Nanoemulsions based edible coatings with potential food applications

ABSTRACT

With the advancement in the technologies and globalization, people are now focusing more on healthy products such as fruits, vegetables having good nutrition capability with enhanced shelf life. The edible coating is one such technique that can be used to enhance the shelf life of fruits, vegetables, and other food products. Various types of edible coatings such as protein based, lipid based, pectin based, and chitosan based are used in food products. However, edible coatings sometimes provide unpleasant flavor and toxicity to the food products. Thus, to overcome this problem newly developed design of active edible coatings to encapsulate the lipophilic active compounds in the form of nanoemulsions plays a vital role in increasing the functionality of the food product and acts as a newer approach in edible packaging. Many active ingredients are used to preserve the quality and provide nutritional benefits to food products such as antimicrobial activity by nanoemulsions containing essential oils. Therefore, the major aim of this review is to study and highlight the newer alternative approach of using nanoemulsions based edible coatings in food products. It also emphasizes on the recent advances for improving the quality and shelf life of food products.

KEYWORDS:

• Edible coating
• nanoemulsions
• fruits
• vegetables
• antimicrobials

Introduction

In the present era, due to the advent in technology, urbanization, and changing lifestyle of the people, consumers are now demanding more healthy and safe food products which can pose positive benefits on the health. In view of this, edible coating, a mild treatment on the surface of food plays a vital role in enhancing their shelf life without affecting the natural properties of the food products. The edible coatings contain effective functional and medicinal properties which is essential for providing benefits to the food product [2]. Also, nowadays the food industry is facing a major challenge in enhancing the shelf life, maintaining the quality and stability of food products such as fruits and vegetables. So, nanoemulsions in edible coatings act as a preservation technique that can provide additional benefits such as antioxidant and antimicrobial properties beyond basic nutrition [3]. Nanoemulsions fall under nanotechnology which is an emerging technology that can help in incorporating food-grade delivery systems with essential ingredients such as vitamins, antimicrobials, flavors, colors, and nutraceuticals [4]. In order to retard the microbial degradation of food products, natural antimicrobial agents are used such as essential oils (lemon, clove, mandarin, oregano essential oils) which can be encapsulate in a nanometric delivery system to provide quality, stability, and antimicrobial action in ready-to-eat fruits and vegetables. It helps to increase the shelf life without altering the natural flavor, taste, and aroma of fruits and vegetables [5].

These coatings on the surface of food products can be easily applied by any of the following methods such as rubbing, spraying, and immersion. These coatings possess functional and medicinal benefits as these are made from natural sources such as polysaccharides, proteins, organic compounds, inorganic compounds, and essential oils. But due to advancement in technology, it leads to the introduction of nanosystems such as nanoemulsions, nanocomposites, and nanoparticles. Out of these, nanoemulsions act as a new generation edible coating technique which can prevent the release of the natural compounds in the environment by acting as a barrier after transforming and incorporating them in the form of microcapsules [6].

Various studies have been done on the edible coating on food products such as fruits and vegetables using nanoemulsion technology. #One such study was done by Jaiswal et al. [37] and Prakash et al. [24] on the sodium-alginate focused edible coating by citral nanoemulsion in order to enhance the shelf life of fresh-cut pineapples. Results revealed that 0.5% and 1% concentration resulted in a lower rate of respiration, enhanced color, and decrease in microbial growth during the storage period of 12 days. However, 1% concentration of citral nanoemulsion in sodium alginate-based edible coating decreases the sensory characteristics and textural properties of the pineapples, whereas 0.5% concentration of citral nanoemulsion also helps in reducing the microbial growth of artificially inoculated Listeria monocytogenes and Salmonella typhimurium. Thus, 0.5% citral nanoemulsion in sodium alginate edible coating provides greater opportunities for enhancing the shelf life of the fresh-cut pineapples.

However, there is still a need for awareness about the nanoemulsions utilized in the edible coating to enhance the functionality and shelf life of the food products. Keeping this in mind, this study highlights the application of nanoemulsions as a new generation edible coatings. It focuses on the edible coatings, its advancement with nanoemulsions, nanoemulsions formation along with its significance and applications.

