Introduction

Food perishables are susceptible to oxidation and microbiological contamination, which can modify their sensory qualities, causing them to lose important nutrients, which in turn could lead to suffering of huge economic losses by the Industry. With a view to minimize such losses and improve the products’ safety, storage stability and quality, artificial preservatives and antioxidants are being added during food processing. Propyl gallate, butylated hydroxytoluene and butylated hydroxyanisole are examples of synthetic antioxidants that are less expensive and more stable. However, questions are being raised about their safety, due to the fact that long-term consumption of various food products that contain these artificial preservatives and antioxidants have the potential to cause allergies, gastrointestinal problems or even cancer in individuals.

Maintaining the consumer acceptance for a product, while also being functional and available for sale to the consumers at reasonable prices are the primary issues that influence any researcher to find an alternative solution. One of the ways to meet the above mentioned requirements is to utilize the beneficial plant compounds. Essential oils and phenolic compounds are the most frequently employed bioactive compounds for food preservation that have been shown to be effective against microbial contamination. Most of the EOs and their constituents have been admitted as safe food additives by the Food and Drug Administration (FDA).

Essential oils (EOs), which are primarily made up of the aromatic and volatile molecules that are found in all portions of plants, are well-known for their potent anti-bacterial properties. The anti-bacterial and antioxidant properties of EOs are attributable to their volatile components, which also hinder the growth of microorganisms and prevent lipid oxidation and peroxidation. The main essential oils with proven antimicrobial properties are obtained from certified crops, such as lavender, thyme, cinnamon, oregano, basil or rosemary. The number of distinctive compounds in EOs varies from ten to hundreds, with 2-3 major bioactive components present in high quantities (20–70%). These include terpenoids (phenols, ethers, ketones, aldehydes, alcohols and esters), phenylpropanoids and terpene compounds (e.g., limonene, p-cymene and terpinene). Terpenes or terpenoids are well-known for being effective anti-bacterial, anti-fungal, anti-viral, and antiprotozoal agents. However, the direct application of essential oil into the food products limits its sensory acceptability, due to high odour and sensitivity of volatile compounds oxidation that has lower stability during the processing operations. One of the ways is Nano emulsion of essential oil.

Importance of Nano Emulsion in the Food Industry

The use of nanotechnology in the field of food science has drawn a lot of attention in recent years. In the Food Industry’s delivery system, Nano emulsions have generated considerable interest. Nano emulsion is biphasic dispersion of two immiscible liquids, either water in oil (W/O) or oil in water (O/W) stabilized amphiphilic surfactant molecule creating nano-sized droplets. In nano emulsion, an oil phase is generally dispersed in an aqueous phase, in which each oil droplet is surrounded by a thin interfacial layer of surfactant molecules that helps in stabilizing the nano emulsion system. Nano emulsion has better functionality, because of controlled release, high stability, protection against degradation and enhanced bioavailability.

In addition to enhancing the bioavailability of the bioactive ingredients that have been encapsulated, nanoemulsion-based delivery systems also promote food stability. Nanoemulsions have very small particle sizes of less than 100 nm and are transparent or translucent colloidal systems that are kinetically stable and thermodynamically unstable. Nanoemulsions are the perfect choice for the Food Industry, owing to their small size, enhanced surface area and susceptibility to physical and chemical modifications. Food-grade Nano emulsions are important, because they improve encapsulation effectiveness, digestion, bioavailability and targeted delivery. Nanoemulsions have the aforementioned advantages over traditional emulsions, which is why they are important in the food business. Different methods have been used in the preparation of Nano emulsion as described in Fig. 1.

Component for Nano emulsion

A Nano emulsion typically comprises of three components, namely aqueous phase, oil phase and emulsifier/stabilizer, which create a significant impact on development of Nano emulsion. The addition of an emulsifier, even in small amounts is crucial for the creation of Nano emulsions, because it lowers the interfacial tension between the water and oil phases and increases the stability of the final product.

