the selected millets, foxtail millet contains the highest protein ( Table 1 ). Foxtail millet also contains a high amount of stearic and linoleic acids, which helps in maintaining a good lipid profile.
• Finger millet has the highest carbohydrate content among the selected millets. However, carbohydrates consist primarily of slowly digestible starch, dietary fiber, and resistant starch and thus offer a low glycemic index compared to most common cereals such as rice and wheat [ 27 ]. Finger millet contains around 7% protein ( Table 1 ), which is less than that of other millets, but it has a good amino acid score and contains more threonine, lysine, and valine than other millets. Subsequently, micronutrients such as Ca, Fe, Mg, K, and Zn, as well as B-vitamins, especially niacin, B6, and folic acid, are abundantly available.
• The nutritional value of little millet is comparable to other cereal and millet crops. It contains around 8.7% protein and balanced amino acids, and it is a rich source of sulphur-containing amino acids (cysteine and methionine) and lysine, which is lacking in most cereals [ 28 ]. It is generally considered to induce a lower glycemic response due to the presence of abundant dietary fiber, resistant starch, and slowly digestible starch [ 29 ]. It is also a good source of micronutrients such as Fe, P, and niacin. Recently, many value-added products have been prepared using little millet to capitalize on the health benefits of little millet.

3.2. Antinutrient Profile of Millets

Antinutrients are phytochemical compounds that plants produce naturally for their defense. These antinutritional factors hinder nutrient absorption, leading to reduced nutrient bioavailability and utilization [ 32 ]. When consumed uncooked, products containing antinutrients and chemical compounds may be detrimental or even pose health issues in humans, such as micronutrient malnutrition, nutritional deficiency, and bloating. Plant-based foods mainly contain antinutrients such as tannins, phytates, oxalates, trypsin, and chymotrypsin inhibitors [ 33 ]. One of the disadvantages of millets is a higher concentration of antinutritional factors compared to wheat and rice. Finger millet contains polyphenols, tannins (0.61%), phytates (0.48%), trypsin inhibitors, and oxalates, which may interfere with the bioavailability of micronutrients and protein digestibility. The goitrogenic compounds in pearl millet are derivatives of phenolic flavonoids, such as C-glycosyl flavones, and their metabolites are responsible for the development of off-odors in the flour during storage [ 34 ]. Antinutritional factors due to metal chelation and enzyme inhibition capacity decrease nutrients bioavailability, mainly of minerals and proteins. However, in recent years, antinutritional factors such as polyphenolic compounds have been reported as nutraceuticals for their contribution to antioxidant properties [ 1 ]. Most secondary metabolites that function as antinutrients may cause extremely detrimental biological reactions, while others are actively used in nutrition and pharmacologically active drugs. The need of eliminating antinutrients is fulfilled by pretreatment or processing techniques of food grains, such as debranning, soaking, germination, fermentation, and autoclaving. These methods add value to food by enhancing the bioavailability of a few cations such as Ca, Fe, and Zn and also the proteins absorption [ 8 ].

4. Mechanical Processing for Millets

Because global food security is at risk, effective utilization of available millet crops to develop an affordable, palatable, and nutrient-rich product is the need of the hour. Millet grains must be processed to remove inedible portions and convert them into cooked and edible form. Therefore, processing is a crucial task, as it increases the bioavailability of nutrients and organoleptic properties and decreases antinutrients [ 1 ]. Processing involves multiple techniques such as dehusking/decortication, milling, soaking, germination, fermentation, malting, cooking, and roasting. These operations cause changes in physicochemical attributes that alter the nutrition, function, and physical characteristics of food [ 15 ]. Processing may be of two types, namely, primary and secondary processing. Processes such as cleaning, washing (soaking/germination), dehulling, milling (into flour and semolina), and refining to remove the undesired seed coat and antinutritional factors are termed as primary processing, while secondary processing involves converting primary processed raw materials into “ready-to-cook” (RTC) or “ready-to-eat” (RTE) products by flaking, popping, extrusion, and baking [ 1 ]. The traditional processing technologies include debranning, milling, roasting, soaking, steaming germination, popping, flaking, ready-to-eat salted grains, and fermented products [ 35 , 36 ]. These processing techniques aim to convert grains into edible forms, with an extended shelf life, improved texture, specific flavor, taste, as well as improved nutritional quality and digestibility [ 37 ]. Millet consumption and utilization can be increased by processing them into various by-products, which also reduces the phytate and tannin levels, increases the minerals and amino acids bioavailability, and improves starch and protein digestibility [ 38 ]. Processing imparts specific morphological, anatomical, or modulated changes in these bioactive compounds present in whole grains. The processing methods may have positive as well as negative impacts on the nutrient and antinutrient profile. Various research studies on millet processing have shown positive results on the effective usage of millets in a variety of traditional and convenience health foods. Significant levels of phytates, tannins, phenols, and trypsin inhibitors decrease nutrient bioavailability and quality, limiting maximum utilization of nutritional potential in millets [ 1 ]. Certain millets contain higher concentrations of unsaturated fatty acids; hence rancidity and off-flavors occur in millet flour during storage due to lipolysis followed by oxidation of “de-esterified fatty acids” [ 32 ]. Thus, understanding the influence of processing on nutritional properties is extremely important for effective utilization of millets. It also assists in choosing an appropriate processing technique for millets to maximize nutrient availability, improve palatability, and increase shelf life. The changes in nutritional composition and digestibility with respect to different mechanical processing methods are discussed ( Table 2 ) and summarized ( Figure 2 ).

Inference on nutritional properties changes during different processing methods.

Changes in millets nutritional properties with respect to processing methods.

5. Effect of Processing on Nutritional Properties of Millets

5.1. proteins.

Millets are a rich source of proteins and are widely consumed by vegans. They are regarded as an excellent plant protein with negligible amounts of saturated fats compared to animal proteins. The presence of antinutrients inhibits protein digestibility; hence, reducing the antinutrients level is important. Simple techniques such as dehulling, milling, soaking, and heating decrease the antinutrient levels and increase the in vitro protein digestibility. The impact of various processing methods on the protein digestibility of foxtail millets has been studied [ 20 ]. The alkaline cooking, fermentation, germination (40 h at 25 °C), and popping of foxtail millet resulted in improved protein quality. In another study, pan-frying showed increased protein content in proso millet by 9.5% [ 18 ]. The puffing or popping of kodo millet increased the protein concentration from 7.92 to 8.12% [ 53 ]. The separation of starch granules from the protein matrix during thermal treatment, as well as the destruction of antinutritional components such as trypsin inhibitors and phytate acid, resulted in enhanced protein digestibility as a result of heat treatment or high pressure.

Protein digestibility in cereals, millets, and legumes has been shown to improve throughout the germination and fermentation processes. The germination of foxtail millet resulted in an increment in the protein concentration due to the synthesis of new amino acids [ 39 ]. Similar results for the increase of protein during germination of two cultivars of pearl millet, namely Gadarif (11.4% to 13.2%) and Gazeera (14.4% to 16.3%) were observed [ 54 ]. A study [ 55 ] showed that following germination, the protein concentration of pearl millet increased from 14% to 26%, whereas another study [ 43 ] reported the increased protein in proso millet after sprouting for 96 h. A research study on the impact of fermenting pearl millet flour with pure cultures revealed enhanced protein efficiency ratios, true and apparent protein digestibility, and utilizable protein values [ 55 ]. In another study, the combined effect of germination, fermentation (12 h and 24 h, respectively) and dry heating of pearl millets resulted in improved “in vitro protein digestibility” (IVPD), indicating that fermentation enhances protein digestibility [ 54 ]. The natural fermentation of pearl millet may significantly enhance the protein content [ 47 ]. During fermentation, antinutritional factors such as phytate gets degraded and the insoluble protein get converted to soluble protein due to the synthesis of proteolytic enzymes by microflora [ 56 ]. The simple technique of soaking pearl millet for 24 h resulted in increased protein due to the mobilization of stored nitrogen [ 46 ]. Similarly the malting of pearl millet (24 h soaking, followed by 18 h germination) significantly enhanced the protein [ 43 ]. These reports suggest that the soaking, malting germination, and fermentation processes lead to an increment in the total protein and improved protein digestibility, and thus can be used as an effective processing treatment in the development of protein-rich foods. Because these processes do not necessitate sophisticated equipment, they can be employed at the domestic level as well, assisting in the fight against protein–energy malnutrition, which is primarily a concern in underdeveloped nations.

Decortication removes about 12% to 30% of the outer husk, bran, and germ portion of grains, limiting the significant loss of proteins and amino acids such as histidine, lysine, and arginine. According to a study [ 49 ], dehulling of pearl millet up to 17.5% had a significant impact on the nutritional contents, increasing protein and digestibility. However, dehulling beyond this point, a substantial decrease in protein occurred. In another study [ 57 ] on the milling of pearl millet, bran-rich milled grains showed the highest percentage of IVPD. Similar improvements in millet’s IVPD were reported by other authors [ 53 ]. Since most of the polyphenolic compounds and antinutrients which precipitate proteins and reduce protein digestibility are present in the hull of millets, the decortication process substantially eliminates them and result in improved protein digestibility.

