# Microencapsulation of Natural Food Antimicrobials: Methods and Applications
## 3 Microencapsulation of Natural Food Antimicrobials
### Propolis  
Propolis, or bee glue, is an adhesive and resinous material collected by honey bees (_Apis mellifera_ L.) from various parts of plants and buds and leaves of trees. The main property of propolis is its inhibitory activity against microorganisms in addition to many other biological activities, such as antioxidant, anti-inflammatory, anesthetic, anticarcinogenic and anticariogenic properties [92; 93; 94]. Owing to its great properties, propolis, as a natural additive, can be safely used in food and pharmaceutical industries despite of some disadvantages, such as its unpleasant taste, off-odor and alcohol solubility [95], which could be overcome by microencapsulation [23; 31; 96; 97].  
The biological activities of propolis are attributed to the presence of several bioactive compounds, such as phenolic compounds, terpenoids, steroids, amino acids and vitamins [93]. Propolis extract (PE) displays antimicrobial activity against both Gram-positive and Gram-negative bacteria, indicating its possible use as a natural food preservative [84; 86]. Several research studies have demonstrated that Gram-positive bacteria are more sensitive to PE than Gram-negative ones due to differences in bacterial cell structure [98; 99]. In fact, the high permeability of Gram-positive bacteria cells is related to the presence of 90-95% peptidoglycan in cell walls, thus allowing the penetration of antimicrobial agents into the cell, whereas lipopolysaccharidesaccharides (LPS) from the outer membrane of Gram-negative bacteria create resistance toward natural antimicrobial active compounds [84]. Spray-dried microparticles of PE (2.5 and 5.0% \(w/v\)) with pea protein as a carrier (2.0% \(w/v\)) have shown bacteriostatic and even bactericidal effects against the Gram-positive bacteria _Listeria monocytogenes_ and _S. aureus_. Unlike PE, these propolis microparticles did not show any activity against either Gram-negative bacteria _S. Typhimuriumium_ or _E. coli_[84]. The antimicrobial activity of pure PE was shown to be higher than that of encapsulated PE, which could be due to the lower PE concentration in the microparticles [23; 84; 86]. The inhibitory activity of microencapsulated propolis in soybean protein isolate-pectin coacrvates with encapsulant agents and core concentrations between 2.5 and 5.0 g/100 mL against _S. aureus_ was about 200-400 \(\upmu\)g/mL, and for the free one, this ranged between 50 and 100 \(\upmu\)g/mL [23]. Thus,higher concentrations of propolis should be encapsulated to obtain antibacterial activity comparable to that of the free antimicrobial.  
The overall antimicrobial activity of encapsulated propolis is influenced by several factors, such as the type and concentration of the wall material used for encapsulation, the amount of PE substances within the dry particles and the presence of various biological compounds in the pure PE. However, no detectable growth of _S. aureus_ was found for either free propolis or spray-dried propolis-gelatin microcapsules, confirming that spray drying was effective at preserving the properties of the active agent [96].