# Nanoencapsulation of Essential Oils as Natural Food Antimicrobial Agents: An Overview
## 4 Nanoencapsulation versus Microencapsulation  
In order to enhance the applicability of active compounds for food products, or to protect them against deteriorating environmental conditions, encapsulation is considered a viable alternative [46]. In general, encapsulation is defined as a carrier matrix in which a bioactive substance is entrapped, which allows one to control the rate of bioactive release [47]. A comparison of the functionality of micro- and nanoencapsulation has been reported by [48], as shown in Figure 1.  
Particle size has been shown to be an important factor affecting the functional characteristics of capsules [49]. Nanoencapsulation is considered to be the use of a carrier with a size of less than 1 micron (1000 nm), possessing different properties than ordinary encapsulation. According to some specific regulations, however, particularly in the pharmaceutical field, the size of carriers should be less than 100 nm for them to be considered  
Figure 1: Advantages of nano- and microencapsulation [48]. Reprinted with permission from Ref. [48]. Copyright 2018 Elsevier. as nanocapsules [50]. The nanometric size of delivery systems can increase the surface area and consequently the dispersion in the food matrices. They are easier to disperse and to suspend in water to form uniform and stable colloidal suspensions and may have good sustained-release effects in comparison to microsized delivery systems. Based on their smaller size, nanocapsules have the potential to increase the passive cellular absorption mechanisms, promoting the effective release of active substances inside the target cells, and consequently increasing the efficiency of active substances and their bioavailability. Meanwhile, nanoparticles may penetrate into the tissues (such as the liver) through the capillaries, and are absorbed by the cells in the tissues; thus, the active substance can be efficiently delivered to the target cells in the body [51]. In the case of emulsions-based delivery systems, there are some interesting physical properties that can be used to distinguish nano- and microemulsions. Microemulsions generally exhibit multiple scattering of visible light, which means they have an opaque white appearance. Conversely, nanoemulsions are much smaller than visible wavelengths, and therefore, appear almost optically transparent [52]. This character can be considered as a very advantageous feature for nanoemulsions in the beverage industry. Despite the numerous technologies for the encapsulating of biologically active compounds which have been studied, only a few techniques, namely spray-drying and freeze-drying, are widely applied in the food industry [53]. Typically, emulsification technology is the first step of encapsulation. There are two types of emulsification techniques used to produce encapsulation systems: a top-down approach and a bottom-up approach. The top-down methods involve changing large structural materials into small structures by reducing the size, and shaping the structure through external mechanical destructive forces. Top-down methods generally include high-pressure homogenization, microfluidization, and microchannel homogenizers [48]. They are commonly used for the encapsulation of hydrophilic and hydrophobic compounds, but have less control over the particle size and structure of the produced emulsion, and are only suitable for a limited type of matrix [54]. The bottom-up approach generally includes self-assembly, phase inversion, and spontaneous emulsification, which are affected by factors such as pH, temperature, concentration, and ionic strength. Meanwhile, low-energy methods are also used as preparatory steps for other nanoencapsulation methods (e.g., spray-drying, complex coaecervation, extrusion, electro-spinning, and electro-spraying [55]). They allow better control of the properties of the capsules and consume less energy. However, low-energy methods require large amounts of stabilizers and are used with limited types of oils and surfactants. [56].