5.2. Carbohydrates

Carbohydrates of the millets range around 60–75%, with foxtail millet containing the minimum carbohydrate and little millet containing the maximum carbohydrate ( Table 1 ). Starch is the principal carbohydrate of the millets like other cereals. The amount of available carbohydrates in food grains is affected by various domestic processing and cooking methods such as soaking, sprouting, pressure cooking, autoclaving, and so on [ 1 ]. The carbohydrate content of foxtail millet increased significantly, by 1.29% [ 58 ]. By contrast, the carbohydrates of pearl millet flour increased non-significantly during the first 24 and 48 h of germination but decreased significantly after 72 h [ 45 ]. The increase in carbohydrates during the germination of foxtail millet is associated with the decrease in moisture, ash, crude protein, and fat, because the carbohydrate levels depend on these attributes of the grains [ 58 ]. The effect of fermentation and germination on the carbohydrates of pearl millet revealed that germination greatly increases the total soluble sugar concentration, as well as the reducing and non-reducing sugar concentration. When homogenized and autoclaved, the germinated slurry substantially increased the soluble sugars and decreased starch [ 49 , 59 ]. The main reason for reduced starch could be due to the starch hydrolysis during the germination and autoclaving process, resulting in a higher concentration of soluble sugars. In a similar study, fermented pearl millet grains also showed lower levels of starch and higher levels of soluble carbohydrates than native pearl millet grain [ 60 ]. Another study revealed a significant rise in the total amount of sugars in proso millet during germination, which could be attributed to starch breakdown [ 61 ]. These results indicate that the germination and fermentation processes improve the carbohydrate digestibility by breaking down the complex starch into simple soluble sugars. This shows the importance of germination and fermentation in the development of energy-dense, easily digestible food products such as infant formula. A study [ 62 ] reported the effect of decortication and hydrothermal processing on finger millet. They observed that decortication significantly increased the total carbohydrates by around 16%. The reduction in carbohydrates due to decortication is apparent due to the removal of the seed coat. However, no change in total carbohydrates due to hydrothermal treatment was reported, but a slight change in amylose fraction was noted. Furthermore, due to leaching during steeping and the Maillard process during steaming, the sugar concentration reduced from 1.085 to 0.71 g/100 g after hydrothermal processing. These results indicate that carbohydrates behave differently with different processing techniques. An extensive study [ 32 ] on the starch digestibility of pearl and proso millet revealed that parboiling significantly reduced the total starch by 5–10% due to starch leaching out during soaking and boiling process. They also observed that parboiled proso and pearl millet had a reduced readily digestible starch fraction (18.2–19.1% to 17.4–18.3%) and thus a lower glycemic index by 1.6–3.9%. These results suggest that parboiling can significantly reduce starch digestibility and therefore can be utilized to formulate products for metabolic diseases such as diabetics and obesity.

5.3. Dietary Fiber

The millet bran fraction is a major and abundant source of dietary fiber, which is characterized as complex polysaccharides that are not readily available. Therefore, removal of the bran fraction during decortication/dehulling results in substantial reduction in fiber component. It was reported that dehulling of about 12% to 30% to remove the kernel is suitable for millet grains as it does not result in significant loss of fiber. However, dehulling of grains beyond 30% results in the substantial loss of dietary fiber [ 37 ]. Since most of the millets are consumed in their decorticated form, it is very important to control the extent of dehulling so as to maximize the fiber content. A study [ 20 ] on the impact of milling on the fiber components of foxtail millet revealed that the insoluble dietary fiber content of lignin, cellulose, and hemicellulose in the milled fraction was lower than that of whole millet flour, while in foxtail millets the fiber content increases significantly with increasing germination time [ 39 ]. This is perhaps due to a change in the structure of the seeds’ cell wall polysaccharides, which may affect the tissue histology and disrupt protein carbohydrate interactions. In addition, the results of cell wall biosynthesis leads to increased production of dietary fiber. A study of solid-state fermentation (SSF) on pearl millet with Rhizopus oligosporus and Yarrowia lipolytica [ 63 ] increased the soluble dietary fiber by 176%. Another study revealed that, fermenting the dietary fiber from foxtail millet bran with Bacillus natto enhanced the soluble dietary fiber (DF) content by 10.9% and increased the ratio of soluble DF to insoluble DF by 16.8% [ 64 ]. Following fermentation, cellulose and hemicellulose breakdown resulted in more porous structure polysaccharides, which explains the changes in DF. Similarly, malting pearl millet for 24 h boosted the fiber level from 0.77% to 0.87% [ 44 ]. A study [ 65 ] on maize and finger millet-based extruded product showed that the non-starchy polysaccharides reduced from 2.5 g/100 g for raw blend to 1.5 g/100 g for unfermented-extruded blend. The values were further reduced to 0.9 for fermented blends and 1.4 g/100 g for blends treated with lactic or citric acid (different molarities) prior to extrusion. It was also observed that high extrusion temperatures and severe mechanical shear disrupt glycosidic networks and weak bonds between polysaccharide chains of dietary fiber polysaccharides, resulting in a reduction in total NSP. Similarly, the thermal processing of biscuits prepared from pearl millet flour resulted in a change in crude fiber content from 1.26% to 1.75% [ 63 ]. Roasting of pearl millet grains at different times and temperatures reduced crude fiber content. Other thermal processes such as puffing and popping on millets resulted a decline in crude fiber by 1.71% and from 18.9 to 15.8 g/100 g, respectively [ 66 ]. This could be mainly attributed to the fact that the outer grain layer has the majority of the fiber that is exposed to thermal degradation. To summarize, the reports suggest that dehulling and milling (debranning) operations reduce dietary fiber, while high temperature extrusion processes lead to thermal degradation of dietary fiber. Dietary fiber, particularly that accumulated in the outer bran layer, plays a vital role in reducing type 2 diabetes and constipation. For a healthy millet diet, it is important to discourage millers from polishing millets and to advise consumers to prefer whole millets (unpolished) and their by-products.

5.4. Minerals

Millets are an abundant source of minerals such as K, Mg, Fe, Ca, and Zn, along with vitamins that are mainly accumulated in the aleurone, germ, and pericarp [ 1 ]. Soaking millet grains prior to cooking helps to reduce antinutrients while also improving mineral bioavailability. Millet grains soaked in water were shown to have reduced Zn and Fe content, which might be attributed to minerals leaching into the soaking water [ 67 ]. Soaking millet grains boosts the “in vitro solubility” of minerals such as Fe and Zn by 2–23%. Soaking the millet grains in hot water (45 to 65 °C) with a pH of 5–6 resulted in a significant increase in bioavailability and a decrease in phytic acid [ 68 ]. The mineral content in pearl millet flour was affected by germination and fermentation [ 49 ]. Germination of foxtail millet improved and modified the nutrient profile by increasing the mineral compounds availability [ 20 , 49 ]. Germination increased the availability of minerals by the catabolism process of antinutrients such as saponins and polyphenols, which inhibit the mineral bioavailability [ 39 ]. A similar increase in the mineral concentration in germinated foxtail millet was reported [ 69 ]. Germination also activate phytase-specific phosphatases enzyme called phytases, which hydrolyze phytate into inositol and orthophosphate and release minerals. Therefore, increased levels of minerals such as Mg (101.16 to 107.16 mg/kg), sodium (Na) (63.34 to 69.45 mg/kg), Ca (17.43 to 25.62 mg/kg), and Fe (16.01 to 54.23 mg/kg) were reported for foxtail millet [ 39 ]. The mineral content of kodo millet increased from 232.82 to 251.73 mg/100 g after 36 h of germination at 38.75 °C [ 41 ]. According to [ 70 ], fermentation improved the availability of Ca by 20%, Fe by 27%, and P and Zn by 26%. Bleaching pearl millet for 90 s increased Fe availability from 2.19 to 3.29 mg/100 g in vitro [ 49 ].

The decorticated millet grains decreased the total mineral content: Ca by 40%, Fe by 50%, and Zn by 12%; however, it increased the bio-accessibility of the minerals Ca (15 g/100 g), Fe (26 g/100 g), and Zn (24 g/100 g) [ 53 ]. The decortication process reduces the antinutrients, which inhibit mineral bioavailability by creating complexes. The antinutrient level reduction leads to an improvement in the bioavailability of minerals [ 53 ]. Another study discovered that the whole grain flour of foxtail millet after milling was mineral-rich, while the polished grain flour showed reduced mineral content but with a higher protein content [ 20 ]. Semi-polished pearl millet has been shown to significantly reduce ash content (1.5% to 1.3%), which represents the noncombustible portion of minerals. The decrease in the ash content was associated with removal of bran. Minerals such as Ca and P, along with antinutrients, are accumulated in the bran fraction of pearl millet [ 70 ]. However, semi-refining reduces the phytate content, which results in improved in vitro bio-accessibility of Fe and Ca. Milling and sieving of finger millet caused a reduction in some minerals such as Fe (6.52 to 3.29 mg), Zn (2.50 to 1.98 mg), and Ca (404.3 to 294.8 mg) [ 71 ].

The total Fe content of roasted pearl millet grains increased by 274 percent, which was due to leaching from the roasting iron-pan into millet samples during the high-temperature roasting process [ 72 ]. Similar studies on finger millet roasting increased the minerals such as Ca (337.31 to 341.24 mg/100 g) and Fe (3.45 to 3.91 mg/100 g) [ 73 ]. Foxtail millets processed through solid-state fermentation (SSF) were rich in important minerals and amino acids [ 63 ]. The mineral content was enhanced when fermented foxtail millet flour was incorporated with a single strain of L. acidophilus [ 20 ]. Studies also indicate that pure culture fermented products increase the bioavailability of minerals [ 53 ].

The dark gray color of pearl millet grains restricts their usage in food preparation. This drawback can be overcome by treating millet grains with organic acids (fumaric, acetic, and tartaric acid) or natural acidic materials (tamarind). Various researchers have studied the effect of acid treatment. A study on acid treatment, which includes soaking the grains in 0.2 N HCl solution for 24 h, subsequent washing, blanching (98 °C for 30 s), and sun-drying (2 days), significantly improved the P, Ca, and Fe extractability [ 74 ]. This increase in HCl extractability was accompanied by an increase in mineral bioavailability. When compared to native grains, pearl millet treated with acid for 18 h significantly improved the in vitro Fe bio-accessibility. The Fe concentration decreased because of the leaching of minerals naturally accumulated in the pericarp portion during processing [ 49 , 53 ]. The millet-based composite flour incorporated with skimmed-milk powder and vegetables showed a substantial increase in Zn (2.1–4.2 mg/100 g), Ca (143.6–667.8 mg/100 g) and Cu (0.5–0.9 mg/100 g), but no significant changes in Fe (3.4–3.6 mg/100 g) and Mg (4.3–4.4 mg/100 g) [ 75 ]. The report suggests that the majority of minerals are accumulated in the germ and bran layer which will be lost during dehulling and sieving operations. However, the process of germination and fermentation was found to increase the mineral content to some extent which could be exploited to develop value-added products.

