## 4 Recent Progress in Microencapsulation Techniques for Natural Bioactive Compounds
### Spray-Based Techniques  
This is the most common and widely adopted technique for the encapsulation of bioactive compounds. It is carried out by spray drying, spray congealing, or spray freeze drying methods [4]. The spray drying method is a popular and well-established method in which the core particles are dispersed in a polymer solution, and spraying is performed in a hot chamber [25]. This is an easy to operate, flexible, fast, and economical technique to produce microcapsules [25; 106]. This procedure consists of an initial dissolution and emulsification of dispersion of the core compound in the carrier material solutions, which is sprayed into a hot chamber [25]. The solvent evaporates, and the wall material coats around the core, leading to the formation of microcapsules of the polynuclear or matrix type. Therefore, this technique consists of the atomization of an emulsion or solution of core-carrier agents and their posterior recovery of the dehydrated microcapsules. Highly encapsulation-efficient and good-quality microcapsules are generally obtained with this procedure. However, this technique also has some limitations, such as the high temperatures that are required to evaporate the solvent, the use of water-soluble carriers, etc. [25]. To solve some of these problems, the spray-freeze drying technique was proposed. The spray-freeze drying method is suitable for heat-sensitive core materials. Under this method, solvent is immediately sublimed (lyophilized) from atomized droplets. Droplets are sprayed into a cold chamber filled with liquid nitrogen. These droplets are sublimed at ambient pressure in a fluidized bed, thus producing encapsulates with a matrix microstructure. Higher core loads lead to thinner shells, resulting in leakage of core material droplets on the surface of encapsulates, as core droplets adjacent to the shell are more susceptible to degradation [107; 108]. The spray drying technology produces encapsulates with very high solubility and reconstitution attributes, low water activity, and easier transport and storage, but the non-availability of perfect wall materials and the agglomeration of microcapsules are still limiting the use of this technology [109]. [PERSON] et al. [96] observed the loss of several heat-labile active ingredients during the operation of spray drying at a high temperature process. In addition, the yield during the spray drying process remains low due to the loss of dried particles in the drying vessel, poor control over the size of droplets, the constraints of common wall materials, which causes higher evaporation losses and less dry matter, low glass transition temperature, and the stickiness of materials with high sugar contents [110]. The choice of wall material for microencapsulation is very crucial to obtain higher encapsulation efficiency and higher stability of microcapsules [3; 25]. Since almost all the spray drying processes in the food industry are carried out in the aqueous feed formulations; the wall material essentially should be soluble in water [4]. The most commonly used wall materials are low molecular weight carbohydrates, milk proteins, gelatin, and hydrocolloids such as gum acacia [3]. Microencapsulation also masked the unpleasant smell of these extracts, making them suitable for nutraceuticals and food applications. Alginates as a carrier and delivery system for natural bioactive compounds have been studied extensively. It can be cross-linked with various cations to modify the characteristics of matrices for multiple applications including food. Even some enzymes such as cellulases and hemicellulases have been encapsulated using alginate matrices [64]. Gums possess excellent film-forming and emulsion stabilization properties, hence they are used during the microencapsulation of a variety of bioactive compounds. Gum arabic produces stable emulsion with many oils at a wide pH range and hence is a better wall material for the encapsulation of lipids.