Introduction

Application of nanotechnology is an increasing trend in the field of nutraceutical, healthcare as well as in food. It includes formation of nanoemulsion, synthesis of nanoparticles, widely used in drug delivery system, nanopackaging in food industry and many others such as cosmetics, pharmaceuticals and material synthesis. Generally, nanoemulsions are kinetically stable liquid-in-liquid dispersions having droplet sizes ranges in order of 100 nm. Due to their small size, useful properties such as high surface area per unit volume, robust stability, optically transparent appearance and tuneable rheology are observed. For their preparation, various techniques are used such as application of high- and low-energy methods, including high-pressure homogenization, ultrasonication, phase-inversion temperature and emulsion inversion point, as well as recently developed approaches such as bubble bursting method, high-shear mixers (Gupta et al. 2016; Mohammadi et al. 2016; O’Sullivan et al. 2018).

A number of food ingredients and additives have been emulsified and are available in the market. Depending upon the parameter to be incorporated into the food material such as food fortification or enhancing stability, appearance, taste or texture, various types of active constituents have been employed such as vitamins, minerals, flavorings, acidulants, preservatives, colorings and antioxidants (Mcclements and Jafari 2018). Proteins, fat and sugar molecules are also emulsified and added to food materials for increased uptake and absorption or improved bioavailability or additives such as benzoic acid, citric acid, ascorbic acid, vitamins A and E, isoflavones, beta-carotene, lutein, omega-3 fatty acids and coenzyme-Q10 (Ostertag et al. 2012). Different constituents to be emulsified are discussed further in each section (Fig. 1).

Fig. 1
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Basic components of nanoemulsion

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Nanoemulsion mainly consists of three components—oil, water and surfactant. A correct mix of these components determines the stability and property of emulsion. Various types of surfactants are used in food industries which have been approved by the Federal Drug Administration (FDA), but no single surfactant can be used for all food products. Thus, based upon the molecular characteristics a surfactant is chosen to give the desired effect. The surface-active nature of the component plays an important role in stabilizing the emulsion; hence, the surfactant used acts as stabilizing agent, emulsifying agent, solubilizing agent according to molecular characteristics and solution conditions, such as pH, ionic strength and temperature (McClements et al. 2017). The ratio of oil to water is very important criteria for fabrication of nanoemulsion. As oil molecules possess hydrophobic nature, higher solubility and dispersibility in aqueous media which reduce the organoleptic properties of the food system, there is growing interest in designing structured delivery systems to improve the dispersion stability and antimicrobial activity with examples being nanoemulsions, microemulsion and liposomes with the help of different methods as mentioned above (Sugumar et al. 2015a). Components used in the formation of nanoemulsion are either hydrophilic or lipophilic. Most of the bioactive components, flavors and pigments are lipophilic in nature (Fig. 2). This article is an abridged version of the chapter published by Dasgupta and Ranjan (2018) in the series Environmental Chemistry for a Sustainable World (http://www.springer.com/series/11480).

Fig. 2
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Lipophilic components used in the preparation of nanoemulsions and nanoencapsulation with descriptive ingredients added to enhance their properties

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Oil phase

There has been growing interest in the development of food-grade colloidal delivery systems for encapsulating flavors, colors, micronutrients, nutraceuticals and antimicrobials which are lipophilic in nature (Mcclements 2018). There are many oil phase components used for fabrication of food-grade nanoemulsions which are shown in Fig. 3. The choice of oil is a highly important aspect in the case of food-grade materials or generally recognised as safe (GRAS).