5.5. Vitamins

Millets when polished/debranned contain a lower nutritional value since the bran and germ components of refined millet flour are eliminated, resulting in a loss of vitamins. Millets are considered superior to wheat, sorghum, and maize in terms of vitamin content and other nutrients that include fats, proteins, and minerals ( Table 1 ). Vitamins along with minerals are naturally accumulated in the aleurone, germ, and pericarp.

Millet grains are high in vitamins such as riboflavin, thiamine, niacin, and folic acid [ 76 ]. It has been noted that the germination and fermentation processes in pearl millet affect the vitamin content of the grains. Improved vitamin levels (thiamin) after the fermentation process were reported [ 49 ]. Little millet decortication resulted in a 67% reduction in vitamin E [ 77 ]. The milling affects the bran portion of the millet grains, which reduces vitamins that are mainly accumulated in the outer bran layer of grains. Milling pearl millet grains resulted in a considerable decrease in vitamin B and a modest reduction in vitamin E, but milling and sieving of finger millet flour tends to decrease vitamins such as thiamine (0.552 to 0.342 mg/100 g) and riboflavin (0.243 to 0.196 mg/100 g) [ 71 ]. The germination of finger millet showed increased vitamin C content, from 0.04 to 0.06 mg/100 g [ 66 ]. Similarly, increased levels of vitamins (thiamine, niacin) after germination and probiotic fermentation were reported [ 49 , 55 ]. The elevation of some vitamins levels, especially thiamine, niacin, and riboflavin, was observed during finger millet fermentation [ 78 ]. Biscuits prepared by replacing refined wheat flour with 45% of foxtail millet flour resulted in an increased value of vitamin content such as niacin (1.41%) and thiamin (0.1836%), except riboflavin (0.09%) [ 79 ]. The nutritional and storage characteristics of nutritious millet food of the West African region were studied. It was found that vitamin B2 concentration was likely reduced by 31.4%, 34.3%, and 45.7% after the processing of grain to a meal, flour, and fura, respectively [ 55 ]. The studies on milling or dehulling suggest that the vitamins are lost during these processing operations as the majority of vitamins are accumulated in the outer layer of millets. The availability of important vitamins can be improved by germinating the millets and developing by-products from germinated millets.

Fats are necessary for calorie supply, brain development, and the absorption and transport of vitamins A, D, E, and K in the body. The germination time has an impact on fat content. For instance, the raw and optimized flour of germinated foxtail millet had 4.4% and 3.6% fat, respectively which was substantially lower than the non-germinated sample. This is due to the fact that the fat is used as an energy source throughout the germination process, which leads to the reduction after germination [ 39 ]. A study to investigate the effect of high-pressure soaking on the nutritional characteristics of foxtail millet revealed that the fat content is reduced by 27.98% [ 40 ]. This was attributable to the enzymatic activity that creates free and soluble nutrients throughout the germinated phase in foxtail millets. Similarly, another study reported that malting of pearl millet for 24 h resulted in a reduction in fat by 6.34 to 5.55% [ 44 ]. During germination the increased enzyme and fat consumption as an energy source might explain the reduction in fat content. According to a study on the influence of different cooking techniques on the characteristic changes of foxtail millet [ 18 ], the fat content was highest in the roasted sample (3.2 g), followed by the raw (2.9 g), pressure cooked (2.8 g), germinated (2.6 g), and boiled sample (1.9 g). The effect of pearl millet fermentation on crude fat, reduced its value from 2.25 to 1.70% [ 63 ]. Another study on fermentation of pearl millet reported an increase in crude fat content from 1.83 to 3.71% [ 37 , 49 ]. Germination of foxtail millet was found to reduce the fat content, which is related to lipid hydrolysis and fatty acid oxidation that occurs during germination [ 55 ]. The foxtail millet grains were germinated at 30 °C and little millet at 35 °C for 24 h after overnight steeping, then tray dried at 60 °C for 6 h and milled for further analysis. The fat content reduced by 17.84% in foxtail millet and increased in little millet by 25.95% [ 58 ]. This was due to the changes in energy values since the fat content includes approximately double the energy values of protein and carbohydrate.

Thermal processing of biscuits made from pearl millet flour resulted in a percentage change in crude fat content from 2.25 to 18.77% [ 63 ]. Another study focused on thermal processing such as pan cooking and microwave heating on proso millet results showed a decreased level of fat content from 3.24 to 2.3 g/100 g (pan cooking) and from 3.24 to 3.05 g/100 g (microwave cooking), while for little millet, fat content decreased from 1.91 to 1.56 g/100 g (pan cooking) and from 1.91 to 1.79 g/100 g (microwave cooking) [ 52 ]. Similarly, roasting decreased the crude fat content by 0.71%, puffing and popping decreased fat content by 0.06% and 1.3–0.63 g/100 g, respectively [ 66 ]. The study on the popping of foxtail millet reported having lower value of crude fat content than raw millet [ 55 ]. Bleaching of pearl millet for 90 s resulted in a greater drop in free fatty acids level from 44.56 to 20.59 mg/100 g [ 49 ].

The use of roller mills for the production of low-fat pearl millet grits was investigated, and it was observed that decortication, tempering, and milling using finer corrugated rollers offered an average output of 61% grits (from whole grains) and 1.2% fat content [ 49 ]. By contrast, another study stated that decortication of pearl millet had no significant changes in fat content. It was also observed that when moisture content and milling time increase, the fat, ash, and fiber content reduces [ 55 ]. Development of composite millet flour had a higher rate of oil and water absorption capacity than that of millet flour [ 75 ]. The oil absorption capacity (OAC) and water absorption capacity (WAC) of the composite flour of different millets increased from 59.2% to 77.9% and from 117% to 225%, respectively. The OAC refers to flour protein’s capacity to physically bind fat through capillary attraction, which is essential since fats function as flavor retainers and improve the mouthfeel of foods. The studies provide sufficient evidence on degradation or denaturation of fat at high temperature processing (cooking and popping) as well as reduction in fat content during milling, malting and fermentation processes. The simple processing techniques such as soaking, germination and malting could be the ideal option for manufacturers to develop low-fat food products from millets. The high temperature processing would damage the fat quality and might reduce the taste and flavor of the processed foods.

6. Conclusions

Millets have an energy value similar to staple cereals. Additionally, they provide more significant health benefits due to their high fiber, minerals, vitamins, macro- and micronutrients, and phytochemicals and can help combat chronic disorders. Making millets part of a regular diet can provide an affordable, complete, and healthy meal. It was observed that during germination and fermentation of millets, the dietary fiber, mineral, and vitamin content of most millets improved. Simple processing techniques such as soaking, germination/malting, and fermentation can help tackle the problem of protein–energy malnutrition by improving protein digestibility and the bioavailability of the minerals. However, it was observed that decortication, dehulling, milling, extrusion resulted in a reduction of total proteins, total dietary fiber, and micronutrients. Thus, care should be taken during the decortication of millets, as excessive dehulling can result in lower fiber content and loss of micronutrients due to the loss of nutrient-rich bran and germ portion.

Looking into the variability of the impact of processing on the nutritional characteristics of millets, there is still a need to focus on optimizing the processing techniques for minor millets to make them more acceptable without compromising the health benefits. Moreover, to combat food insecurity and malnutrition, awareness needs to be created at both commercial and household levels regarding the impact of processing methods on the nutritional properties of millets and the health benefits of millets.

Acknowledgments

This paper is contribution number 22-178-J from the Kansas State University Agricultural Experiment Station.

Author Contributions

Conceptualization, supervisor lead, writing—original draft preparation, writing—review and editing, N.A.N.G.; conceptualization, writing—review and editing, K.S.; writing—review and editing, P.V.V.P.; writing—original draft preparation, writing—review and editing, Y.B.; writing—original draft preparation, writing—review and editing, B.P.N.; writing—original draft preparation, writing—review and editing, C.G. All authors have read and agreed to the published version of the manuscript.

This research received no external funding.

Institutional Review Board Statement

Informed consent statement, data availability statement, conflicts of interest.

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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• Ultra-processed foods...

Ultra-processed foods linked to higher mortality

Linked research.

Association of ultra-processed food consumption with all cause and cause specific mortality

• Related content
• Peer review
• Kathryn E Bradbury , senior research fellow 1 2 ,
• Sally Mackay , senior lecturer 1
• 1 Department of Epidemiology and Biostatistics, School of Population Health, University of Auckland, Auckland 1023, New Zealand
• 2 Centre for Translational Health Research (TRANSFORM), Faculty of Medical and Health Sciences, University of Auckland, Auckland 1023, New Zealand
• Correspondence to: K E Bradbury k.bradbury{at}auckland.ac.nz

Debate about the “ultra-processed” concept must not delay food policies that improve health

As research into ultra-processed food gains momentum, 1 so too does the debate. 2 3 4 Foods that fall into the ultra-processed category according to the Nova classification are heterogeneous and include carbonated soft drinks, confectionary, extruded snack foods, distilled alcohol (spirits), and mass produced packaged wholegrain bread. 5 Ultra-processed foods are typically high in energy, added sugar, saturated fat, and salt, and a major criticism of previous studies is that they have not disentangled the effects of processing, per se, from the nutrient profile of food products. The linked paper by Fang and colleagues (doi: 10.1136/bmj-2023-078476 ) addresses this concern and others, in their evaluation of the relation between ultra-processed food consumption and mortality in two large US cohort studies. 6

Fang and colleagues found a modest increase in the risk of total mortality with higher ultra-processed food consumption 6 ; however, this association was no longer apparent after overall diet quality was taken into account. They also showed that the association between ultra-processed food consumption and mortality was somewhat stronger when distilled alcohol, which is a well established risk factor for premature mortality, 7 was included in the ultra-processed category and somewhat weaker when packaged wholegrain products were included in the ultra-processed category. 6 Further adjustment for pack years of smoking (rather than current smoking status only) greatly attenuated the association between ultra-processed food consumption and respiratory mortality. 6 Thus, future studies must adjust more fully for lifetime smoking exposure or present results in non-smokers to reduce the impact of residual confounding.

