# Nanoencapsulation of Essential Oils as Natural Food Antimicrobial Agents: An Overview
## 5 Main Strategies for Nanoencapsulation of Essential Oils
### Lipid-Based Nanoencapsulation
#### 5.2.1 Nanoemulsions  
Nanoemulsions (Figure 2A) are nanoscale droplets of a multiphase colloidal dispersion formed by dispersing one liquid in another immiscible liquid by mechanical agitation [102]. Depending on the hydrophobic or hydrophilic nature of the antimicrobial agent, an O/W or W/O emulsion system can be used to stabilize the oil-soluble or water-soluble agent, respectively. Emulsions containing nanodroplets with a size range less than 100 nm have properties different from those of conventional emulsions, especially the viscosity, color, and dispersion [52]. The emulsion stability is also different. In fact, while microemulsions are thermodynamically stable and form spontaneously, nanoemulsions are kinetically stable [103]. On the other hand, nanoemulsions are generally prepared by both top-down (high-energy) and bottom-up (low-energy) approaches, while microemulsions are produced by low-energy approaches only [104]. Top-down approaches use mechanical energy to form nanodroplets; however, low-energy methods are based on changes in process parameters, such as temperature and phase composition, to achieve nanoemulsions with less energy input [105].  
For high-energy methods, the most generally used methods are high-shear mixing, high-pressure homogenization, microfluidization, and microchannel homogenization [2]. The emulsification process is divided into two stages: (i) breaking down the coarse droplets into smaller droplets, and (ii) absorbing the emulsifier onto the newly formed interface to prevent re-coalescing [106]. Nanoemulsions obtained by high-energy methods are thermodynamically unstable colloidal systems dispersed in the second phase in the form of 50-1000 nm droplets. Since the droplets of the nanoemulsions are small, they have kinetic stability under certain conditions, and phase separation, flocculation, coalescence, or precipitation in the system will not occur for a long time. [PERSON] and [PERSON] [107] used ultrasonic as the external energy source, and used the response surface method to optimize the nano-emulsification of orange peel EOs. The results showed that when the ultrasonic power was 94%, the ultrasonic time was 138 s, and when the ultrasonic treatment temperature was 37 \({}^{\circ}\)C, the prepared orange peel EOs nanoemulsion can reach a particle size of 12.68 nm. [PERSON], [PERSON] et al. [108] successfully incorporated lemon myrtle EOs (LMEO) into a water system by using ultrasonication to form a nanoemulsion. The minimum average droplet size achieved for LMEO was 16.07 \(\pm\) 0.13 nm, and LMEO nanoemulsions showed higher antibacterial activity than LMEO alone. For low-energy methods, the most commonly used techniques are phase inversion temperature, emulsion phase inversion, and spontaneous nanoemulsification. Nanoemulsions obtained by low energy are thermodynamically stable systems formed by the action of oil phase, water phase, and a large amount of emulsifier or surfactant. Meanwhile, the compatibility is better, and the droplets are uniform and small in size [109]. A limitation of this approach is that a large amount of organic solvent surfactant is required to prepare the emulsion, so the application of the low energy method is limited in the food field. [PERSON] et al. [110] used Span-80 and Tween-80 as surfactants and glycerin as a co-surfactant to prepare grapefruit EOs oil-in-water type nanoemulsions. The particle size was about 21 nm, and the stability was good. [PERSON] et al. [111] used casein and soy lecithin to finely emulsify thyme oil by nanoemulsification and obtained a thyme oil nanoemulsion with better antibacterial activity than free oil. In another study, the authors successfully prepared a thyme EOs nanoemulsion with a particle size of 150-250 nm using triglyceride as an additive [112].