Fig. 3
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Oil phase components used in fabrication of food-grade nanoemulsions

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Flavor oils

Oil-in-water nanoemulsions and microemulsions are two types of colloidal dispersions suitable for encapsulation and delivery of lipophilic components within the beverage industry (Rao and McClements 2012a). Generally, flavor oil contains characteristic aroma profiles with volatile constituents which are used as flavoring agents in the food and beverage industries, e.g., lemon, orange and peppermint oils. Additionally, it has been suggested that the use of natural flavor oils extracted from medicinal plant to develop health beneficial food or nutraceuticals may be termed as Nano-Ayurveda. Many phytonutrients are externally added in nanoemulsions in order to incorporate its biological activities, as reported by Jintapatanakit et al. (2018a). Additionally, folds/forms are developing in the flavor oil according to the differences in isolation and processing procedures; e.g., flavor oil extracted from cold processing is termed as single-fold oils, and further the use of different steps mainly distillations determines the folds of it. The fold of the flavor oils determines the chemical composition and physicochemical properties which ultimately has similar effects on nanoemulsions, its activities and properties (Lang and Buchbauer 2012; Baldissera et al. 2013). Notably, it has been established in our laboratory that higher-fold oils are suitable to develop nanoemulsions (Dasgupta et al. 2016b), and similarly it can be supported by the statement that lower-fold oils are suitable for microemulsion fabrication (Rao and McClements 2011; Jeirani et al. 2013).

Cloud oil

The cloud oil is mainly composed of non-flavor oils, e.g., terpene oils or triacylglycerol. Triacylglycerol oils are usually derived from natural resources as vegetable, corn, canola and sunflower oils, while terpene oils are typically extracted and purified by distillation of naturally occurring flavor oils. Nanoemulsion fabricated using cloud oils are known as cloud nanoemulsion as it shows a cloudy/milky appearance Because of very low water solubility, these oils are stable at Ostwald ripening—unlike flavor oil. Normally, they do not participate directly to increase the flavor properties of nanoemulsions, instead they obliquely manipulate nanoemulsions’ flavor because of their abilities to detach the flavor molecules among the oil, water and headspace regions (Vijayalakshmi et al. 2014; Jintapatanakit et al. 2018b; Teng et al. 2018). Generally, this kind of emulsion has wide application in beverage industry. The emulsion which provides only turbidity is termed as cloud emulsion (or clouding agent), and the other that provides both cloudiness and the aromas is denoted as flavor emulsion (Stounbjerg et al. 2017). Cloud oil physicochemical properties—mainly density, melting conditions and viscosity—have a critical role in fabrication and constancy of cloud oil nanoemulsions. The cloud oil density has important influence on their long-standing stability which further decides the creaming velocity of the nano-sized droplets. Its melting characteristics determine the partial crystallinity of cloud oil droplets and finally can be vulnerable for droplet aggregation and phase separation of nanoemulsions. Its viscosity controls the ability to fabricate small-sized droplets throughout the homogenization process, and the higher viscosities generally lead to large-sized droplets (Dickinson 2010).

Nutraceutical lipids

The nutraceutical lipids—polyunsaturated fats, carotenoids, fat-soluble vitamins and fat-soluble antioxidants, phytosterols and fat-soluble colorants—have attracted the food market for its incorporation in commercial nano-foods. They are incorporated into nanoemulsions, fabricated by high-/low-energy approaches such as high-speed homogenization, sonicators, phase-transition methods, and/or encapsulated into the foods (Anandharamakrishnan 2014). Since the nutraceutical lipids are prone to several factors for its chemical or physical degradation, so, using the same in commercial nano-food is still a major challenge (Akhavan et al. 2018). It can be noted that specific lipid nutraceutical has specific confronts which ultimately depend on their physicochemical properties—chemical stability solubility, melting point and oil–water partition coefficient (McClements et al. 2009; McClements and Li 2010). Henceforth, they should be fabricated with extreme care and the fabrication protocol should be selected wisely to maintain the specific physicochemical properties of particular nutraceutical lipids used in the nanoemulsions (Stellavato et al. 2018).

Polyunsaturated fats

The potential health benefits of polyunsaturated fatty acids (PUFAs) are the major driving force to grow the interest of their use in food and beverage sector, especially ω − 3 fatty acid. It is noted that it lowers the risk of different types of cancers, cardiovascular disease, as well as it improves the brain functioning (Rubio-Rodríguez et al. 2010; Ryckebosch et al. 2012; Filipović et al. 2015; Ranjan et al. 2016a; Sánchez-Salcedo et al. 2016; Azizi et al. 2018). These properties have attracted the food manufacturers to incorporate such PUFAs and fortify the nanoemulsions based foods. As the nano-level droplet size ranges, it will provide efficient delivery and activities (Katouzian et al. 2017). It can be noted that PUFAs are highly vulnerable for oxidative degradation and ultimately cause problem in long-term storage; additionally, they are extremely susceptible at smaller size of nanorange (van de Rest and de Groot 2014). It is advised to follow the preventive measures and protocol to have the long-term stability of PUFA-based nanoemulsions which include deactivation of pro-oxidants, i.e., oxygen and transition metals, controlling quality of the initial ingredient, interfacial engineering and addition of antioxidants to have stable nanoemulsions.