The potential mechanisms put forward to explain observed associations between ultra-processed food and health outcomes are also heterogeneous and include over-consumption due to the energy density; fat, sugar, and salt content; potential deleterious effects of certain additives; and contaminants from packaging. 1 Combining heterogeneous foods into a single exposure variable does not help to progress our understanding of the potential harm, if any, of specific additives, processing, or packaging techniques, beyond any harmful effects of the poor nutrient profile of food products. Note that the Nova food processing categorization system classifies foods on the basis of not only the level of processing and the presence of additives but also on the purpose of those additives. 5 From an aetiological perspective, the purpose of a food additive is irrelevant—either it is harmful for health or it is not.

Expert bodies such as the Joint Food and Agriculture Organization/World Health Organization Expert Committee on Food Additives (JEFCA) exist to evaluate individual food additives for safety and to determine the potential carcinogenicity of foods and their components. The International Agency for Research on Cancer (IARC) and the World Cancer Research Fund (WCRF) both concluded that alcohol and processed meat cause cancer in humans. 8 9 10 Both alcohol and processed meat, as defined by IARC, span both the “processed” (for example, beer and wine; salted, dried, and cured meat) and “ultra-processed” (for example, distilled alcohol; sausages and hot dogs) Nova categories. 5

Fang and colleagues sensibly conclude that not all ultra-processed food needs to be universally restricted and that careful deliberation is needed when considering whether to include recommendations about ultra-processed food in dietary guidelines. 6 Most dietary guidelines already implicitly emphasise the consumption of less processed foods. 11 In countries where affordable, mass produced packaged wholegrain products such as breads are a recommended dietary staple and a major source of fibre, adding a sweeping statement in dietary guidelines about avoiding ultra-processed foods is not helpful.

Recommendations to avoid ultra-processed food may also give the impression that foods that are not ultra-processed are healthy and can be freely consumed. This is problematic—for example, the IARC and WCRF have concluded that red meat (categorised by the Nova system as “unprocessed or minimally processed”) is probably increases the risk of bowel cancer. 8 9 In addition to effects on health, beef and lamb come from ruminant animals, which produce methane—a greenhouse gas that has a particularly potent warming effect over the short term. 12

Our global food system is dominated by packaged foods that often have a poor nutritional profile. 13 This system largely serves the goals of multinational food companies, which formulate food products from cheap raw materials into marketable, palatable, and shelf stable food products for profit. 13 We should not let the debate on the usefulness of the ultra-processed food concept delay the implementation of evidence based interventions such as the WHO’s “best buys” for health. 14

Several countries have already implemented and demonstrated the effectiveness of best buys and other interventions to better serve population health. These include the restriction of marketing of unhealthy foods to children and the addition of warning labels on nutritionally poor food products, 15 taxes on sugar sweetened beverages, 16 and bans on partially hydrogenated oils that are a source of industrial trans fat. 17 Our focus should be on advocating for greater global adoption of these and more ambitious interventions and increasing safeguards to prevent policies from being influenced by multinational food companies with vested interests that do not align with public health or environmental goals.

Competing interests: The BMJ has judged that there are no disqualifying financial ties to commercial companies. The authors declare the following other interests: KEB’s spouse is a brewer at Steam Brewing Company, Auckland, New Zealand; SM is co-chair of the food expert group of Health Coalition Aotearoa, a non-commercial advocacy group against harmful commodities including unhealthy food; she was on the organising committee of the Nutrition Society of NZ and Australia conference in 2023; the conference received sponsorship from food companies whose products were screened for alignment with national dietary guidelines. Further details of The BMJ policy on financial interests is here: https://www.bmj.com/sites/default/files/attachments/resources/2016/03/16-current-bmj-education-coi-form.pdf ."

Provenance and peer review: Commissioned; not peer reviewed

• ↵ Percival R, Warner A, Rayner M. Table Debates, Series 5: Is the Ultra-processed Food (UPF) concept useful, and for what goals? 2024. https://tabledebates.org/letterbox/is-the-ultra-processed-food-concept-useful .
• ↵ British Nutrition Foundation. The concept of ultra-processed foods (UPF): Position statement April 2023. 2023. https://www.nutrition.org.uk/media/swdophda/upf-position-statement-april-2023.pdf .
• ↵ Science Advisory Committee on Nutrition. SACN statement on processed foods and health. 2023. https://assets.publishing.service.gov.uk/media/64ac1fe7b504f7000ccdb89a/SACN-position-statement-Processed-Foods-and-Health.pdf .
• Monteiro CA ,
• Rossato SL ,
• Zaridze D ,
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• ↵ World Cancer Research Fund/American Institute for Cancer Research. Diet, nutrition, physical activity and cancer: a global perspective. Continuous Update Project Expert Report 2018. https://www.wcrf.org/diet-activity-and-cancer/ .
• Bouvard V ,
• Guyton KZ ,
• International Agency for Research on Cancer Monograph Working Group
• ↵ International Agency for Research on Cancer. Personal habits and indoor combustions. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans Volume 100E. 2012. https://publications.iarc.fr/122 .
• Machado P ,
• Lacy-Nichols J
• Godfray HCJ ,
• Aveyard P ,
• Garnett T ,
• Swinburn BA ,
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• ↵ World Health Organization. Updated Appendix 3 of the WHO Global NCD Action Plan 2013-2030. 2022. https://cdn.who.int/media/docs/default-source/ncds/mnd/2022-app3-technical-annex-v26jan2023.pdf .
• Taillie LS ,
• Bercholz M ,
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• Scarborough P ,
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• Harrington RA ,
• McKelvey W ,
• Curtis CJ ,

• Open access
• Published: 11 May 2024

Natural approach of using nisin and its nanoform as food bio-preservatives against methicillin resistant Staphylococcus aureus and E.coli O157:H7 in yoghurt

• Walaa M. Elsherif 1 , 2 ,
• Alshimaa A. Hassanien 3 ,
• Gamal M. Zayed 2 , 4 &
• Sahar M. Kamal 5  

BMC Veterinary Research volume  20 , Article number:  192 ( 2024 ) Cite this article

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Metrics details

Natural antimicrobial agents such as nisin were used to control the growth of foodborne pathogens in dairy products. The current study aimed to examine the inhibitory effect of pure nisin and nisin nanoparticles (nisin NPs) against methicillin resistant Staphylococcus aureus (MRSA) and E.coli O157:H7 during the manufacturing and storage of yoghurt. Nisin NPs were prepared using new, natural, and safe nano-precipitation method by acetic acid. The prepared NPs were characterized using zeta-sizer and transmission electron microscopy (TEM). In addition, the cytotoxicity of nisin NPs on vero cells was assessed using the 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The minimum inhibitory concentrations (MICs) of nisin and its nanoparticles were determined using agar well-diffusion method. Further, fresh buffalo’s milk was inoculated with MRSA or E.coli O157:H7 (1 × 10 6 CFU/ml) with the addition of either nisin or nisin NPs, and then the inoculated milk was used for yoghurt making. The organoleptic properties, pH and bacterial load of the obtained yoghurt were evaluated during storage in comparison to control group.

The obtained results showed a strong antibacterial activity of nisin NPs (0.125 mg/mL) against MRSA and E.coli O157:H7 in comparison with control and pure nisin groups. Notably, complete eradication of MRSA and E.coli O157:H7 was observed in yoghurt formulated with nisin NPs after 24 h and 5th day of storage, respectively. The shelf life of yoghurt inoculated with nisin nanoparticles was extended than those manufactured without addition of such nanoparticles.

Conclusions

Overall, the present study indicated that the addition of nisin NPs during processing of yoghurt could be a useful tool for food preservation against MRSA and E.coli O157:H7 in dairy industry.

Peer Review reports

Introduction

Using of bacteriocins such as nisin alone or combined with other natural materials such as essential oils, could be represented as a useful candidate for improving the microbiological quality and maintaining the sensory properties of milk and milk products [ 1 , 2 ]. The utility of nisin as a bio preservative in food industry has been approved and this bacteriocins was effective enough to extended shelf life in regions with inadequate preservation facilities such as developing countries [ 3 ]. Nisin is a natural water-soluble antibacterial peptide (AMP) composed of 34 amino acid residues produced by Lactococcus lactis. It has the ability to inhibit the growth of some foodborne pathogens and many of Gram-positive spoilage bacteria [ 4 , 5 ]. This antibacterial peptide is generally regarded as a safe food preservative by the joint Food and Agriculture Organization and World Health Organization (FAO/WHO), also by the US Food and Drug Administration (FDA) [ 6 , 7 ]. Based on aforementioned permissions, it is widely commercialized as a safe and natural food preservative in the food industry in more than 50 countries around the world [ 8 ].

The antibacterial activity of nisin in food is depending on several factors such as its solubility, pH and structural properties of target bacteria. It could exhibit potent antimicrobial activities against many species of Gram-positive pathogens, while it has little effect against Gram-negative bacteria, yeast and fungi due to their outer membrane barriers [ 9 ]. The exact antibacterial mechanism of nisin is attributed to the passage of nisin through the cell wall of bacteria and its interaction with lipid II, which considered as an essential element in the bacterial cell wall [ 9 ].

