## 4 Recent Progress in Microencapsulation Techniques for Natural Bioactive Compounds
### Fluidized Bed Coating  
This technology is a modified version of the spray drying method and involves the use of coating onto a fluidized bed of solid particles [180]. It is mostly used in the pharmaceutical industry. In this technology, solid and liquid core materials are encapsulated by absorbing into porous solid material. It consists of three processes such as suspending the solid materials in the air, followed by the spraying of liquid material to encapsulate these solid particles, and at last hardening the shell by cooling or solvent vaporization. The core material is attached to encapsulants by hydrogen bonds or liquid bridges and electrostatic, hydrophobic, and hydrophilic interactions [6], and surface tension and viscosity are very important parameters of encapsulant solution. These three processes are repeated until the desired thickness of the wall is achieved. By using this technique, the microencapsulation of ascorbic acid has been performed by using polymethyl acrylate and ethyl cellulose. [PERSON] et al. [181] fluidized and microencapsulated ascorbic acid by using hydrophobic coating materials as top-spray and polymethacrylate coating material.  
[PERSON] et al. [6] used a fluidized bed spray coating for the encapsulation of _Rosmarinus officinalis_ extract containing various bioactive polyphenols such as carnosol, rosmarinic, carnosic, and caffeic acids by using whey protein concentrate and gum arabic as encapsulant solutions. Th authors reported encapsulation efficiencies of more than 75% upon drying air temperature of 70 \({}^{\circ}\)C for carnosol, rosmarinic acid, and carnosic acid, and for caffeic acid, it was possible by using less viscous whey protein concentrate solution. However, a highly viscous solution such as gum Arabic resulted in lower encapsulation efficiency (57%) due to the poor entrapping of caffeic acid. [PERSON] et al. [182] reported a 95% encapsulation efficiency when encapsulating L-methanol using a gelatin emulsion as an encapsulant solution and noted a lower encapsulation efficiency upon using a gelatin solution with higher viscosity. Various authors recommended an optimum surface tension of encapsulants solution as 32-49 mN/m as it facilitates a perfect microstructure, uniform wall thickness, and higher encapsulation efficiency [5; 182].  
Air is the most commonly used fluidizing and drying medium, and air temperature has a direct impact on the quality and characteristics of microstructures; for example, high temperature causes pores, cracks, and uneven wall thickness and coating due to the drying of droplets prior to their contact with the surface of the core material [5; 183]. The desirable quality of microstructures could be achieved by increasing the coating cycles, theproper flow of coating solution and its temperature, air velocity, atomizing pressure, the core to encapsulants ratio, and the use of coating material with desirable attributes such as lower viscosity, higher solubility, lower permeability, etc. [98, 183, 184]. [PERSON] et al. [185] encapsulated _Lactobacillus paracasei_ by using molten wax with ethylcellulose solution (7% \(w/w\)) and noted a higher survival rate in the case of wax-coated particles at an inlet temperature 45 \({}^{\circ}\)C and an outlet temperature of 25 \({}^{\circ}\)C. Authors further reported a lower survival rate upon coating with ethylcellulose due to crack formation in the wall. The main limitation of this method remains a complex and lengthy process, uncontrolled aggregation of particles due to wet-coated material and the presence of liquid particles among particles [180, 186].