Carotenoids

Carotenoids are intensely colored liposoluble pigments synthesized by plants and microorganisms and are present in many foods, particularly fruit, vegetables, and fish which are used as colorants to provide colors, especially red, yellow and orange. Many of the researches have claimed their health benefits too. Oxygen-containing carotenoids called as xanthophylls—e.g., lutein and zeaxanthin—have been claimed to have properties to decrease age-related cataract and macular degeneration. Similarly, carotenoids without oxygen known as carotenes—e.g., β-carotene and lycopene—have the potential to decrease the risk of prostate cancer. Nevertheless, there is some limitation with the carotenoids incorporation into many food and beverage products. These include poor water solubility due to their high hydrophobicity, melting point and susceptibility to chemical degradation as they are highly unstable (Chaari et al. 2018). Therefore, carotenoids have a poor intake in the body. Due to their properties, carotenoids have the growing interest to be used in food and beverages; in recent days, the interest is to use them in nanoemulsions (Bou et al. 2011; Arunkumar et al. 2013; Jo and Kwon 2014; Uzun et al. 2016).

Vitamins

To improve the nutritional value of nanoemulsion-based foods, there has been increased interest to incorporate vitamins (mainly A, D, E and K) and nutraceuticals (e.g., flavonoids, curcumin and sterols) (Hormann and Zimmer 2016). Vitamin E is a natural antioxidant, and its most bioactive form is α-tocopherol which is having wide applications in cosmetics, pharma and food industries (Campardelli and Reverchon 2015). Researchers have claimed that the nanoemulsified vitamin E is used in drug delivery system for reducing diabetes, cancer, cardiovascular diseases, etc. (Li et al. 2015). For these reasons, there has been huge interest in fortifying many foods and beverages with vitamin E. In a recent study, it has been cited that the alpha–tocopherol nanoemulsion is used for the wound healing (Bonferoni et al. 2018). Vitamins are being emulsified to increase its bioavailability in organisms. As it is unstable in nature, nanoemulsions are prepared with the help of nonionic surfactant, water-soluble co-solvent such as glycerol, and sometimes different oil phases are used followed by continuous stirring and homogenization (Saberi et al. 2013; Ozturk et al. 2015; Dasgupta et al. 2016b; Morais and Burgess 2014; Zheng et al. 2016).

Other vitamins such as vitamin C and D are also loaded with emulsifier and mixed surfactant with different oil phase components to produce nanoemulsion having a droplet diameter of 2.0–3.0 μm which was stable for more than 30 days (Khalid et al. 2013a, b; Ziani et al. 2012). A patent has been filed—US20130189316 A1—for nanoemulsion of vitamin K that can therapeutically replace phytonadione injectable emulsion (vitamin K in aqueous solution with a strong detergent to solubilize it). The drug sometimes causes hypersensitive reaction if injected intravenously or intramuscularly (Andrew 2013).

Vitamin E is becoming lost during processing, utilization and storage of the commercial food and beverage products—oxidation is the major cause for vitamin E instability (Cheng et al. 2016). To achieve higher oxidative stability, vitamin E acetate has been advised to be used in commercial food and beverages rather than vitamin E. It can be noted that in the gastrointestinal tract, action of pancreatic esterase causes the breaking of vitamin E acetate to vitamin E (Mayer et al. 2013; Yang and McClements 2013). The higher lipophilic behavior of vitamin E restricts its direct dispersion into aqueous solutions (Fuchs-Godec and Zerjav 2015). Further, its efficient delivery is necessary, wherein it should be transported using colloidal system for which a number of previous studies have proved the efficient emulsion-based delivery system, e.g., emulsions (Gonnet et al. 2010), microemulsions (Zhou et al. 2015) and nanoemulsions (Mehmood 2015). For enhancing the biological and physicochemical properties of vitamin E in the drug delivery system, D-α-tocopheryl polyethylene glycol succinate (vitamin E TPGS or TPGS) has been approved by FDA as a safe adjuvant which is a water-soluble derivative of natural vitamin E. It possesses multiple advantages in drug delivery such as high biocompatibility, enhancement of drug solubility, improvement of drug permeation and selective antitumor activity (Yang et al. 2018).