There are some obstacles that can hinder the antimicrobial efficacy of free nisin as a food bio preservative such as its ability to interact with food components (e.g. proteolytic enzymes, phospholipids, fatty acids and proteins), high pH and many other food additives. These factors could drastically reduce or completely diminish the antimicrobial effect of nisin [ 10 ]. Hence, different strategies were developed to improve the preservative efficacy of nisin such as liposomes [ 11 ] and nanoparticles [ 12 ]. However, these reported techniques are not suitable for applications in food industries due to the utility of inorganic solvents and chemical compounds, in addition to they are expensive and complicated. For these reasons, alternative organic chemicals and solvents or green synthesized nanoparticles were developed to overcome the inactivation of free nisin by many food components through protecting nisin and releasing it in sustained manner [ 13 ]. For instance, acetic acid, a well-known biocompatible organic acid, has no adverse effects, no dietary restrictions and it is generally recognized as a safe food additive. This organic acid is commonly used, as a natural preservative, in the preservation of food especially in cheese and dairy products where it inhibit the development of bacteria, yeast and fungi [ 14 , 15 ]. Besides acetic acid, tween 80 has a great potential to stabilize nanoparticles dispersion through formation of a protective coat around the nanoparticles, so it was used in food without adverse health effect [ 16 , 17 ].

Application of nisin in dairy industry was reported in more than 55 countries due to its prominent antimicrobial, technological characteristics, safety, stability and flavorless. Commercially, nisin was used in several food matrices to ensure safety, extend shelf life, and to improve the microbial quality either through addition of nisin directly in its purified form or through its production in situ by live bacteria [ 18 , 19 , 20 ]. For instance, nisin was added as a bio-preserving ingredient in some kinds of cheese [ 21 , 22 , 23 ], skim milk and whole milk [ 24 , 25 , 26 , 27 ]. Nisin has a potent antibacterial effect against spore-forming bacteria that are the main spoilage concerns in the food industry [ 26 ]. However, several factors such as neutral pH [ 4 ], Fat% [ 25 ], protein% [ 28 ] as well as calcium and magnesium concentrations that can reduce the antimicrobial efficacy of nisin were reported when used directly in dairy foods [ 15 , 29 , 30 ]. Certain previously reported strategies, such as encapsulation and nano-encapsulation of nisin, were applied to increase the antimicrobial efficacy of nisin in dairy industry [ 31 , 32 ]. . Importantly, there is no available data about the use of nisin or nisin NPs as antimicrobial agents during yoghurt preparation.

Accordingly, the current study was designed to prepare nisin NPs by simple nanoprecipitation technique using natural, biocompatible and safe materials. Also the aims of this study were extended to investigate the antibacterial effect of obtained nanoparticles on MRSA and E.coli O157:H7 during manufacturing and storage of yoghurt. Additionally, the effect of the used nisin NPs on the organoleptic properties of yoghurt was addressed.

Materials and methods

Acetic acid (Merck Co., Germany), nisin (Sigma Aldrich from Lactococcus lactis , potency ≥ 900 IU/mg, purity ≥ 95%, CAS Number 1414-45-5), Brain Heart Infusion (BHI) (BBL 11,407, USA), phosphate buffer saline (PBS) (Oxoid, Basingstoke, UK) were purchased and used as received. Polyethylene glycol sorbitan monooleate (Tween 80) was purchased from Sigma Aldrich. Additionally, Mueller Hinton agar (M173) was purchased from HiMedia (Pvt., India), and LAB204 Neogen Company. While, 0.5 McFarland Standard (8.2 log 10 CFU/ml) (Cat. No. TM50) was purchased from Dalynn Biologicals Co. The deionized water was obtained from the Molecular Biology Unit, Assiut University, Egypt.

Preparation of nisin nanoparticles

Nisin (2 mg/mL) was completely dissolved in 100 mL of 0.1 M aqueous acetic acid solution with the aid of sonication using cold probe sonication (UP100H Hielscher Ultrasound). Then, 50 mL of deionized distilled water was gradually added to the nisin solution while maintaining the pH value within the range of 2.5 to 3. Further, 0.01% tween 80 was added as a stabilizer and the mixture was constantly stirred at 25 oC for 7 h to eliminate acetic acid as much as possible. Finally, the nanoparticles suspensions were then sonicated for 5 min before stored at refrigerator temperature for further use. The obtained nanoparticles were examined for size, shape, antibacterial activity and stability after six months.

Characterization of the prepared nisin NPs

Dynamic light scattering (dls).

The prepared nanoparticles was characterized by DLS at a fixed scattered angle of 90° using a Zetasizer, ZS 90 (3000 HS, Malvern Instruments, Malvern, UK) at the Nanotechnology Unit, Al-Azhar University at Assiut, Egypt. Measurements were taken at 25 °C and Zetasizer® software (version 7.03) was used to collect and analyze the data [ 33 ].

Fourier-transform infrared spectroscopy (FTIR)

FTIR was performed at the Chemistry Department at the Faculty of Science, Assiut University. This experiment was used to identify the functional groups and the fingerprint of the molecule. Samples were prepared by compressing potassium bromide with either free nisin or NNPs into small discs. The produced discs were then scanned using FTIR spectrometer (FTIR, NICOLET, iS10, Thermo Scientific) in the wave number ranged from of 4000 to 500 cm − 1 [ 34 ].

High resolution transmission electron microscopy (HRTEM)

The morphology of the prepared nisin NPs was determined using HRTEM (JEM2100, Jeol, Japan) at the Electronic Microscope Unit, National Research Center, Egypt. The sample was diluted with deionized water, and a small drop of nisin NPs was dropped onto 200-mesh copper coated grids at room temperature and negatively stained with uranyl acetate for 3 min. Excess liquid was removed using Whatman filter paper and samples were dried at room temperature [ 35 ].

Bacterial strains and inoculum preparation

The tested pathogens (MRSA and E. coli O157:H7) were previously isolated from dairy products (milk, cheese and yoghurt) samples by culture method and identified using conventional biochemical method and PCR at a certified food lab, Animal Health Research Institute (AHRI), Egypt [ 36 , 37 ]. These isolates were inoculated in trypticase soy broth (Himedia, India) and incubated at 37˚C for 24 h, then co-cultured on selective agars such as MRSA agar base (Acumedia, 7420, USA) and Sorbitol MaCconkey agar (Himedia, India) [ 38 , 39 ] for MRSA and E. coli O157:H7, respectively. The isolates were inoculated in BHI broth and incubated at 37 °C for 24 h until turbidity was comparable to a 0.5 McFarland turbidity standard. Before inoculating bacteria in milk, the inoculum was washed twice in PBS and then re-suspended in skim milk.

Determination of minimum inhibitory concentration (MIC) of free nisin and nisin nanoparticles against MRSA and E. Coli O157:H7

To determine the MIC of nisin NPs against MRSA and E.coli O157:H7, the agar well diffusion method was used according to Suresh et al. [ 40 ] with minor modifications. In brief, 0.1 mL of the previously prepared bacterial suspensions was spread on Mueller Hinton agar plates and left for 10 min to be absorbed. Then, 8 mm wells were punched into the agar plates for testing the antimicrobial activity of nanoparticles. One-hundred µl of different concentrations of free nisin and nisin NPs (from 0.0313 mg/mL to 2 mg/mL) were poured onto the wells. One well in each plate contained 100 µL of sterile deionized water was kept as a negative control. After overnight incubation at 35 ± 2 °C, the diameters of the inhibition zones were observed and measured in mm [ 41 ]. Each concentration was performed in triplicate.

Assessment of nisin nanoparticles cytotoxicity

The biocompatibility and the cytotoxicity of the nisin NPs were evaluated using a MTT assay against a Vero cell line after culture at 37 °C in a humidified incubator with 5% CO 2 in Dulbecco’s Modified Eagle’s Medium supplemented with 10% Fetal Bovine Serum. The cells were seeded into a 96-well plate at a density of 1 × 10 4 cells/well overnight before treatment. Different dilutions (0.5×MIC, MIC, 2×MIC, 4×MIC) of optimized nisin NPs were added to the seeded cells. Cells without nanoparticles served as control group. After 72 h, the consumed media was replaced with phosphate buffered saline, 10 µL from 12 mM MTT stock solution was added to each well and cells were incubated for 4 h at 37 °C. Next, 50 µL DMSO was added to dissolve formazan crystals and then the absorbance was measured at 570 nm using a BMG LABTECH®-FLUO star Omega microplate reader (Ortenberg, Germany). All experiments were performed in triplicate.

Antibacterial efficacy of the free nisin and nisin NPs against MRSA and E. Coli O157:H7 during manufacturing and storage of yoghurt

Fresh milk was heated at 85 °C for 5 min in water bath then suddenly cooled. The prepared inoculums were added to the warmed milk (41 ºC) in a count of 10 6 CFU/mL. The inoculated milk was divided into four parts for further use as following, part 1 is the positive control (contained MRSA or E. coli O157:H7 only, one jar each), part 2 (contained MRSA or E. coli O157:H7 with nisin NPs at MIC and 2×MIC, two jars each), part 3 (contained MRSA or E. coli O157:H7 with free nisin at MIC and 2×MIC, two jars each) and part 4 (negative control; free from pathogens and contained free nisin or nisin NPs only, one jar each). After inoculation of the different treatments, yoghurt was manufactured according to Sarkar [ 42 ] by adding 2% yoghurt starter culture ( Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus ) at 41 °C to milk. The prepared yoghurt was placed in a constant-temperature incubator at 40 °C until pH reached 4.6 to 4.5. Finally, the obtained products were stored at refrigeration temperature (4 ± 1 °C) for 5 days. Samples were collected just after manufacturing of yoghurt and every 2 days during storage, then tested for the count of MRSA using MRSA agar base media [ 43 ], and E. coli O157:H7 using Sorbitol MacConkey (SMAC) agar plates [ 44 ]. In addition, pH values were determined in the examined samples as previously described by Igbabul et al. [ 45 ]. In brief, 10 g o f yoghurt sample was dissolved in 100 mL of distilled water. The mixture was left to equilibrate at room temperature. Then, the pH of the samples was then measured by a pH meter (Microprocessor pH meter, pH 537, WTW, Germany).