Edible coating

The edible coating is a thin layer of protective edible material applied over the surface of foods (Figure 1). This coating provides similar properties to that of modified atmosphere packaging in protecting the food products from oxygen, light, microorganisms, moisture to enhance their shelf life [7]. The barrier properties of edible coatings can be determined by water vapor permeability and oxygen permeability methods. On the other hand, the protective function of edible coatings can be determined by their tensile properties, namely Young’s Modulus, tensile strength, and elongation at break [8].

Figure 1. Conventional strategy of edible coating of fruits [7]

The type of edible coatings required depends upon the characteristics and factors affecting the quality of food product such as to prevent deterioration from oxidation, oxygen level needs to be monitored and also sometimes there is a need for the reduction of respiration rates and production of ethylene in fruits and vegetables [6]. Out of the materials utilized in the edible coatings, some are water-soluble and some are hydrophobic substances. Water-soluble substances are those which easily fused inside a protein matrix to form edible coatings, such as vitamins, probiotics, prebiotics, antimicrobial proteins, and peptides. On the other hand, hydrophobic substances are those which demand additional efforts and techniques for merging and stabilizing inside the aqueous matrix to form edible coatings [2].

One of the major benefits of edible coating is to provide a glossy-finished surface to the food products along with the replacement of the plastic packaging with natural and environmentally safe edible coatings which helps in retarding the waste disposal issues and thus protect the environment [8]. Commercially, different edible coatings are utilized for different products such as coatings from oils, waxes, resins; chocolate coatings employed for confections; corn zein coatings for candy, nutmeats and pharmaceuticals; gelatin coatings for pharmaceuticals; collagen casings coatings for sausages and cellulose ether coatings for food ingredients [9]. Among fruits and vegetables, whole and fresh-cut can be coated which include apple, grapefruit, passion fruit, avocado, lime, peach, lemon, orange, melons, tomato, cucumber, bell pepper, fresh-cut pear, fresh-cut apple, fresh-cut peach, fresh-cut lettuce, fresh-cut cabbage, fresh-cut potato, fresh-cut tomato, fresh-cut muskmelon, fresh-cut cantaloupe, minimally processed carrot, and minimally processed onion [10]. Although edible coatings offer many advantages but still it exerts various disadvantages to the food products especially postharvest fruits and vegetables. For example, they provide rancid flavor due to oxidation of lipids, they contain their inherent color which might be unappealing for the consumers, they also require special compounds in order to meet the appropriate effectiveness of the bioactive ingredients used in the coating material. Therefore, in order to eradicate this problem, nanoemulsion acts as an emerging and novel technology to enhance the quality, stability, and shelf life of food products [12].

Nanoemulsions as advanced edible coatings

Nanoemulsions are the oil-in-water or water-in-oil emulsions which help in enhancing the physicochemical properties of edible coatings on various food products such as nanoemulsion in the pectin-based edible coating on fresh-cut orange slices helps in enhancing its quality and sensory characteristics [13]. Among the various nanoencapsulating systems, nanoemulsions with oil-in-water emulsions are emerging and new generation edible coatings because they can be easily fused with food-grade ingredients and provide more scope for scale up in food industries with the help of high-pressure homogenization technique. These nanoemulsions are of ≤100 nm of size, in which the nanometric droplets are immersed in an aqueous continuous phase and each nanometric droplet is surrounded by a film or layer of food-grade ingredient [14]. Nanoemulsions are very well different from conventional emulsions. Conventional emulsions are unstable and they break over a period of time. Also, the particle size of conventional emulsion ranges between 100 nm and 100 µm which is very high than nanoemulsions. Nanoemulsions are also known as mini-emulsion and can be regarded as conventional emulsions whose particle size ranges from 10 nm to 100 nm [15–17]. Also, nanoemulsions are optically clear as compared to conventional emulsions which are opaque. Nanoemulsions are also having greater bioavailability and chemical reactivity than conventional emulsions [18]. Nanoemulsions are considered as generally regarded as safe (GRAS) and to maintain the stability of the oil-in-water emulsion, emulsifiers are used such as proteins (whey protein isolate), phospholipids (soy, dairy lecithin), polysaccharides (gum arabic) and surfactants (sugar esters, polyoxyethylene) to reduce the interfacial tension by electrostatic/steric stabilization process [19]. These small size droplets help in increasing the surface area of the droplet and thus enhance the functional characteristics of encapsulated ingredient [20].

