Preparation Methods

Nanoemulsions could be fabricated utilizing different or combined techniques. Generally, there are two main categories known as either high-energy or low-energy approaches depending on the energy input requirements (Jafari, He, & Bhandari, 2006; Jafari, He, & Bhandari, 2007a, 2007b).

High-energy techniques use a high mechanical energy. Their main advantage is the better control of size distribution and composition of the final emulsions (Jafari, He et al., 2007, 2007b; Shamsara, Muhidinov et al., 2015). Typically, the droplet size is controllable as it decreases at higher energy input and duration (Miastkowska, Banach et al., 2017), although there are some reports about droplet recoalescence during a high-energy emulsification (Jafari, Assadpoor, He, & Bhandari, 2008). Normally, high-energy methods alone do not result in oil droplets of very small size (<100 nm). In particular, application of natural biopolymer emulsifiers with large molecular weights hinders the formation of very small droplets. High-energy emulsification methods could be implemented by the following equipment:

  • • high-speed and high-shear homogenizers (rotor—stator devices);
  • • high-pressure valve homogenizers;
  • • microfluidizers; and
  • • ultrasound-based devices.

On the other hand, low-energy emulsification methods depend on the surfactant properties and the oily phase. Their main advantages are low-energy consumption and more straightforward scale-up of the process since the costs are minimized as well as employing simple production methods (Zhu, Zhang et al., 2015; Dasgupta, Ranjan, Mundra, Ramalingam, & Kumar, 2016). Low- energy emulsification drawbacks, regardless of the method, can be the requirement of large amounts of surfactants, typically needed in some cases. Natural surfactants have to be further investigated for food applications. Moreover, emulsifiers that are natural biopolymers, such as proteins or polysaccharides, can only be used to form nanoemulsions by high-pressure methods.

Briefly, the low-energy emulsification techniques can be distinguished in two main categories: phase inversion by simple phase mixing, and phase inversion by changing the hydrophilic—lipophilic balance through altering the system conditions. Accordingly, two commonly low-energy emulsification methods are the phase inversion and the spontaneous emulsification. For the first group, phase inversion temperature and phase inversion composition are applicable (Jafari, Paximada, Mandala, Assadpour, & Mehrnia, 2017).

It should be noted that another important and novel method of low-energy emulsification for producing nanoemulsions is membrane emulsification.

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