Organoleptic assay of manufactured yogurt

Pathogen-free yoghurt jars (negative control) were prepared with two concentrations of either free nisin or nisin NPs (MIC and 2×MIC) as previously mentioned to be used for organoleptic evaluation. Thirty-five panelists were selected in teams of different ages, sex and education. The perception of consumers toward samples with two concentrations of nisin NPs was recorded. Consumers were asked to evaluate the color, flavor, mouth feel, appearance, and overall acceptability (OAA) of the prepared yoghurt samples containing nisin NPs [ 46 ]. The scale points were excellent (5); very good (4); good (3); acceptable (2); and poor (1).

Statistical analysis

One-way analysis of variance (ANOVA) was performed using the SPSS program (SPSS Inc., Chicago, IL, USA, 18) to determine the statistical significance of differences between groups. Results with P  < 0.05 were considered statistically significant. The microbiological and cytotoxicity assay data were prepared using Excel software version 2017. While, the FTIR results were performed using Origin Lab 2021 for graphing and analysis. All experiments were carried out in triplicate.

Characterization of the prepared nanoparticles

The freshly prepared nisin NPs had 26.55 nm size and PDI 0.227 as determined by zetasizer. While, the diameter of the same after 6 months at refrigeration temperature was 86.50 nm with a PDI equal to 0.431 (Table  1 ). These results indicated that reasonable small-sized particles of nisin were obtained by precipitation technique using acetic acid. The small size of the prepared particles and the small PDI range (from 0.2 to 0.4) indicated a mono size dispersion and a good stability of the prepared nisin NPs.

The size and morphology of the freshly prepared nisin NPs and after 6 months of storage were measured by HRTEM are presented in Fig.  1 . Both freshly prepared and stored nisin NPs were approximately uniform in size with adequate distribution of particles. The shape of the particles was nearly spherical with slightly a bit of agglomeration just after 6 months of storage. The average size of freshly prepared nisin NPs was 7.35 nm while, after 6 months was 15.4 nm. The size of particles determined by TEM is usually smaller than the dynamic particles determined by zeta-sizer because TEM determine the actual particle diameter while zeta-sizer determine the particles diameter with adjacent moving layers of solvents.

The TEM images of freshly prepared nisin NPs (A) and after 6th months of storage (B)

Figure  2 showed the FTIR of pure and nisin NPs; both spectrum showed the characteristic peaks of nisin at 3425, 1599 and 1493 cm − 1 corresponded to O-H stretching of COOH, C = O stretching of amide I and N-H bending amide II. Bands 1530 cm − 1 in free nisin indicated the stretching of amid II and which, increased to 1549 cm − 1 in nisin NPs that indicated increase the H- bond in nano form than free one. The results of FTIR spectrum confirmed that the formation of nisin NPs did not result in any chemical changes or interaction of nisin with used the materials. These results also demonstrated the suitability of the applied method for the preparation of chemically stable and small-sized nisin NPs.

The FTIR of pure nisin and nisin NPs

Assessment of Nisin nanoparticles cytotoxicity

In the present study, Veros cells were exposed to nisin NPs for 48 and 72 h, and the cytotoxicity was measured by MTT assays. Results showed that the MIC did not exhibit an anti-proliferation effect (Fig.  3 ). Interestingly, even at very high concentrations (4xMIC), there were no cytotoxicity effect as the percentage of viable cells reach 92% and 89.98% after 48 and 72 h, respectively. The obtained findings confirmed the safety and good biocompatibility of the prepared nisin NPs at MIC level.

Cytotoxicity and cell viability of different concentrations nisin NPs using Vero cells after 48 and 72 h using MTT assay

MIC of free nisin and nisin NPs against MRSA and E. Coli O157:H7

The efficacy of the free nisin and prepared nisin NPs against MRSA and E. coli O157:H7 was investigated using agar well diffusion assay (Table  2 ). Nisin and its nanoparticles showed potent antibacterial effect against MRSA than E. coli O157:H7. The MICs of nisin and nisin NPs toward MRSA were 0.0625 and 0.0313 mg/mL, respectively. While, 0.125 mg/mL was the MIC of both nisin and nisin NPs against E. coli O157:H7. Of note, growth inhibition zone was not observed against MRSA at 0.0313 mg/mL of nisin, and toward E. coli O157:H7 at both 0.0625 and 0.0313 mg/mL nisin (Table  2 ). On the other hand, the prepared nisin NPs could produce inhibition zones against MRSA with a mean diameter ranged from 25.4 ± 2.1 mm to 7.1 ± 0.89 mm at concentrations of 2 to 0.0313 mg/mL, respectively. Also, the nisin NPs showed anti- E. coli O157:H7 activity at different concentrations of 2, 1, 0.5, 0.25 and 0.125 mg/mL with average size of 20.1, 15.4, 12.7, 9.5 and 7.2 mm of the inhibitory zones, respectively. There were no inhibition zones against E. coli O157:H7 at 0.0625 and 0.0313 mg/mL of nisin NPs. Overall, the obtained findings indicated that the most effective MICs of nisin and nisin NPs for both organisms were 0.125 mg/mL (Table  2 ).

Antibacterial effect of nisin and nisin NPs against MRSA and E. Coli O157:H7 during manufacturing and storage of yoghurt

Figure  4 presented the antibacterial activity of nisin against the examined foodborne pathogens (MRSA and E. coli O157:H7). Here, nisin at 0.125 and 0.25 mg/ml could induce antibacterial effect against MRSA (3.3 and 3 log 10 CFU/g, respectively) after 24 h of yoghurt storage. However the effect was not higher as in case of nisin NPs (2.3 and 1 log 10 CFU/g) at the same concentrations and time of storage. While, the inhibitory impact of the free nisin on E. coli O157:H7 was observed after 24 h (3.7 log 10 CFU/g) and 3 days (3.8 log 10 CFU/g) of storage at the concentrations of 0.25 and 0.125 mg/mL, respectively. The pathogens were still detected till the end of the experiment in nisin treated yoghurt (Fig.  4 ).

Antibacterial effect of free nisin (A) and nisin NPs (B) on MRSA and E.coli O157:H7 during manufacturing and storage of yoghurt

On the other hand, there was a clear reduction in mean count of MRSA and E.coli O157:H7 in the laboratory-manufactured yoghurt supplemented with different concentrations (0.125 and 0.25 mg/mL) of nisin NPs. A complete inhibition of MRSA was observed after 24 h and at the 3rd day of storage by 0.25 and 0.125 mg/mL of nisin NPs, respectively (Fig.  5 ). While, E. coli O157:H7 was undetectable at the 5th day of storage with 0.25 mg/mL nisin NPs, however it was still detected till the end of the experiment in either yoghurt inoculated with 0.125 mg/mL nisin NPs or in the positive control group (Fig.  4 ). Taken together, the antimicrobial count tests revealed that the free nisin is not effective as the nisin NPs at same time points during processing and storage of yoghurt.

During storage, the pH did not change significantly between different treatments. However, the negative control group showed little decrease in pH in comparison to other groups at the 3rd and 5th day of storage (3.5 and 3, respectively).

Evaluation of pH levels during processing and storage of yoghurt inoculated with different concentrations of free nisin or nisin NPs

Organoleptic evaluation of the laboratory-manufactured yoghurt

Figure  6 clarified that there was no difference in the sensory properties between the different groups (contained 0.125 or 0.25 mg/mL nisin (Fig. 6A) or nisin NPs (Fig. 6B)) in comparison to the control group. The OAA of yoghurt inoculated with 0.125 mg/mL and 0.25 mg/mL of free nisin was 3 and 2.5, respectively (Fig. 6A). While, the control samples had the highest score in mouth feel (4.5), followed in order with yoghurt loaded with 0.125 mg/mL and 0.25 mg/mL nisin NPs (3.8 and 2.7, respectively). Additionally, the overall acceptability (OOA) of control, 0.125 mg/mL and 0.25 mg/mL nisin NPs groups was 4, 3.7 and 3, respectively (Fig. 6B). Such findings indicated the high acceptability of yoghurt containing different concentrations of nisin NPs than those inoculated with free nisin.

Organoleptic properties of yoghurt inoculated with different concentrations of free nisin and nisin NPs

The current study elucidated for the first time the inhibitory effect of free nisin and nisin NPs on two of the most common foodborne pathogens (MRSA and E. coli O157:H7) during processing and storage of laboratory manufactured yoghurt. Strikingly, adding of nisin NPs to yoghurt could induce much higher antibacterial effect on MRSA and E. coli O157:H7 with high consumer acceptability than free nisin. Accordingly, nisin NPs could be a useful and effective bio-preservative candidate against MRSA and E. coli O157:H7 in dairy industry.

The present study revealed that nisin NPs was prepared by a novel and safe method using natural material such as acetic acid which is commonly applied in food products. Chang et al. [ 47 ]. prepared ultra-small sizes of nisin NPs by nanoprecipitation method using HCL while we obtained much smaller particle size of NNPs using acetic acid which is more safer, less toxic and accepted by consumers. The particle size determined by TEM is smaller than the size measured by DLS this difference could be attributed to the removal of solvent and shrinking of nanoparticles during the drying of nisin NPs samples for TEM investigations. In addition, DLS measures the hydrodynamic diameter of the dispersed moving particles with the surrounding moving layers of solvents [ 48 , 49 ].

