# Microencapsulation of Natural Food Antimicrobials: Methods and Applications
## 2 Encapsulation Methods
### Alginate Microbeads  
Alginate is a linear polysaccharide consisting of variable proportions of \(\beta\)-D-mannuronic acid and \(\alpha\)-L guluronic acid linked by \(\beta\) 1-4 glycosidic bonds and considered as GRAS. Alginate biopolymer could be derived from seaweed or from _Pseudomonas aeruginosa_ strains as an exopolysaccharideacharide [74]. It is one of the most studied polymers for the microencapsulation of active compounds due to its biodegradability, biocompatibility and ability to form heat-resistant hydrogels in the presence of bivalent cations such as calcium in an aqueous medium. The application of alginate beads for microencapsulation has some advantages, such as being a mild process, its easy lab-scale production and ability to encapsulate a wide variety of  
Figure 4: Incorporation of different hydrophilic bioactive agents into the hydrophilic interiors (W2) of liposomes [66].  
compounds (hydrophobic, hydrophilic, heat labile, liquid, viscous oil... ). Alginate capsules containing bioactive substances can be used as delivery systems by which the release of the agent occurs through a diffusional process in pores and might be accelerated by the degradation of the polymeric network [75]. Recently, the ionic gelation method has been used for the production of alcohol-free alginate-propolis capsules. The release of propolis from alginate beads was higher in solutions at pH 7.4 than at pH 1.5 and pH 6.8, indicating that the bioactive substances of propolis might reach the upper part of the large intestine [31]. Calcium alginate microbeads were produced by emulsion extrusion technology to microencapsulate clove, thyme and cinnamon EOs. A high loading capacity of EOs in microbeads (up to 23%) was obtained for 2% sodium alginate, 0.5% calcium chloride and 20 min of crosslinking. Increasing the amount of sodium alginate from 0.5 to 2% improved the EE due to the formation of dense microbead structures possessing cohesive pores that entrap oil droplets. In addition, the microencapsulation in alginate beads conserved the antifungal activity of EOs for about 8 days of storage, contrary to free oils, which lost their activity after two days of storage [29].  
The porous structure and poor physical properties of alginate microcapsules, which lead to the rapid diffusion of water and other fluids in and out of capsules, are considered as the main disadvantages of using this process for preserving biological properties of encapsulated compounds and reaching longer food shelf-lives [76; 77]. Regarding the mentioned drawback of this method, researchers have used alginate in combination with other materials for the microencapsulation of bioactive agents [28; 30]. Microencapsulation of nisin has been optimized within sodium alginate as a primary wall material and guar gum as a filler under different applied pressures [30]. Despite the low EE of nisin (36.65%) under the optimal conditions (sodium alginate concentration: 2% \(w/v\), guar gum concentration: 0.4% \(w/v\), air pressure: 0.5 bar), the encapsulation within the alginate-guar gum matrix can offer the possibility of a controlled release and sustained activity. By increasing alginate concentration, more structurally dense packed gels could be produced, resulting in increased nisin entrapment. At alginate concentrations higher than 2%, the solution became viscous, which causes difficulty in the encapsulation process [30]. Alginate-cellulose nanocrystals (CNC) microbeads were used for the microencapsulation of nisin. Molecular characterization by ATR-FTIR (attenuated total reflectance-Fourier transform infrared) showed high interactions between nisin and calcium-alginate-CNC microbeads, resulting in an effective preservation of antimicrobial activity of peptide during storage [28].