Antioxidants, flavors and colorants

Bioactive substances are commonly lipophilic components such as antioxidants, vitamins, phytosterols, fatty acids and probiotics, which are potentially unstable against environmental stresses; encapsulation and emulsification are attractive technologies frequently used to slow down their oxidative degradation, increase their activity and functionality, enhancing the shelf life of nanoemulsion-based foods (Rabelo et al. 2018). It can be noted that the chemical degradation of the above-discussed sensitive nutraceuticals—mainly PUFAs and carotenoids—can be avoided using lipophilic antioxidants (McClements 2016; Sotomayor-Gerding et al. 2016). Few of the active lipophilic antioxidants used at industrial level are: ascorbylpalmitate, rosemary extracts, gallic acid, tert-butyl hydroquinone (TBHQ), butylatedhydroxy toluene (BHT), butylatedhydroxy anisole (BHA), and alpha-tocopherols (Budilarto and Kamal-Eldin 2015; Mohammadi et al. 2016).

There are many lipophilic colorants which are naturally available and used in food—such as paprika, lycopene, β-carotene and other carotenoids. Because of their origin from natural sources, they can be used as pigments or colorants in food (Meléndez-Martínez et al. 2015; Kiokias et al. 2016). These natural colorants are unstable, hence prepared with the help of emulsifier and stabilizer (Patil et al. 2017) such as β-carotene, lycopene, lutein and astaxanthin which are prepared using different carrier materials.

β-carotene has been emulsified by various methods, such as lipid carrier/liposomes (Pardeike et al. 2009), casein micelles (Dalgleish 2011) or by β-lactoglobulin complexes (Ron et al. 2010) which form stable emulsion with minimum droplet size of 8–15 nm (Poonia et al. 2016). Other carotenoids being emulsified include lycopene, lutein and astaxanthin. Lycopene emulsions have been prepared by high-pressure homogenizer, and its thermal stability is evaluated in emulsion systems (Ax et al. 2003; Boon et al. 2008). Astaxanthin was also prepared by high-pressure homogenization with mean diameter of the dispersed ranged from 160 to 190 nm (Kim et al. 2012). Another study includes improving stability of acidic model beverage astaxanthin by loading nanostructured lipid carriers along with α-tocopherol and EDTA as antioxidants. This nanoemulsion shows particle size 94 nm with better stability (Tamjidi et al. 2018). Nanoencapsulated lutein is developed using chitosan to improve its bioavailability. It was observed that lutein absorption was higher from nanoencapsules than mixed micelles (Arunkumar et al. 2013).

Plant essential oil and active compounds

Plant oils are mainly used as oil phase for making food-grade nanoemulsions rather than encapsulating. The detailed application of plant essential oil is summarized in Table 1. Phytosterols and phytostanols are the plant-derived lipid which shows bioactivity (Otoni et al. 2014; Bhargava et al. 2015; Topuz et al. 2016). It has the potential to inhibit the dietary cholesterol, because of its capability to reduce the low-density cholesterol as well as total cholesterol, so there is a growing interest in food and beverage industry (Nicolosi and Wilson 2015). It is an experimentally established fact that 1.6 g phytosterols per day intake is able to reduce the LDL by 10% (Augustin and Sanguansri 2014). Few properties such as tendency to form crystals, low oil solubility, low water solubility and high melting point are the main hurdles while incorporating phytosterols and phytostanols into nanoemulsified food products and their susceptibility for oxidative degradation Esterification of phytosterols with polyunsaturated fatty acids can be done to overcome these hurdles. The esterified esterification of phytosterols will be digested in gastrointestinal tract and produces free fatty acids and phytosterols (Smoliga and Blanchard 2014; McClements 2015; McClements et al. 2015; Yao et al. 2015; Zou et al. 2015).

