Biopolymer-Based Nanoencapsulation of Food Antimicrobial Agents
Biopolymers have found tremendous applications in nanoencapsulation of antimicrobial agents. Various sources serve as origins of biopolymers, which could be either animal or plant based, in their native form or being modified for specific goals such as controlled or target delivery. Commonly used biopolymers for nanoencapsulation include protein-based materials like dairy, plant proteins and carbohydrate-based biopolymers like starch and gums.
Protein-Based Biopolymers
Proteins are biopolymers comprising amino acids, adopting different molecular structures determined by amino acids sequence and environmental factors like pH, ionic strength, heat, pressure, etc. Proteins from various sources could be drastically different in terms of emulsification capability, solubility, stability, and their impacts on texture, flavor, color of food matrix that they will be incorporated in. To function as an encapsulation material for antimicrobial agents, only proteins have balanced hydrophobic/hydrophilic portions/amino acids sequence that can be used to minimize the overall free energy between dispersed phase (antimicrobial agents) and continuous phase (solvent) that generally have significant polarity differences.
Whey proteins have been used as an effective encapsulant for various hydrophobic antimicrobial agents such as essential oils for preparation of stable dispersions in aqueous systems like milk. By forming conjugation with maltodextrin, whey-maltodextrin conjugate was able to encapsulate and stabilize thymol, inhibiting growth of Escherichia coli O157:H7, and Listeria monocytogenes in both reduced fat milk and apple cider while not affecting the transparency of the beverage. (Bhavini et al., 2012) The anti-listerial activity can also be improved significantly by utilizing the synergistic effect of thymol and propylene glycol after being encapsulated in whey-maltodextrin conjugate, resulting in much faster and improved bactericidal effect in skim, reduced and whole fat milk, compared with free thymol applied at same levels (Pan et al., 2014a).
Caseins are dominant dairy protein, comprising 80% of the total bovine milk protein, with the rest of being mainly whey and other minor globular proteins. Different from whey protein which is globular protein with defined three-dimensional structure, caseins are considered to have random coil spatial structure with flexible chains in aqueous solution. There are four types of caseins forming micelles in their native state in milk. After being depleted of calcium ions, which serve as bridges for keeping the integrity of casein micelles, caseins can be transformed into caseinate. Because of the balanced hydrophobic and hydrophilic amino acid portions, casein or caseinate has found valuable uses as a natural biopolymer for nanoencapsulation. Through direct high shear homogenization of thymol mixed in caseinate solution, nanoencapsulated thymol showed transparent appearance and significantly improved anti-listerial activity in milk of different fat levels (Pan et al., 2014a).
Zein is a group of prolamine proteins insoluble in water. Zein has been used extensively for nanoencapsulation of bioactive compounds due to its high portion of hydrophobic amino acid residues (>50%) that provides more binding sites for hydrophobic molecules, as well as its superb film formation property and resistance to water enables its applications in antimicrobial food packaging. Liquid-liquid dispersion or antisolvent method (which will be covered in later sections) is commonly used for preparation of zein nanoparticles. Spherical particles are formed with hydrophobic antimicrobial agents like essential oils entrapped into lipophilic chamber of zein nanoparticles (Parris et al., 2005). To improve the water redispersibility of antimicrobial encapsulated in zein nanoparticles, additional layers of coatings could be applied with material of better water binding capacity such as sodium caseinate and chitosan (Chen and Zhong, 2014; Zhang et al., 2014b). However, in cases where organic solvent is not desired, dispersibility of zein nanoparticles could also be improved by “green” method without involvement of organic solvent by utilizing the pH-dependent self-assembly properties of biopolymers (Pan and Zhong, 2016a).