Nanoemulsion in edible coatings can be prepared by using two different methods. One method involves single-step preparation of nanoemulsion by mixing all the components into a coarse solution and then homogenizing it to form nanometric-sized droplets. The other method is a two-step preparation of nanoemulsion in which the aqueous solution of components is prepared and afterward it is mixed with biopolymer solution [23]. Various studies have been done on the utilization of nanoemulsions in increasing the shelf life and quality of food products such as fruits and vegetables (Table 1). For many postharvest fruits such as papaya, mango, strawberry, nanoemulsions can be used as an edible coating as it has the ability to enhance the dispersion of active ingredients. Nanoemulsion prepared from chitosan or nutmeg seed oil as an edible coating on the strawberry helps in maintaining the quality and reducing the microbial growth up to 5 days of storage period [25]. For the preparation of nanoemulsions in edible coatings, essential oils are the most widely used bioactive component due to its eco-friendly characteristics [26]. Essential oils are concentrated hydrophobic liquids which possess aromatic flavor and provide defense mechanism in the form of antimicrobial activity toward pathogenic microorganisms, UV light, and insects. But their usage is limited due to their low stability and extreme flavor which affect the sensory characteristics of the ultimate food product. Thus, encapsulating essential oils in nanoemulsions helps to overcome this limitation by enhancing their stability and reducing the effect of extreme flavor on sensory characteristics of food products [28].

Table 1. Nanoemulsions in edible coatings in different food products

Type of food products	Conditions	Nanoemulsion	Properties affected	Researchers
Chicken fillets	Extension of shelf life	Curcumin-cinnamon essential oil, curcumin-garlic essential oil, curcumin-sunflower oil	Microbiological, sensory, physical, chemical and rheological properties, droplet size, morphology of nanoemulsion	[1]
Green beans	Preservation	Mandarin essential oil	Microbial analysis, antimicrobial effect, firmness, color	[11]
Tomatoes	Destruction of planktonic and sessile cells of food-borne pathogens and enhance the quality characteristics of tomatoes	Sweet orange essential oil	Nanoemulsion characterization, its antimicrobial and antibiofilm properties, tomatoes quality characteristics	[22]
Pork loin	Effect on preservation of pork loin in modified atmosphere packaging	Oregano essential oil and resveratrol	pH, color, lipid and protein oxidation, texture, microbiological analysis	[29]
Fresh-cut cucumber slices	Impact of pulsed light with antimicrobial coatings	Carvacrol	Microbial analysis against Escherichia coli and Listeria innocua	[30]
Plum	Microbial stability and physicochemical storage qualities	Lemongrass oil	Emulsion stability, sensory evaluation, antimicrobial effects, microbiological analysis, color, firmness, weight loss, respiration rate, phenolic compounds	[41]
Grape berry	Shelf life and microbiological safety	Lemongrass oil	Sensory analysis, emulsion stability, firmness, antioxidant activity, total anthocyanin, microbial analysis, color, weight loss, phenolic compounds, total soluble solid content, pH,	[42]
Okra	Shelf life extension	Basil oil	Moisture, color, firmness, physiological loss in weight, antifungal properties, sensory analysis	[50]

Characteristics of nanoemulsions

Nanoemulsions used in edible coatings provide a wide range of benefits such as it can help to decrease the wide transmission of essential components, reduce the interaction with other food matrix ingredients and enhance the antimicrobial, antioxidant, and homogeneity of edible coatings [3]. Due to the small droplet size, they can be easily deposited on the substrates which also allow them for evenly spreading due to their low surface tension and low interfacial tension of the droplets. Also, due to smaller droplet size with elasticity, their coalescence and surface interactions are retarded [31]. Nanoemulsions also possess other advantages to be used in personal care, cosmetics, and healthcare. Due to the small droplet size, creaming or sedimentation is prevented upon storage due to reduction in gravity force and availability of sufficient Brownian motion to overcome gravity [17]. Nanoemulsions are more durable against environmental conditions and possess greater stability with shelf life ranges from moths to several years [32].

Nanoemulsions can help in enhancing the absorption of drugs by utilizing in drug delivery processes due to their large surface area. These are versatile and can be made according to the requirement as they can be prepared from many methods with low energy, high energy, and some are prepared from mechanical devices to break the particles into optimum size. Also, these are nontoxic as they consist of water, oil, and surfactants, which are considered as safe to consume. Also, these have the advantages to be used in a wide range of applications such as in food, cosmetics, drug delivery, and pharmaceuticals [33].