The result of FTIR was in consistent with that of Flynn et al. [ 50 ]. Herein, we found that the -OH stretching peak of nisin NPs displayed a greater intensity than that of free nisin, which indicated a stronger hydrogen bonding formation within nisin NPs. In case of free nisin, the peak at 1620 cm − 1 corresponding to COO − was shifted to 1610 cm − 1 in nisin NPs indicating that the hydrogen bonding was increased within nisin NPs. In contrast, the amid II band in free nisin appeared at 1530 cm − 1 became more obvious at 1549 cm − 1 in nisin NPs which was in agreement with Webber et al. [ 51 ]. . Band of amide I at wave number of 1632 cm − 1 could be due to the change in the structure of free nisin when converted into nisin NPs by using natural acetic acid.

In food chain, nisin has been approved for use in over 50 countries due to its safety and its potent antimicrobial activity without inducing microbial resistance [ 52 ]. Of particular note, the FAO/WHO Codex Committee and US FDA allow using nisin as a food additive in dairy products at a concentration up to 250 mg/kg [ 1 , 53 ]. Moreover, European Food Safety Authority [ 54 ] reported that nisin has been shown to be non-toxic to humans and it is safe as a food preservative for dairy and meat products. In the current study, the examined organisms (MRSA and E. coli O157:H7) have been involved in many food outbreaks worldwide as well as their resistance to many antibiotics, considered a challenge to be controlled [ 55 , 56 , 57 ]. Therefore, the present study could be a useful alternative strategy to avoid the possible health hazards of these organisms after consumption of yoghurt using either nisin or nisin NPs as natural food preservatives.

The obtained results revealed that the MICs of nisin and nisin NPs against MRSA were lower than that of E. coli O157:H7. This could be due to the ability of nisin to penetrate the cell wall of Gram-positive bacteria, however, it is difficult for nisin to penetrate the outer membrane barrier of Gram-negative bacteria [ 58 ]. Nisin could destroy bacteria through two mechanisms, either by making pores in the plasma membrane or by inhibiting the cell wall biosynthesis through binding to lipid II [ 59 , 60 , 61 ]. Importantly, the obtained results in the current study showed that that MIC of nisin NPs against MRSA was lower than that of pure nisin. Similarly, Zohri et al. [ 62 ] reported that the MICs of nisin and Nisin-Loaded nanoparticles was 2 and 0.5 mg/mL after 72 h of incubation period with the S. aureus samples, respectively. In addition, Moshtaghi et al. [ 63 ] examined the antibacterial effect of nisin on S. aureus and E. coli at different pH values and they found that the MICs against S. aureus were ranged from 19 to 312 µg/mL of nisin at pH levels from 8 to 5.5, respectively. While for E. coli , the MICs were from 78 to 1250 µg/mL at the same range of pH, respectively [ 63 ].

Interestingly, nisin inhibited the pathogenic foodborne bacteria and many other Gram-positive food spoilage microorganisms [ 13 ]. In the present study, evaluation of the kinetic growth of MRSA and E. coli O157:H7 based on the total counts in the laboratory manufactured yoghurt revealed that nisin NPs was able to inhibit more effectively the growth of such foodborne pathogens than free nisin during manufacturing and storage of yoghurt. These findings were in concurrent with those obtained by Zohri et al. [ 62 ] who demonstrated that nisin-loaded chitosan/alginate nanoparticles showed more antibacterial effect than free nisin on the growth of S. aureus in raw and pasteurized milk samples. Additionally, nisin Z in liposomes can provide a powerful tool to improve nisin stability and inhibitory action against Listeria innocua in the cheddar cheese [ 64 ]. In our study, nisin NPs showed a complete inhibition of MRSA after curdling of yoghurt and reduced the survivability of E. coli O157:H7 when applied at two different concentrations during storage of such product. Nisin NPs with high specific surface area could be easily attached to the target cell surface leading to increased permeability of the cell membrane, and finally cause bacterial cell death. Furthermore, nisin NPs were thermo-tolerant because of the internal non-covalent interactions in the nanoparticles [ 4 , 65 ]. Additionally, the decline in the mean count of the examined pathogens (MRSA and E.coli O157: H7) in the current study may be due to the effect of low pH (high acidity) of yoghurt that leads to shrinkage and death of the bacterial cells [ 66 ]. Similarly, Al-Nabulsi et al. [ 67 ] reported that the combination of a starter culture, low temperature, and pH ( ∼ 5.2) had inhibitory effects on the growth of S. aureus .

The effect of adding different levels of nisin and nisin NPs on OAA scores of yoghurt was recorded and the obtained results were in agreement with Hussain et al. [ 68 ], Radha [ 3 ], and Gharsallaoui et al. [ 4 ] who reported that a Nigerian fermented milk product had acceptable sensory scores till 25th day of storage when loaded with nisin at 400 IU/mL. Additionally, Chang et al. [ 47 ] said that the thermal treatments are known to cause undesirable changes in the sensory, nutritional and/or technological properties of milk. Taking advantage of the antimicrobial action of nisin NPs against several spoilage and pathogenic microorganisms, this innovative non-thermal food preservative offers the inactivation of microorganisms with minimal impact on the quality, safety, nutritional values and acceptability of dairy products.

Overall, as the demand for preservative-free food products increased, natural antimicrobials have gained more and more attention because of their effectiveness and safety. Consequently, the current study investigated that the addition of nisin NPs to milk for manufacturing of yoghurt can be used as an innovative preventive measure to inhibit the contamination with foodborne pathogens. However, further researches are required to determine the effective and safe dose of nisin NPs for application in other dairy products.

The present study prepared nisin NPs using acetic acid by precipitation method and the obtained particles were small in size with good stability and consumer acceptability. The antibacterial effect of nisin and nisin NPs against MRSA and E. coli O157:H7 in yoghurt was impressive. Additionally, the studied nanoparticles did not affect the sensory and textural characteristics of the finished product. Hence, this study could be useful for yoghurt makers and dairy products factories through using this novel preservation technology to inhibit the growth of MRSA and E. coli O157:H7, in yoghurt and dairy products, and subsequently avoid food spoilage and foodborne diseases.

Data availability

All data and materials are available here in the current study.

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Acknowledgements

The authors thank the nanotechnology research and synthesis unit at animal health research institute, Assiut, Egypt for their help in preparation of nanomaterials.

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W.M.E., A.A.H., G.M.Z., and S.M.K. conceived and designed the experiment. W.M.E., A.A.H., G.M.Z., and S.M.K. collected the experimental data. W.M.E., A.A.H., and S.M.K. performed the microbiological analysis. A.A.H. and G.M.Z. performed the preparation and analysis of nanoparticles. W.M.E. and S.M.K. performed the statistical analysis. All authors interpreted the data. W.M.E. wrote the first draft of the manuscript. All authors reviewed the manuscript.

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Elsherif, W.M., Hassanien, A.A., Zayed, G.M. et al. Natural approach of using nisin and its nanoform as food bio-preservatives against methicillin resistant Staphylococcus aureus and E.coli O157:H7 in yoghurt. BMC Vet Res 20 , 192 (2024). https://doi.org/10.1186/s12917-024-03985-1

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Study: Little Difference Between E. Coli Growth on Leafy Greens in Forward vs. Source Processing Conditions

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Forward processing of leafy greens crops does not significantly increase the food safety risk posed by Escherichia coli , suggests a recent study led by Xiangwu Nou, Ph.D, a U.S. Department of Agriculture Agricultural Research Service (USDA-ARS) scientist.

Leafy greens grown in California and Arizona are often transported over long distances to other facilities for fresh-cut processing and regional marketing, a practice called “forward processing.” The alternative to forward processing is “source processing,” which is when raw commodities are grown locally to their fresh-cut processing facilities. The shipping distances involved with forward processing raise questions about possible food safety implications, which Dr. Nou sought to investigate in his study. “Our results indicate the concerns [about forward processing] might not be warranted,” he said in a Center for Produce Safety (CPS) Research Report.

Specifically, the study began with an analysis of randomly selected transportation data from industry partners to determine crucial variables that would affect raw leafy greens during forward processing. The researchers also collected new temperature, humidity, and barometric pressure data from romaine lettuce- source and forward-processing facilities. Samples were also collected from freshly harvested lots of romaine that were source- and forward-processed at different facilities, both before the goods’ shipments to the processing facilities and after their arrivals.

The romaine samples were examined for product quality, and were assayed to determine overall microbial load present on the lettuce. Significantly, a larger overall microbial load was measured on the forward-processed romaine; however, after a laboratory experiment in which romaine was inoculated with E. coli  O157:H7 and stored under conditions simulating source and forward processing, the researchers found that slightly less pathogens survived under forward-processing conditions than source-processing conditions.

The fact that the microbial growth observed on forward-processed leafy greens was not reflected in the behavior of E. coli  under corresponding laboratory conditions suggests to Dr. Nou that lettuce microbiome plays a role in foodborne pathogen growth (or lack thereof). Future analysis is required to better understand the microbial dynamics on leafy greens.

Dr. Nou was joined on the project by co-investigators Yaguang Luo, Ph.D. and Patricia Millner, Ph. D. from USDA-ARS, as well as Shirley Micallef, Ph.D. from the University of Maryland. The project was funded by CPS and its findings will be presented at the 2024 CPS Research Symposium, taking place June 18–19 in Denver, Colorado.

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Maize Price Shocks, Food Expenditure and the Mediating Role of Access to Market in Ghana

May 14, 2024 - Edward Martey, Justina Adwoa Onumah, Frank Adusah-Poku

The existing empirical literature on the impact of food price shocks on food consumption has primarily concentrated on market-purchased foods, offering limited insights into home-produced foods and food quality. Addressing this gap, our study employs panel data from Ghana to investigate the relationship between exposure to positive maize price shocks and price variability and household consumption patterns of nutrient-dense and less nutrient-dense diets, considering both market purchases and home production. Our findings indicate that maize price shocks lead to a reduction in households' consumption of purchased nutrient-dense and less nutrient-dense food groups, while increasing the consumption of home-produced nutrient-dense food groups. The effects of maize price shocks on diet consumption vary across household types, primary crop cultivation, and wealth status. Additionally, access to markets emerges as a crucial mechanism through which maize price shocks influence households' consumption of nutrient-dense and less nutrient-dense diets. The implications of our study underscore the significance of enhanced market access and policy interventions aimed at mitigating food price increases to improve food nutrition security.

 maize price shocks; food consumption; nutrient-dense diets; market access; Ghana

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EDITORIAL article

This article is part of the research topic.

