Water-solubility of chitosan and its antimicrobial activity

Chitosan samples with different molecular weights were prepared by depolymerization with hemicellusase, and water-soluble half N-acetylated chitosan samples were obtained by N-acetylation with acetic anhydride. The action of chitosans with molecular weights Mw from 1.4×103 to 4.0×105 on the growth of Staphylococcus aureus, Escherichia coli and Candida albicans was explored by microcalorimetry. The water-soluble half N-acetylated chitosans and chitooligomers had no significant antimicrobial activity. Moreover, water-soluble chitosans and chitooligomers promoted the growth of C. albicans. In contrast, water-insoluble chitosan in acidic medium exhibited inhibitory effect against these microorganisms. The water-insoluble chitosans with Mw around 5×104 were the optimum for antimicrobial action in these tested samples. The antimicrobial mechanism of dissolved water-insoluble chitosan was hypothesized as forming an impervious layer around the cell. The results suggest that optimum chitosan as food preservative should be water-insoluble chitosan from mild depolymerization of native chitosan.