Nanoemulsions are different from microemulsions and macroemulsions in various aspects such as in shape, size, stability, and formation. The size of nanoemulsions ranges from 20 to 500 nm whereas in macroemulsions it ranges from 1 to 100 µm and in microemulsions it ranges from 10 to 100 nm. All the three types of emulsions are spherical in shape. In the formation of emulsions, nanoemulsions and macroemulsions are formed from low- as well as high-energy methods. In contrast, microemulsions are only formed from low-energy methods. In terms of stability, nanoemulsions are thermodynamically unstable and kinetically stable. Whereas microemulsions are thermodynamically stable and macroemulsions are thermodynamically unstable [34].

Food-grade nanoemulsions preparation

In the preparation of food-grade nanoemulsions, three major components are used which include oil phase, aqueous phase, and stabilizers. The oil phase in the formulation of nanoemulsions is usually derived from triglycerides such as triacylglycerols, diacylglycerols, monoacylglycerols, essential oils, mineral oils, fatty acids, and other nutraceuticals [35,36]. Out of these, it is recommended that triacylglycerol oils such as flaxseed, sunflower, safflower, olive, corn, soybean, and fish oil, should be utilized in the food industry due to its low price, increased functional and nutritional properties. The aqueous phase used in the formulation of nanoemulsion consists of water, polar components such as alcohols, carbohydrates, proteins, minerals, acids, and bases. The concentration and type of these components reveal the polarity, refractive index, pH, density, the strength of the aqueous phase which ultimately affects the nanoemulsion stability and physicochemical properties [16]. The usage of only two phases, i.e. oil and aqueous phase, and their intermixing lead to the development of temporary emulsion which breaks down easily. So, in order to prevent the collapse of nanoemulsion and maintaining their stability stabilizers such as emulsifying agents are used. Emulsifying agents are majorly divided into two classes including surfactants such as spans, tweens, and hydrophilic colloids such as acacia, bentonite, and veegum. These agents are nontoxic, reduce the surface tension, and prevent the coalescence resulting in a stable nanoemulsion [37].

Methods of formation of nanoemulsions

Nanoemulsions can be formed by using two methods on the basis of energy input, namely low energy and high energy. Figure 2 represents the formation of nanoemulsions. Low-energy methods include the method of spontaneous emulsification, phase inversion composition, phase inversion temperature, and high-energy methods include high-pressure homogenization, microfluidizer homogenization, and ultrasonication [38].

Figure 2. Formation of nanoemulsions based edible coating by different methods

Low-energy methods

Low-energy methods for the formation of nanoemulsions are more efficient and effective than high-energy methods but low-energy methods are not utilized for all types of oils and emulsifiers. In low-energy methods, the nanometric droplets are formed spontaneously when the oil-water and emulsifier mixture is hindered in terms of its composition or environment [16]. In this method, very little energy is required for the formation of nanoemulsion but the magnetic stirrer used should be fast moving in such an order that it cannot break the droplets [39].

Spontaneous emulsification

The spontaneous emulsification method is a very simple method that requires no expensive equipment irrespective of whatever difference is there in the two phases and the experimental conditions [40]. In the spontaneous emulsification method of preparing nanoemulsion, two liquids are mixed. One liquid remains in the aqueous phase while the other liquid is a mixture of oil, surfactant, and a water-miscible solvent. The emulsions of these two liquids are made after mixing them at room temperature. When both liquids (thermodynamically stable) are mixed, they lead to the formation of the nonequilibrium state resulting in the transfer of hydrophilic materials from the oil to the water phase. This increases the interfacial area and converts the droplets into the nanometric form [43].

Phase inversion composition

Phase inversion composition (PIC) is a nanoemulsion formation method that enhances the curvature by altering the conformation at room temperature. This method utilizes Gibbs free energy of emulsions to change the phases which leads to the inversion of surfactant’s curvature between positive and negative phase [40]. Various studies have been conducted on the formation of nanoemulsions by phase inversion composition. One such study was conducted by Gonçalves et al. [35] and Pagan et al. [27] to develop citral nanoemulsions to examine its antimicrobial activity through the phase inversion composition (PIC) method. The results revealed that citral nanoemulsion was more effective as compared with the conventional form of citral.

