## 3 Microstructure of Encapsulate  
During microencapsulation, microcapsules are produced containing active ingredients as core materials surrounded by encapsulated materials such as solids, liquids, or gases. The size of microcapsules varies from 1-1000 \(\upmu\)m depending upon the microencapsulation techniques applied, as well as core materials, wall materials, processing parameters, etc., while when the size of these microcapsules falls lower than 1 \(\upmu\)m, they are known as nanocapsules [3; 96]. For food applications, generally speaking, microcapsules should be lower than 100 \(\upmu\)m in order to avoid impacting the mouthfeel of the food product [25]. Additionally, microparticles can be classified into three types on the basis of structure such as microcapsules with a single core covered by wall material or cores dispersed in continuous matrix or multilayer or multishell complex microcapsules [97]. The size of encapsulates varies with the method used for preparation such as spray coating resulting in encapsulate size ranges from 5-500 \(\upmu\)m, co-extrusion results with encapsulating size of more than 200 \(\upmu\)m, micelle-based encapsulation with 5-20 nm size, liposome-based encapsulation with with 10-1000 \(\upmu\)m, coacervation with 1-500 \(\upmu\)m size, emulsion-based encapsulation with less than 200 nm size, and ionic gelation encapsulation with a 100 nm-100 \(\upmu\)m size of encapsulates [98]. The size of encapsulating has no direct effect on encapsulation efficiency, but it affects the controlled release of active ingredients in addition to physical and chemical properties of encapsulants, as smaller sizes result in larger contact areas for enzymes or releasing media and thus the release of larger amounts as compared to encapsulates with larger sizes [5]. The shell or wall material should preferably be insoluble, non-reactive to active ingredients, have excellent film-forming, and have the desired protective properties against various environmental conditions [3; 99].  
Figure 1: Basic concept of microencapsulation for bioactive compounds.  
The encapsulation method, properties of core and encapsulant materials, and load quantity result in the different microstructures of encapsulates. The microstructure of capsules has an impact on the efficiency and retention of the core material and the encapsulation efficiency and regulates the release of active ingredients [25]. The microstructure of encapsulates varies in the external structure such as rough, porous, hollow, cracked, shrunken, etc., or the internal structures such as the arrangement and configuration of the core material [100]. Recently, several researchers noticed the higher encapsulation efficiency, larger shell thickness, more compact shell, and slower release of ingredients in the case of encapsulates with a lower core load, whereas weak shells such as porous or cracked structures lowered protection and resulted in an early release of encapsulated ingredients. The higher load in microcapsules leads to thinner shells, resulting in lower protection and increased release, whereas fewer core materials in microcapsules usually results in a thicker wall and in the better protection and slower release of core materials [101; 102; 103].