Techniques used for nano emulsion preparation Non-Polar Molecules like Oil Phase

The physical and chemical characteristics of the oil droplets, like solubility in water, viscosity, density, polarity, refractive index and interfacial tension determine the major properties of nano emulsions. Example: Oil from Soybean, safflower, corn, flaxseed, sunflower, olive, algae, fish and plant’s essential oil.

Polar molecules like Aqueous Phase

The production of a food-grade aqueous phase for nano emulsion, a variety of polar molecules such as carbohydrates, proteins, acids, minerals or alcoholic co-solvents with water can be employed.

Stabilizer

To increase the stability of Nano emulsions, some important stabilizers can be added, like emulsifiers, texture modifiers, ripening retarders and balancing agents. Stabilizers such as proteins (For eg. whey protein isolate, gelatin or soy protein isolate), sugars (such as high-fructose corn syrup or sucrose), polysaccharides (such as carrageenan, xanthan, pectin, or alginate) and polyols (such as sorbitol or glycerol) can be used to improve the viscosity. Small-molecule surfactants like Tweens and Spans, amphiphilic polysaccharides like gum Arabic or modified starch, phospholipids like soy, egg or dairy lecithin and amphiphilic proteins like caseinate or whey protein isolate are the most commonly used emulsifiers in the Food Industry for the formulation of nanoemulsions. To balance the densities of liquid emulsions, balancing agents like dense lipophilic compounds (ester gums, sucrose acetate and isobutyrate) can be used. The process of Nano emulsion is described in Fig. 2.

Application of NE in Food Preservation

In this context, nanoemulsion is a key nano-vehicle that has been used in the Food Industry to improve physical properties, the efficacy of foodstuffs, enhance food safety by appropriate packaging and preservation of qualities of food materials with anti-bacterial, anti-fungal and antioxidant agents and most importantly delivery of various food components.

Improves Digestibility

The proportion of food that is absorbed into the bloodstream through the gastrointestinal tract is determined by the food’s digestible quality. Nano emulsions can be used to improve it, because its small droplet sizes enable them to increase the surface area of food-related components. As a result, food nano emulsion improves digestible functionality for food ingredients. Studies have been conducted on the impact of food nano emulsions on digestion. In one study, curcumin was added to the oil portion of a nano emulsion and the digestibility of the mixture was compared to curcumin taken orally. The nano emulsified curcumin had a higher digestibility, because this nano emulsion makes the simplification of the lipid digestion stage in the GI tract.

Process of Nano emulsion preparation Fortification and Stability Improvement of Beverages

The resulting nano emulsion offered potential for beverage fortification. In a different study, soybean lecithin, polysorbate 20 and their mixes were used as emulsifiers to generate O/W nano emulsions with vitamin D3 injected into the oil core. These emulsifiers were then used to fortify dairy emulsions. The supplemented whole-fat milk produced had stability for at least 10 days and the size of the droplets was discovered to be less than 200 nm.

Improved Bioavailability

Nano emulsion has been employed to enhance the bioavailability of bioactive substances used in dietary supplements. Bio accessibility and absorption are two ways to increase the bioavailability of bioactive substances by nano emulsions. By changing the particle size, it improved the solubility, as it improved the area-to-volume ratio. By making bioactive more permeable across the GI wall, one can increase the absorption of such substances. This can be accomplished by choosing the right surfactant which has improved the penetration of green tea catechin from stabilized O/W nano emulsions.

Prevent Enzymatic Browning

The human diet includes a considerable portion of apples. These processed fruits are extremely vulnerable to browning, which degrades the organoleptic quality and nutritional value and is mostly brought on by the copper-containing enzyme tyrosinase or polyphenol oxidase (PPO). The nano emulsion containing quercetin had the greatest impact on PPO activity, reducing it by 41% and preventing apple flesh browning by 74% up to six hours.