Phase inversion temperature

Phase inversion temperature results in more stable and uniform nanometric droplets without expensive equipment. In this method, temperature difference affects the temperature-sensitive surfactants by affecting the curvature of their surfactant layer. Temperature-sensitive surfactants are water soluble at lower temperature and resulting in positive curvature of the surfactant layer at the droplet interface. While temperature-sensitive surfactants are oil soluble at higher temperatures leading to the negative curvature of the surfactant layer at the droplet interface. On the other hand, temperature-sensitive surfactants at an intermediate temperature, which is also known as phase inversion temperature, are neither more oil soluble and water soluble. They have equal solubility toward both the phases and resulting in a zero curvature of surfactant layer at droplet interface resulting in the ultimate oil solubilization in bicontinuous phase to finally give nanometric size droplets [44]. One study has been done on the formation of nanoemulsion through the phase inversion temperature method and determining its stability. The results revealed that on increasing the oil solubility and surfactant concentration, the stability of nanoemulsions decreases. This is because of the rise in Ostwald ripening rate which occurs due to the rise in the rate of oil diffusion [45].

High-energy methods

In high-energy methods, intensive disruptive forces are utilized by means of intense energy mechanical devices including high-pressure homogenization, microfluidizer, and ultrasonication. These methods can be utilized for the formation of food-grade nanoemulsions by using any type of oil and emulsifier [16].

High-pressure homogenization

This method of formation of nanoemulsions involves various forces such as hydraulic shear, intense turbulence, and cavitation. The nanoemulsions are formed by passing two liquids, i.e. surfactants and cosurfactants through an orifice in piston homogenizer at a 500–5000psi pressure rate. This method is widely utilized on lab scale as well as industrial scale but it has some limitations such as it requires a large amount of energy to operate and temperature in the homogenizer increases during processing which sometimes destroy the components [46]. In this method, initially, the emulsion mixture comes in contact with high pressure but afterward, it falls into a controlled valve. Although the size of nanometric droplets decreases with an increase in the pressure and number of cycles [40].

Microfluidizer homogenization

Microfluidizer homogenization method is also known as the microfluidization technique, which is used for the formation of nanoemulsions and is quite similar to the high-pressure homogenization method. In this method, the emulsion is pumped through a small orifice and droplets of emulsion are rotating again and again inside the equipment in order to achieve the optimum droplet size. Afterward, with the effect of high shear on the emulsion droplets, it leads to the formation of nanoemulsions [38]. Microfluidizer, in mechanical terms, is similar to a high-velocity static mixer which does not constitute any movable joints. Also, this method can be employed at the laboratory scale as well as industrial scale [33]. The benefit of utilizing his method for the formation of nanoemulsion is that it will provide very narrow nanoemulsion droplets as compared to other methods [39].

Ultrasonication

The ultrasonication method for the formation of nanoemulsion is efficient in reducing the particle size of nanoemulsions. In this method, a sonicator probe (also known as sonotrodes) is used which possess piezoelectric quartz crystal to break the particles into smaller size and according to the electric voltage can expand or contract the particle size. The tip of the sonicator probe creates mechanical vibration inside the liquid emulsion and leads to the formation and disintegration of vapor cavities inside the liquid emulsion. This ultimately results in the formation of nanoemulsions [33]. This method produces very fine nanoemulsions which are more stable than other techniques. The main reason behind the small droplet size is the usage of high-intensity waves in the ultrasonication method which disturb the forces between oil and water mixture [38].