Table 1 Recent research updates on plant oil-based nanoemulsions or nanoencapsulation for achieving different activities
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Minerals

Minerals exist in food material in various forms such as ions, compounds, complexes or as chelates. Like vitamins, minerals also play an important role in body metabolism, and their solubility depends upon their functional properties. To overcome the deficiency of vitamin and minerals, direct fortification into food may induce chemical degradation, change the level of bioavailability or decrease the sensory quality of food products such as undesirable color and flavor change. This direct addition may accelerate the undesirable chemical reaction which lowers down the nutrition profile of the food product. To solve this hurdle, water-in-oil-in-water (W/O/W) emulsions as encapsulation system is a suitable alternative (Prichapan and Klinkesorn 2014).

High concentrations of minerals, especially Ca2+, can have an adverse effect on the aggregation stability of O/W emulsions containing electrostatically stabilized droplets due to electrostatic screening and ion-binding effects. Certain mineral ions such as iron and copper ions may also promote undesirable chemical reactions such as lipid oxidation that leads to product deterioration. In these systems, it is usually necessary to add chelating agents to sequester the mineral ions and prevent them from causing chemical instability. Certain types of minerals influence the functional properties of other food ingredients. For example, the ability of many biopolymers to thicken or gel a solution is strongly dependent on the type and concentration of mineral ion present (Mungure et al. 2018). Careful selection and control of the mineral ions present in food emulsions are therefore equally important when formulating a successful product (McClements and Li 2010; Piorkowski and McClements 2014).

Weighing agents

Weighing agents are the additives that are added to nanoemulsions in order to minimize the separation of oil droplets. It can be noted that the densities of vegetable and flavor oils are significantly lower than those of the aqueous phase as well as the aqueous sugar solutions, and the nanoemulsions containing these oil droplets tend to move upward. The upward movement of oil droplets in aqueous solutions leads to ringing—oil droplet ring formation at the product surface. The best way to minimize ringing can be possible by using the hydrophobic higher density materials other than the oil droplets. These higher density materials are the weighing agents which increase the density of oil droplets and match with aqueous phase. Generally, weighing agents are used to prevent the creaming in nanoemulsion with low-density oil phase. In addition, such weighing agent reduces the turbidity of the emulsion and increases their cloudiness as well as the refractive index of the emulsion. Ester gum, protein, sucrose acetate isobutyrate (SAIB) and damar gum are used as a weighing agent for increasing the density of oil phase (Stounbjerg et al. 2017).

Emulsifiers

Emulsifiers are surface-active molecules which are mainly used for easing the droplet breakup using top-down approach—thus helping in smaller droplet size formation—and ever for prevention of aggregation—further maintaining long-term stability (Ranjan et al. 2014, 2016a, b; Dasgupta et al. 2015, 2016a; Komaiko et al. 2016; Jain et al. 2018). During the fabrication of nanoemulsions, the emulsifiers get absorbed in the O/W interface which ultimately reduces the interfacial tension causing ease in disruption of droplets. After facilitating the droplet disruption, the emulsifiers must play a role of a protective coating around the oil droplets to prevent aggregation and coagulation. It can be importantly noted that the concentration of emulsifiers should be appropriate to cover all the O/W interfaces, and rate of coating around the oil droplets by emulsifiers must be faster (Komaiko et al. 2016; Artiga-artigas et al. 2018).