High Value Utilization of Waste in Food Processing

Editorial: High Value Utilization of Waste in Food Processing Provisionally Accepted

• 1 Xinyang Normal University, China
• 2 Jimei University, China
• 3 Florida Agricultural and Mechanical University, United States

The final, formatted version of the article will be published soon.

High value utilization of waste in food processing is an important aspect of sustainable development and environmental protection (Ravindran & Jaiswal, 2016).It involves converting various waste materials generated during food processing into valuable products or resources, thereby reducing the environmental impact of waste disposal and promoting economic growth (Saba et al., 2023). One significant area of high value utilization is the recycling of food waste into compost or biofertilizers (Hamilton et al., 2015). Organic waste from food processing, such as vegetable scraps and fruit peels, can be composted to create a rich soil amendment that can be used in agriculture. This not only reduces the need for chemical fertilizers but also improves soil health and crop yields. Another example is the extraction of valuable components from waste streams (Chan et al., 2018). For instance, oils and fats from cooking waste can be recovered and used as biofuels or in the production of soaps and detergents.Similarly, proteins and carbohydrates from food waste can be extracted and used in the production of animal feed or bioplastics. Furthermore, waste from food processing can also be used to produce energy (Pagliano et al., 2017). Through processes such as anaerobic digestion or incineration, organic waste can be converted into biogas or energy-rich fuels that can be used to power factories or provide heat for buildings. In addition to these direct utilizations, food waste can also be used in innovative ways to create new products. For example, some companies are developing technologies to convert food waste into bioplastics, which can be used as an alternative to traditional petroleum-based plastics (Acquavia et al., 2021).Overall, high value utilization of waste in food processing offers numerous benefits, including reducing environmental pollution, conserving resources, and promoting economic growth. Advanced technologies including multiple fermentation, countercurrent extraction, electrodialysis, capsule embedding, and artificial intelligence production lines, can effectively optimize the processing technology as a result of advancements in food processing technology (Shen et al., 2019). Future efforts should continue to intensify research and development, promote the widespread application of these technologies, and contribute to the construction of a green, low-carbon, and circular economic model. The study by Feng et al. focused on analyzing dietary fibers (DF) extracted from papaya peel (PP) and seed (PS) using three extraction methods: acidic, enzymatic, and alkaline. Characterization of these DF samples was conducted using various techniques including SEM, FT-IR, XRD, thermal and rheological analyses, and monosaccharide composition determination. Results indicated that DF extracted via the acidic method exhibited looser and more intricate structures, while enzymatically extracted DF demonstrated superior thermal stability. Additionally, the extraction method influenced the monosaccharide composition of the DF. Notably, DF extracted through the acidic method demonstrated elevated functional and antioxidant properties. This study provides a comprehensive analysis of DF derived from papaya peel and seed using multiple extraction methods. The application of advanced analytical methods provides a detailed insight into the structural and functional attributes of these fibers. The discovery that acidic extraction produces DF with remarkable functional and antioxidant properties holds importance, indicating potential health advantages upon integration into food formulations. Overall, the research is well-designed and executed, paving the way for further exploration of papaya-derived DF in functional food applications.The study by Fernando et al. centered on enhancing the extraction of phenolic compounds from avocado Hass peels in Peru, a region producing a substantial quantity of avocado by-products. The primary objective of the study was to assess the impact of various process variables on the extraction yield, total phenolic content, total flavonoid content, and antioxidant capacity of the extracts. Additionally, the investigation involved the analysis of phenolic compounds and the evaluation of antioxidant capacity. The findings of the study indicated that the extraction conditions exerted a significant influence on the desired outcomes, particularly in the case of unripe avocado peels extracted using 40% ethanol at 49.3°C and a specific solvent-to-feed ratio, which resulted in the highest levels of phenolic and flavonoid content, as well as potent antioxidant properties. The main phenolic compounds identified in the extracts were vanillic acid and 4-hydroxyphenylacetic acid. The findings highlight the potential of avocado peels as a valuable source of natural antioxidants, offering a sustainable and healthier option to synthetic antioxidants in the food industry. This study not only contributes to reducing waste but also presents opportunities for enhancing the value of agricultural by-products, thereby benefiting both the environment and the economy.The study by Sumeth et al. focused on the development of an expanded snack utilizing Riceberry rice flour and dried coconut meal, a byproduct of coconut milk processing, via a twin-screw extruder. A factorial design was utilized to examine the effects of varying feed moisture levels and quantities of dried coconut meal on the physicochemical, functional, and sensory characteristics of the snack.Key findings included the impact of moisture and dried coconut meal on viscosity, expansion, color, texture, and antioxidant properties of the extruded snacks. The findings are valuable for the food processing industry, specifically in the realm of enhancing the utilization of agricultural byproducts such as dried coconut meal. In addition to analyzing physicochemical properties, the research also delves into sensory attributes, a key factor in consumer approval. Overall, the research is well-designed, comprehensive, and has practical implications for the development of healthy and sustainable snack options.The study by Chen et al. presented a novel approach for the production of extracellular polymeric substances (EPS) and iron or copper complex from glutinous rice processing wastewater, which is both environmentally friendly and economically viable. This study initially isolated an EPS-producing bacterium from marine sources utilizing glutinous rice processing wastewater, subsequently investigating the application of EPS as a carrier for the synthesis of EPS-iron (EPS-Fe) and EPS-copper (EPS-Cu) complexes. Furthermore, this study determined the optimal conditions for synthesizing EPS-Fe and EPS-Cu, followed by a comprehensive characterization of both compounds. The results demonstrated the potential of EPS-Fe and EPS-Cu as a new type of comprehensive essential trace mineral supplement, offering a safer alternative to inorganic trace minerals. The study also highlighted the antioxidant and antiproliferative properties of these complexes, suggesting their possible use in health supplements or functional foods. Overall, this work offers a promising approach to mitigating environmental contamination from glutinous rice processing wastewater by producing valuable organic metal complexes. This, in turn, could enable the widespread production and utilization of organic metal complexes as dietary supplements, feed additives, or fertilizer enhancers.The study by Zorana et al. conducted an analysis of goat whey sourced from two distinct origins: the market (produced in small and large dairy facilities) and a laboratory (derived from goat milk heated at different temperatures). The samples were subjected to comprehensive analysis including gross composition, pH, protein content, mineral composition, and microbial examination. Findings revealed that the protein composition of whey was influenced by the applied heat treatment, while the mineral content was determined by the type of coagulation. Particularly noteworthy was the observation that acid whey exhibited significantly higher concentrations of calcium and zinc compared to sweet whey. This study provides a comprehensive analysis of goat whey from different sources and heating conditions. The use of statistical methods such as ANOVA and PCA adds rigor to the data interpretation. The finding that heat treatment does not affect Ca and Mg content in goat whey, unlike cow milk, is particularly interesting and may have implications for the dairy industry.The study also highlights the consistency in quality between small and large scale market goat whey, despite differing legal requirements, which is reassuring for consumers. Overall, the research is well-designed and presents valuable insights into the properties of goat whey.The review by Nashi et al. focuses on date press cake (DPC), a significant by-product of the date honey or syrup industry in the Middle East, North Africa, and Southwest Asia. This article explored the potential uses of DPC in various food and non-food applications, discussing its chemical composition, nutritional value, functionality, current usages, as well as limitations and future trends. Given the large amount of waste generated in date processing, finding value-added uses for DPC is environmentally and economically beneficial (Al-Khalili et al., 2023). The article successfully outlines the current state of DPC utilization and the need for further research to fully harness its potential. By discussing limitations and future trends, it paves the way for further exploration and innovation in this field. Overall, this is a valuable resource for researchers and industry professionals interested in sustainable waste management and value-addition in the date palm industry. In Summary, the 6 contributions published in this research topic provide insights into the high value utilization of waste in food processing. These articles explored the potential application value of various food processing by-products, including papaya peel and seed, avocado Hass peels, dried coconut meal, glutinous rice processing wastewater, goat whey, and date press cake. As technologies continue to advance, we can expect to see even more innovative and sustainable solutions for managing and utilizing food processing waste in the future.

Keywords: food processing, Waste utilization, sustainable production, Advanced technology, Environmental protection

Received: 25 Apr 2024; Accepted: 09 May 2024.

Copyright: © 2024 Chen, Wang, Wu, Tan and Li. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Dr. Zhipeng Li, Jimei University, Xiamen, 361021, Fujian Province, China

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CREDIT 24/04: Climate shocks, household food security and welfare in Afghanistan

Abstract .

The increasing impact of natural disasters (floods, earthquakes, landslides, and avalanches) in Afghanistan, notably flooding and similar climate shocks, poses a growing concern as vulnerability to climate change intensifies the potential severity of these impacts in future.  This paper uses two household surveys (2011/12 and 2013/14) combined with other data to assess the effects of climate shocks (especially floods) on the welfare of agricultural households, allowing also for conflict and price shocks. We evaluate the impacts of shocks on several measures of food security, dietary diversity, household food consumption spending, farm revenue and income comparing affected to non-affected households. The analysis is based on endogenous switching regressions (ESR) and propensity score matching (PSM) allowing for selection bias and addressing endogeneity. Floods are the main shock and have significant adverse effects on food security and welfare indicators. For example, the estimated average treatment effect in 2013-14 implies a decrease of about a third in food consumption expenditures, with similar reductions in household income and farm revenue.  The findings highlight the need for better disaster risk reduction and planing strategies to support affected populations to respond to and recover from climate shocks.

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Hayatullah Ahmadzai and Oliver Morrissey

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