The most important function of α-tocopherol in the nano emulsions could be the regeneration of quercetin to prolong its antioxidant activity. Citral NE incorporated in Fresh cut pineapples as edible coating to maintain the microbial quality and extend the shelf life of fresh cut pineapple prolonged up to 12 days at 4°C and 90% relative humidity. Nano emulsion of cinnamon essential oil incorporated into apple juice delayed the degradation of total phenol and AA content in apple juice and inhibited PPO activity and preserved the colour of apple juice during 48 hours of storage at 4°C. Citric Acid/ Pomelo essential oil nano emulsion can be utilized to promote the inhibition of enzymatic browning and maintain the quality of banana puree, owing to its reduced oxidative enzyme activity, low pH, strong antioxidant capacity and excellent colour retention.

Food Packaging / Coating Agent

To increase food preservation, the active packaging designed with addition of NE containing anti-bacterial or an antioxidant agent has been used. In comparison to the pectin film, the bio nanocomposite containing the pracaxi oil nano emulsion effectively improved the stability of butter samples against oxidation processes and was a promising strategy for extending the shelf life of butter.

Materialistic lifestyle demands Ready-to-Eat foods, owing to their convenience, better sensorial qualities and fresh-like appearance, leading to its tremendous growth in the market. Further, its importance increased for minimally processed products such as vegetables, fruits and dairy or meat. Application of nano emulsion in the form of edible coating onto these food products not only prolongs their shelf-life, but also prevents microbial growth on the food surface, resulting from peeling and cutting operation. Nano emulsion-based edible coating was developed with alginate as a film-forming agent and essential oils such as thyme, lemongrass or sage oil. This edible film provides different functions as well as physical properties.

The coating of nano emulsion with 6% of ginger essential oil was found to significantly decrease the total aerobic psychrophilic bacteria of refrigerated chicken fillets during 12 days. A multifunctional nano emulsion comprising of sodium alginate, organic essential oil, tween 80 and mandarin fibres was developed to create an edible coating around low-fat cut cheese. The presence of organic essential oil reduces the growth of bacteria, yeast and moulds in cheese during refrigerated storage. The incorporation of mandarin fibre, a prebiotic, enhanced the nutritional value of the coated cheese.

Food Preservation

Food preservation is crucial for maintaining freshness and preserving the nutritional value of Ready-to-Eat foods. According to this perspective, the nano emulsion’s anti-bacterial or anti-fungal components can provide this preservation. The major purpose of using essential oils in nano emulsion is preservation. For the preservation of ultra-filtered Labneh Cheese, Nano emulsion of thyme essential oil was added, which had reduced the pathogenic count in 1 hour and preserved the cheese’s quality for up to 6 weeks.

In a different study, gelatine, lecithin and thymol essential oil were combined to create a nano emulsion that was used to generate fortified milk. The Listeria monocytogenes bacterium colony gradually decreased, as a result of the bacteriostatic activity of thymol, which helped to preserve fortified milk. The Nano emulsified Thymus vulgaris essential oil has been tested to determine the anti-bacterial activity against E.coli and found that the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) against E.coli demonstrated higher anti-bacterial activity, as compared to the pure form of essential oil, which was attributed to the intimate contact between the cell wall of the bacteria and essential oil.

Few essential oils demonstrated anti-fungal activity, which protected foods from fungal infections, similar to that of anti-bacterial activity. Controlling over microbial deterioration of Minas Padrao cheese was demonstrated by a nano emulsion encapsulated with oregano essential, which showed greater anti-fungal efficacy. The outcome demonstrated that the increased solubility of essential oils was responsible for the nano emulsion’s enhanced mycotoxin inhibitory efficacy.