Sweeteners

Nanoemulsions get sweetened by natural (preferably) or artificial compounds which interact with tongue taste receptors, giving sensitivity of sweetness—primarily sugar-based compounds are such compound to give perception of sweetness for mammals (Komaiko and McClements 2015; Puri et al. 2015). Compared to number of such compounds, sucrose has been considered, having sweetness of 100 which is additionally responsible for the increase in density of emulsion, while non-nutritive or artificial sweetener reduces their density (Stounbjerg et al. 2017; Kheynoor et al. 2018). Once compared with sugars (glucose, fructose, sucrose, etc.) and sugar alcohols (sorbitol, xylitol, mannitol and erythritol), the latter is considered useful for the development of reduced calorie product (Carrillo-inungaray et al. 2018; Kheynoor et al. 2018). The reason behind this statement is that the human body breaks down sugar alcohol with slower rate and lesser efficiency, which results in the lesser calories per gram in them. Other than sugar alcohols, many of the high-intensity sweeteners are there with favorable flavor profile, low-/medium-calorie nanoemulsions, e.g., natural sweetener such as Stevia and artificial sweeteners such as: saccharin, aspartame, cyclamate, sucralose, neotame and acesulfame K (Delaveau et al. 2015; Al-Nemrawi and Dave 2016; Baker Jr. et al. 2016). In gelatine-based dessert, generally a sweetening agent is incorporated for a thermodynamically stable emulsion (Komaiko and McClements 2015).

Thickening agents

Thickening agents are the polymeric compounds which are used in nanoemulsions to increase the viscosity of aqueous phase which ultimately leads to a decrease in the rate of droplet creaming, as well as alters the mouthfeel, acting as an emulsifier as well as stabilizing agent. Some emulsifiers are used in combination with thickening agent to improve the mechanical strength of emulsions (Meng et al. 2018). These are the biocompatible polymers which get dissolved in the aqueous phase and cover the larger volume than original polymer chain. The oxidative stability of the polymer and their molecular characteristics (molecular weight, interactions and confirmation) are the main parameters. Also, the addition of thickening agent slows down the rate of flocculation and coalescence in the desired nanoemulsion system (Stounbjerg et al. 2017).

Phase diagram

Emulsion is a complex system which includes micelles, colloids and gel network of components involved in it. Phase diagram is used to determine the existence zone of nanoemulsion. It graphically depicts the ratios of the three variables as positions in an equilateral triangle. Usually, only three components are used to form a nanoemulsion (oil phase, water phase and surfactant), as three different sides of the triangle. The three components are each found at apex of the triangle, where their corresponding volume fraction is 100%. Moving away from that corner reduces the volume fraction of that specific component and increases the volume fraction of one or both of the two other components. These points combine to form regions with boundaries between them, representing the “phase behavior” of the system at constant temperature and pressure (Kumar et al. 2012).

To produce such diagrams, a large number of samples of different compositions are prepared. To construct a phase diagram, oil phase is first mixed with surfactant (and co-surfactant) in different ratios. Then, this mixture is titrated with distilled water until a clear, isotropic and thermodynamically stable dispersion with low viscosity is obtained. The ternary phase diagram is constructed by plotting the different values obtained from the experiment. The titration begins by fixing two components and varying the third component. The titration procedure begins with zero loading of water and ends at a point of 100% water loading. Usually, three different forms of phases are observed while constructing a ternary phase diagram—emulsion state, liquid crystal and coarse emulsion. Emulsion state is the area of our interest and can be easily identified by its transparent appearance. Liquid crystal is a translucent gel-like state where partition between oil and water phase can be still visible. Coarse emulsions are the unstable emulsions and appear as milky white. The boundary lines between the three states are drawn according to their appearance (Wang and Pal 2014).

Conclusion

Nanoemulsions are colloidal nano-sized emulsions, which are fabricated for improving the delivery of bioactive ingredients in food, pharmaceutical and cosmetic industry. These are the thermodynamically stable isotropic system in which two immiscible liquids are mixed to form a single phase by means of an emulsifying agent, i.e., surfactant and co-surfactant as discussed above. The droplet size of nanoemulsion falls typically in the range 20–200 nm. With the help of a number of techniques such as high-energy approaches (high-pressure homogenization, ultrasonication), low-energy approaches (low-energy emulsification, phase-inversion method), it is easy to form the nanoemulsion for achieving stability (Anandharamakrishnan 2014). The above-discussed ingredients are used to prepare nanoemulsion; depending upon the requirement, suitability and solubility of the component in both oil and aqueous phases, they are mixed.