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 M-w from 1.4X 10(3) to 4.0 X 10(5) 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 M-w around 5 X 10(4) 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. (c) 2005 Elsevier Ltd. All rights reserved.