# Microencapsulation of Natural Food Antimicrobials: Methods and Applications
## 3 Microencapsulation of Natural Food Antimicrobials
### Essential Oils  
Limitations in the direct addition of EOs in food products include: alteration in sensory properties due to their very low flavor threshold, being hydrophobic and highly insoluble in water, resulting in low antibacterial activity in moist foods, and being volatile and chemically unstable because of oxidation, volatilization and chemical interactions. The antibacterial activity of EOs is determined through evaluating the minimum inhibitory concentration (MIC) in many studies. In brief, high amounts of EOs should be added directly to food to exhibit the same antimicrobial activity rather than antibacterial assays in vitro because of the probable effectiveness of high amounts of some food components such as fats and proteins in preserving bacterial strains against EO antibacterial activity in some way. The bacteria present in the aqueous phase of food is mostly out of reach of EO, which is dissolved in the lipid phase. Additionally, antimicrobial activity against targeted bacteria can be restricted by low diffusion of bioactive agents in food products with low water activity [82]. Considering demands for effective application of natural antimicrobials such as EOs in food industries, microencapsulation of these compounds can be used as an alternative technique to solve problems relating to their direct addition [11; 83; 84; 85]. Controlled release of antimicrobial agents from the films without any changes in their antimicrobial capacity could be achieved via microencapsulation rather than direct addition of these agents into the film matrix. The release of encapsulated thymol and carvacrol (1%, 2%, 5% and 10% each) by the emulsification method (O/W) from bi-axially-oriented polypropylene (BOPP) films was studied over a course of 28 days at 4 \({}^{\circ}\)C [86]. EOs were more effective against yeast while the highest MIC (375 ppm) was obtained for carvacrol against _Escherichia coli_ O157:H7. The amount of the released agents from films containing antimicrobial microcapsules was higher than that required for the most resistant microorganism _E. coli_ O157:H7 at the level of 5% of each agent after two days and at the level of 10% of each agent after 1 day of storage. The fast release of carvacrol from microcapsules was attributed to its liquid form and noncrystallizable property compared to the solid and crystallizable thymol [86]. Synergistic activity of different compounds present in thyme oil such as thymol and carvacrol (MIC 0.64 mg/mL), compared with pure thymol (MIC 0.73 mg/mL), increased the antimicrobial activity against _E. coli_ K12 [13]). Based on the MIC values, the encapsulation of pure thymol and thyme oil in \(\beta\)-CD by freeze drying enhanced the antimicrobial activity (0.37 and 0.47 mg/mL, respectively) compared to free thymol and thyme oil. However, the antimicrobial activity of microencapsulated thymol/\(\beta\)-CD and thyme oil/\(\beta\)-CD inclusion complexes prepared by the kneading method were not improved compared to free ones. The differences among MICs of antimicrobial agent/\(\beta\)-CD inclusions against microorganisms were related to the method applied for inclusion complex synthesis, different steric conformation of the guest molecule and the rate of agent release from CD [13]. The water solubility of carvacrol encapsulated in CD was improved by increasing both the concentration of hydroxyl propyl-\(\beta\)-CD (HPBCD) in water and the temperature that increased contact between carvacrol and _E. coli_ K12 and _Salmonella Typhimurium_ LT2 in the medium. Encapsulated carvacrol showed an improved inhibition growth, ranging from 60 to 74% compared to the free one against both pathogensdue to improved availability of hydrophobic EOs in the medium through increased water solubility in HPBCD [12].