Applications of nanoemulsions

The main limitation of functional food is its stability, less availability of bioactive components, and its shelf life. Some bioactive components may even deteriorate and undergo oxidation during the storage period. These limitations can be overcome by the inclusion of nanoemulsions in the edible coating of food products. For example, chitosan can be used for the development of nanoemulsions coatings of essential oils such as lemon oil, bergamot oil possessing antimicrobial activity. Also, nanoemulsion prepared from alginate-based edible coating with lemongrass essential oil is used to coat fresh-cut Fuji apples for 2 min for 14 days at 4°C in order to prevent the activity of E. Coli [47–49]. Nanoemulsions have a wide range of applications in food products such as muscle foods. Nanoemulsions in muscle foods can be applied in four different ways. In the first case, nanoemulsion can be dispersed into a coating material and then applied over the finished surface of fish or meat products. Secondly, nanoemulsion can be directly mixed with the finished meat or fish products. In the third way, nanoemulsion can be directly applied over the surface of fish or meat products by dipping or spraying. And the last one involves the incorporation of nanoemulsion into an edible film which can finally be used as a packaging material to package the finished fish or meat products. All of these methods of application possess some advantages as well as disadvantages. The nanoemulsion mixed with the ground meat or fish products helps in evenly distributing the bioactive compounds throughout the product whereas dipping or coating of nanoemulsions helps in preventing spoilage over the surface of the meat or fish products [21]. Also, nanoemulsions can be incorporated in personal care products for providing effective fragrance such as in perfumes that are specially designed as alcohol-free [17].

In the food industry, nanoemulsions help in retarding the unpleasant and unappealing taste of bioactive compounds by evaporation, thus improving the digestibility of food products. In cosmetics, nanoemulsions produced from high energy-based methods help in preventing or removing the irritating surfactants which allow the better retention of bioactive ingredients to the skin [33]. It also has a wide range of applications in the pharmaceutical industry such as malaria diagnosis, cancer treatment, treatment of tumor and coronary artery disease, anti-inflammatory [39].

Conclusion

Nanoemulsion is a novel approach in edible coatings under the heading of nanotechnology which is gaining a lot of attention nowadays in comparison with conventional emulsions. It is due to the reason that nanoemulsions are versatile and possess various advantages in comparison with conventional emulsions. The nanoemulsions provide enhanced quality, stability, and physicochemical properties of edible coatings. It helps in protecting the bioactive components from oxidation and makes them easily and evenly distributed in nanoemulsion resulting in enhanced texture, flavor, nutrition, stability, and shelf life of the finished food products. These can be obtained from low-energy methods such as spontaneous emulsification, phase inversion composition, phase inversion temperature as well as high-energy methods such as high-pressure homogenization, microfluidizer homogenization, and ultrasonication. Nanoemulsion comprises of effective delivery systems of antimicrobial properties possessing bioactive components. There are various applications of nanoemulsions in food products, either through direct mixing in food products or the form of an edible coating. However, there is still a need for further studies regarding the benefits of nanoemulsions in different areas, its potential for scale up in food industries as well as their positive health benefits.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Notes on contributors

Abhishek Dutt Tripathi

Dr. Abhishek Dutt Tripathi is Assistant Professor in Department of Dairy Science and Food Technology, Institute of Agricultural Sciences, Banaras Hindu University, India. He has published 65 research papers and articles in journals of national and international repute. His field of specialization is Food Chemistry, Enzyme Technology, Fermentation Technology and Food Microbiology. He has also published 02 books, 02 manuals and many book chapters associated with food and bioprocess technology. He has received several prestigious national and international awards and also serving as editor and reviewers many international and national journals.

Ruchi Sharma

Ruchi Sharma holds B.Tech in Food Technology from Bhaskaracharya College of Applied Sciences, University of Delhi and M.Tech in Food Supply Chain Management from National Institute of Food Technology Entrepreneurship and Management. She is currently a PhD Research Scholar in Indian Institute of Technology (IIT), Delhi.

Aparna Agarwal

Aparna Agarwal is B.Tech (Dairy Technology), M.Tech and Ph.D. in Dairy  Chemistry from National Dairy Research Institute, Karnal. Her areas of interest include Dairy and Food Chemistry, Functional foods, Bioactive peptides and proteins. She has published several research papers and book chapters in national and international journals She has completed one research project funded by Delhi University Innovation Project on trans fatty acid in cooking oils.

Dr Rizwana Haleem

Dr Rizwana is Associate Professor and has more than 25 years  experience in teaching Food Science and Technology at the Department of Food Technology at Bhaskaracharya College of Applied Sciences, (University of Delhi) India. Presently her research interest is Packaging Technology and  Agro waste based green nano composite development and applications, especially utilization of rice husk as a packaging material  for food. She along with team members received the Teaching Excellence Award for Innovation from University of Delhi , for the innovation project “Agro-Waste Material Management: From Waste to Wealth”.