The to form nanoemulsions in the food industry

The two methods that broadly categorizes the
production of nanoemulsions are mechanical (high-energy) and non-mechanical
(low energy) methods. The high-energy methods include high-pressure homogenization,
ultrasonication and microfluidization (Anton, Saulnier, Beduneau, & Benoit,
2007; Tadros et al., 2004; Pouton and Porter, 2006; Acosta, 2009; Anton et al.,
2009; Leong et al., 2009). Today, high-energy methods are the most widespread
methods used to prepare nanoemulsions as they can be used with  a wide range of emulsifier and oil types (McClements & Rao, 2011). These methods
are capable of producing extremely strong disruptive forces (Tadros et al.,
2004; Leong et al., 2009) which are needed to surpass the reinstating forces
that maintain the spherical shape of the droplets (Walstra, 1993; Schubert and
Behrend, 2003; Schubert and Engel, 2004).

High-pressure
Homogenization

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The most common method used to form nanoemulsions in
the food industry is High pressure homogenization (Schubert and Behrend, 2003;
Schubert and Engel, 2004). High pressure valve homogenizers are best to reduce
the size of droplets in the pre-existing emulsions rather than creating new
emulsions from two separate liquids. They have a pump that makes the coarse
emulsion pass through a narrow valve, where it experiences severe disruptive
forces causing larger droplets to break into their smaller counterparts (McClements & Rao, 2011). The droplet
size can be reduced by increasing the homogenization pressure or the number of
passes of the droplets through the homogenizer.  Also sufficient emulsifier must be present to
cover the surfaces of the new droplets formed to prevent their re-coalescence
(Jafari et al., 2008).

Microfluidisation

Microfluidisation
also forces an emulsion pre-mix through a narrow orifice to facilitate droplet
disruption just like homogenization, the only difference being the presence of assisting
chambers and its design which allows more efficient size reduction of the fed
in droplets. The special microchannel configuration provides for an optimum
formation of nano-scale emulsion droplets, which is also hugely affected by the
operating pressure and the number of passes through the orifice (Constantinides
et al., 2008; Maa & Hsu, 1999; Quintanilla-Carvajal et al., 2010).

Ultrasonication

Ultrasonicators use high intensity ultrasonic waves
to generate intense disruptive forces required for breaking up the oil and
water phases into fine droplets. The energy input is provided using sonicator
probes containing piezoelectric quartz crystal that expand and contract in
response to alternate electric voltage. Tip of the sonicator probe is placed in
the liquid to be homogenized, there it generates intense mechanical vibrations
leading to cavitation effect, i.e. formation, growth and collapse of small
bubbles, eventually leading to droplet disruption.

Formation of nanoemulsions using low-energy methods
relies on the spontaneous formation of oil droplets in the oil-water emulsifier
mixtures when either their composition or environment is altered. Low energy
methods mainly include spontaneous-emulsification and phase-inversion method
(Bouchemal et al., 2004; Tadros et al., 2004; Freitas et al., 2005; Chu et al.,
2007a; Anton et al., 2008; Yin et al., 2009). They are more efficient at producing
smaller droplets than high-energy methods but get restricted by the type of oil
and emulsifier used and it also becomes necessary to use high concentration of
synthetic surfactants.

Spontaneous-emulsification

Spontaneous-emulsification, as the name suggests
involves spontaneous formation of emulsion when two liquids are mixed together.
It can be carried out in various ways, which include varying the environmental
conditions (such as, temperature, pH, ionic strength), or changing the
composition of the two phases or varying the mixing conditions of liquid (such
as, mixing or stirring speed, rate or the order of addition).

Phase-inversion

Phase-inversion methods induces a phase-inversion in
emulsion from water-in-oil to oil-in-water form (or vice-versa). For example,
PIT (phase-inversion temperature), PIC (phase-inversion composition), or EIP
(emulsion inversion point), PIT relies on charges in molecular geometry
(optimum curvature) or relative solubility of non-ionic surfactants with
changing temperature. PIC changes the optimum curvature by altering the
formation of the system which could be achieved either by addition of salt,
water or by altering the pH. EIP involves change in ratio of oil-to-water phase
while the surfactant properties remain constant. One method to achieve this
would be by increasing or decreasing the volume fraction of the dispersed phase
in an emulsion above or below the critical level.

Solvent diffusion method has also been used as a
low-energy method to form nanoemulsions (Anton et al., 2007; Tadros et al.,
2004; Unger et al., 2004). Membrane emulsification is another low energy
process that requires less use of surfactant and is still able to produce
emulsion with fine size droplets.

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