Delivery of Bioactive Components

Various food components such as healthy lipids, vitamins and excipient foods can be better delivered with nano emulsion. Alpha-linolenic acids, bioactive oils rich in tocopherols, omega-3 fatty acids, omega-3 fatty acids rich in algal oil and other healthy lipids are being explored for delivery using a nano emulsion carrier. An alpha-linolenic acid-rich flaxseed oil nano emulsion that is stabilized with whey protein and sodium alginate was developed. An essential source of omega-3 fatty acids supplied to broiler chicken, the resulting nano emulsion exhibited a greater level of omega-3 fatty acids in the meat when compared to bulk flaxseed oil.

Tocopherol-rich in kenaf seed oil was used to create a nano emulsion, which was then stabilized with β-cyclodextrin, Tween 20 and sodium caseinate. When the aforementioned nano emulsion was administered to rats, it was found that, in contrast to the emulsion and bulk oil, the tocopherol level in the bloodstream increased quickly. Both outcomes were linked to the nano emulsion’s smaller droplets’ (more surface area) quicker digestion and absorption. The effectiveness of kenaf seed oil in lowering cholesterol levels in hypercholesterolemia rats was examined and revealed that nano emulsion-based vehicles had the strongest ability to reduce cholesterol, while also reducing liver fat levels.

Conclusion

Food contamination by bacteria and fungi has been linked to an increase in disease occurrences and significant number of deaths among humans worldwide. Utilizing plant-based anti-microbial agents for food preservation has lately grown in importance as a means of addressing these problems. The solubility of essential oils might be significantly improved by adding into the core of nano emulsions with improved antioxidant activity of nano emulsions. Additionally, essential oils’ anti-bacterial and anti-fungal properties in nano emulsions could be effectively used. Furthermore, edible coatings created by nano emulsions encapsulated with essential oils have favourable effects on the prevention of foodborne bacteria and increase the shelf life of foods, which is advantageous for the safety and consumption of fresh and healthy foods.

References:

1. Mehmood, T., Ahmed, A., Ahmad, Z., & Rehman, M. A. (2022). Food-grade nanoemulsions for effective delivery of vitamins. In Bio-Based Nanoemulsions for Agri-Food Applications (pp. 441-449). Elsevier.

2. Abbasi, F., Samadi, F., Jafari, S. M., Ramezanpour, S., & Shargh, M. S. (2019). Ultrasound-assisted preparation of flaxseed oil nanoemulsions coated with alginate-whey protein for targeted delivery of omega-3 fatty acids into the lower sections of gastrointestinal tract to enrich broiler meat. Ultrasonics Sonochemistry, 50, 208-217.

3. El-Sayed, S. M., & El-Sayed, H. S. (2021). Antimicrobial nanoemulsion formulation based on thyme (Thymus vulgaris) essential oil for UF labneh preservation. Journal of Materials Research and Technology, 10, 1029-1041.

4. Bedoya-Serna, C. M., Dacanal, G. C., Fernandes, A. M., & Pinho, S. C. (2018). Antifungal activity of nanoemulsions encapsulating oregano (Origanum vulgare) essential oil: in vitro study and application in Minas Padrão cheese. Brazilian journal of microbiology, 49, 929-935.

5. Noori, S., Zeynali, F., & Almasi, H. (2018). Antimicrobial and antioxidant efficiency of nanoemulsion-based edible coating containing ginger (Zingiber officinale) essential oil and its effect on safety and quality attributes of chicken breast fillets. Food control, 84, 312-320.

6. Silva, H. D., Cerqueira, M. Â., & Vicente, A. A. (2012). Nanoemulsions for food applications: development and characterization. Food and bioprocess technology, 5(3), 854-867.

7. Dasgupta, N., & Ranjan, S. (2018). Food nanoemulsions: Stability, benefits and applications. An Introduction to Food Grade Nanoemulsions, 19-48.

About the Authors:

Authors - Sangeetha K Gnana Moorthy Eswaran Proshanta Guha & Prem Prakash Srivastav

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An editor by day & dreamer at night; passionately involved with both print and digital media; Pet lover; Solo traveller.

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