## 4 Recent Progress in Microencapsulation Techniques for Natural Bioactive Compounds
### Emulsion Based Techniques  
An emulsion consists of at least two immiscible liquids, with one of the liquids being dispersed (dispersed phase) as small droplets in other (continuous phase) [3]. This is a suitable method for the microencapsulation of lipophilic bioactive compounds. The emulsion can be categorized into two types, viz., a single emulsion, usually being an oil-in-water emulsion, used for encapsulating oil soluble core materials and double emulsion, utilized to coat several cores in a single droplet of emulsion [153; 168]. Double emulsions refer to the dispersion of one liquid into other, resulting in double-layered liquid droplets such as water-in-oil-in-water (W/O/W) or oil-in-water-in-oil (O/W/O) emulsions [169]. It has the advantages of compatibility with the food matrix, economic production, higher loading capacity, and the retention and enhancement of bioavailability of bioactive compounds.  
Different types of delivery systems that can be created based on emulsion technique are conventional emulsions (oil-in-water), multiple emulsions (water-in-oil-in-water), multi-layer emulsions (multilayer oil-in-water), solid-lipid particles, and filled hydrogel particles. After the preparation of the emulsion, spray drying may or may not be carried out. Thus, the emulsion preparation is actually the first step for many encapsulation procedures. The encapsulation of various bioactive compounds such as phenolics, fatty acids, anthocyanins, and vitamins has been successfully carried out using the emulsion technique. The potential of emulsion electrospray was evaluated for the encapsulation of epigallocatechin gallate (EGCG). Hydrophilic (H-EGCG) or lipophilized (L-EGCG) catechins were encapsulated either in the aqueous or the oily phase of the emulsions [170]. Similarly, the encapsulation of isoeugenol in spray-dried emulsions with \(\beta\)-lactoglobulin and n-OSA starch as emulsifiers was performed [171].  
A number of lipophilic vitamins and bioactive compounds such as omega-3 fatty acids, carotenoids, phytosterols, flavonoids, and vitamins A, D, K, and E are very prone to degradation upon exposure to heat, oxygen, and light. Furthermore, these bioactive compounds are not easily water soluble and possess poor stability in the human digestive system leading to the limited absorption and bioavailability of these compounds. For increasing their bioavailability and stability, it is preferred to encapsulate these bioactive compounds and incorporate them into aqueous-based food. [PERSON] et al. [103] noted a higher encapsulation efficiency (95%) in encapsulating these compounds by forming an emulsion with a combination of maltodextrin and whey protein as an encapsulant at pH 6.6, having a \(\zeta\)-potential of \(-\)29 mV, which is within the prescribed limits of \(\zeta\)-potential for stable emulsion, which is \(\pm\)30 mV [172; 173; 174; 175]. [PERSON] et al. [103] also noted a higher encapsulation efficiency in the case of highly stable emulsion, however, excessive \(\zeta\)-potential or values close to zero could further breakdown emulsion droplets, leading to an excessive repulsion force or a lack of repulsive force, resulting in flocculation [172]. The use of materials with high gelation efficiency for emulsion formation facilitates a strong structure and thus a better protection of the core material and encapsulation efficiency [176; 177; 178]. Alternatively, a pickering emulsion prepared by incorporating solid particles such as cellulose, nanocrystals, etc., to stabilize emulsion in place of surfactants also improves the stability of emulsions, as indicated in terms of biodegradability and chemical stability. This methodology is especially recommended to encapsulate active ingredients exhibiting lower melting points, such as hydrogenated vegetable oil [179].