Role of Surfactants

To avoid phase partition of an emulsion, surfactants are used. Surfactants upgrade the strength of emulsion because they frame a defensive layer to prohibit aggregation. Surfactants reduce interfacial tension and Laplace pressure that occurs among water and oil molecules (Grigoriev and Miller, 2009). Surfactant molecules incorporate at least one useful gathering that is not just firmly pulled into the mass medium but also have little attraction with the medium. Surfactants could be considered fewer than three major classifications. These are proteins, polysaccharides, and small surfactants (McClements, 2004).

10.3.1 Protein Emulsifiers

Some nourishment proteins, for example, soybean and whey protein segregate and (3-lactoglobulin, are very utilized in the nourishment industry as surfactants because their nutritional values are high and generally recognized as safe (He et al., 2011). Proteins have amphiphilic characters, and they stabilize emulsions via electrostatic repulsion, steric hindrance, and generation of osmotic pressure at the external surface of protein molecules (Charoen et al., 2011).

10.3.2 Polysaccharide Emulsifiers

Gum arabic, some galactomannans, celluloses, modified starches, and few sorts of pectin are the most utilized polysaccharide emulsifiers in different foodstuffs. Depending on their functional groups, polysaccharides are named as non-ionic, anionic, and cationic (Dickinson, 2009). At the point when the monomer arrangement of polysaccharides is compared with proteins, polysaccharides are more uniform, and this provides specific functional and physicochemical properties like viscosity enhancement, binding properties, gelation, and solubility (Chanamai and McClements, 2001).

10.3.3 Small Surfactants

These types of surfactants having both hydrophilic (head group) and hydrophobic parts. These surfactants might be able to be examined under four subgroups, considering their hydrophilic functional groups in aqueous media. They are anionic, cationic, nonionic, and zwitterionic (Gupta and Kumar, 2012):

a. Anionic surfactants: These surfactants bear a negative charge. Anions of alkali metal salts of fatty acids, anions of long-chain sulfonates, sulfates, and phosphates are included in this group.

b. Cationic surfactants: These surfactants bear a positive charge. Generally, they consist of ammonium and pyridinium compounds (Chu et al., 2007).

c. Nonionic surfactants: These surfactants frequently refer to polyoxyethylene mixtures; however, sugar esters, amine oxides, and fatty alkanol amides are also included in this group (Komaiko and McClements, 2014).

d. Zwitterionic surfacltants: These types of surfactants having both positive charges as well as negative charges in the hydrophilic component of the particle. Lecithin (long-chain phosphonyl cholines) belongs to this group (Azeem et al.. 2009).

Nanoemulsion Formation

Arrangement of nanoemulsions requires energy that can be acquired from mechanical or stored chemical energy in the system (Gutierrez et al., 2008). Nanoemulsion preparation can be classified as either high-energy emulsification or low-energy emulsification (Figure Ю.2; Maali and Mosavian. 2013).

High-Energy Emulsification

High-energy emulsification usually involves the utilization of mechanical devices that can generate intense disrupting force to reduce droplet size, such as higher- pressure homogenizers and microfluidizers (Swathy et ah, 2018). Given its many advantages, for example easy scale-up, organic solvent-free, and high efficiency. High-pressure homogenizers are the most broadly used emulsifying device to fabricate stable nanoemulsions in the food-based industries (Reza, 2011). Ordinarily, coarse emulsions delivered by high-shear blenders are directed into a chamber in the homogenizer and afterwards constrained through a limited valve at a high weight (50-200 MPa; Gupta et ah, 2010), which causes exceptionally troublesome powers, for example turbulence, hydraulic shear, and cavitation that are capable of breaking down large droplets into small ones (Lovelyn and Attama, 2011). Microfluidizers are another type of commonly used high-energy emulsifying device. Similar to the high- pressure homogenizer, microfluidizers also work at higher pressure (3-134 MPa; Thakur et ah, 2012). Coarse emulsions are forced into an inlet channel and then separated into two different streams that interrupt on each other intensely in a cooperation chamber, where droplet disruption occurs under solid disorderly forces (Jaiswal et ah, 2015). Microfluidizers can produce fine nanoemulsions with narrow droplet distributions (Dixit et ah, 2008). Ultrasonic homogenizers deliver nanoemulsions by utilizing high-power ultrasonic waves, that is frequency greater than 20 kHz, that are

Formation methods of nanoemulsions

FIGURE 10.2 Formation methods of nanoemulsions.

extremely proficient in diminishing the droplet measure; however, they are more reasonable for little clumps and subsequently broadly utilized in research labs (Mahdi Jafari et al., 2006). An ultrasonic probe in the gadget changes over electrical waves into the extreme mass, which produces exceptional disrupting powers (Kentish et al., 2008). The emulsifying effectiveness altogether relies on the ultrasonication time at various amplitudes (Solans et al., 2005).

Low-Energy Emulsification

Low-energy emulsifications depend on the physicochemical qualities of surfactants and co-surfactants (Anton et al., 2008). Nanoemulsions can be able to impulsively form as the system compositions or ecological situation are adjusted. The two most frequently approaches are spontaneous emulsification and the phase inversion temperature (PIT; Setya et al., 2014). There is an increase in the low-energy approach; that is spontaneous emulsification is a very simple process (Bouchemal et al., 2004). Nanoemulsions are formed by mixing an organic phase that consists of oil, surfactant, and a water solvent and a pure aqueous phase at a particular temperature (Anton and Vandamme, 2009). This approach depends on the fast diffusion of water-miscible solvent like ethanol and acetone from the organic phase into the aqueous phase, which induces great unstable forces at the water or oil interface (Ichikawa et al., 2007). A disadvantage of this approach is that a high solvent/oil ratio is necessary to shape nano-droplets, which largely reduces the oil amount in the final last nanoemulsions (Fernandez et al., 2004). The PIT technique exploits changes in the affinities of nonionic surfactants for oil and water with respect to temperature (Devarajan and Ravichandran, 2011). At a low temperature, the surfactant is fully solubilized in water, which supports the development of oil-in-water (O/W) emulsions.

As the temperature is gradually increased above a particular temperature, the surfactant turns out to be more solvent in oil than in water; then O/W emulsions invert to water-in-oil (W/O) emulsions (Savardekar and Bajaj, 2016). Then an O/W nanoemulsion can be produced by rapidly cooling the system below the PIT. These procedures are reversible, as the temperature becomes raised the presence of clear nanoemulsion will end up turbid once more, which could be an issue in some nourishment and drink applications that need humid treatments (Anandharamakrishnan. 2014).

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