## 4 Recent Progress in Microencapsulation Techniques for Natural Bioactive Compounds
### Extrusion-Based Techniques  
In this technique, the droplets of an aqueous solution of polymer are dropped into a gelling bath solution. Sodium alginate (0.6-3%) is the most commonly used polymer with calcium chloride gelling bath solution. The dropping tool can be a pipette, syringe, vibrating nozzle, spraying nozzle, jet cutter, or vibrating disc. This method is widely used for the encapsulation of volatiles and unstable flavors. In general, conventional extrusion is applied for particle fabrication by forcefully flowing a mixture of core and encapsulants through the nozzle, whereas co-extrusion is applied for highly concentrated and viscous cores or encapsulant incorporation and particle fabrication, leading to the formation of matrix microstructures [163,164].  
Under co-extrusion, the core and encapsulants flow from the same outlet at high frequency, resulting in small encapsulates possessing a mononuclear microstructure, while the flow rate of the encapsulants and core materials and the vibration frequency regulate shell thickness [7; 128; 165]. Over a range of pH levels, alginate exhibits a negative charge and is able to form hydrogel via cross-links with cations. Ca\({}^{+2}\) is the most widely used cation due to the formation of a desirable final product, and adequate cross-linking formation occurs with higher concentrations of alginate and cations, i.e., calcium chloride solution and improved encapsulation efficiency [89]. The extruded encapsulates can be dried by freeze drying or fluidized bed drying for extending storage stability, while freeze drying leads to a porous structure and lowering encapsulation efficiency and poor core defense. Furthermore, alginate beads are more prone to the rapid release of active ingredients, such as during digestion or in an aqueous medium, due to their lower concentration (2-4% alginate hydrogel of high viscosity), leading to a low-density gel network. To overcome this problem of fast release, positively charged biopolymers such as chitosan are used along with alginate [28; 166]. Furthermore, using dietary fiber sources such as microfibers isolated from psyllium and rice along with alginate prior to cross-linking with calcium chloride was noted to significantly improve the encapsulating efficiency and optimum release of active ingredients such as caffeine [166]. [PERSON] et al. [167] utilized combinations of calcium chloride and potassium chloride salts, \(\kappa\)-carrageenan, and alginate for increasing encapsulating efficiency and controlled release due to the protonation and chelating effect of these beads. The more complex cross-linking network is ensured due to the cross-linkage between calcium, alginate, and sulfate groups of \(\kappa\)-carrageenan, whereas potassium ion further increases the cross-linkage of sulfate with the anhydrous-O-3, 6-ring of galactose residue present in \(\kappa\)-carrageenan. These beads were reported to have controlled releases even during digestion, as the lower pH of gastric juice further increases the active site for further increasing cross-links due to the protonation of carboxylic and alginate-\(\kappa\)-carrageenan. However, the higher pH in the intestine resulted in replacing calcium cations with sodium cations, leading to swelling and higher release of the core material.