The effects of the deacetylation degree (DD) and preparation methods for chitosan on antimicrobial activity were evaluated. Chemically prepared chitin (CH-chitin) and microbiologically prepared chitin (MO-chitin) were obtained from shrimp shells. The CH-chitin and MO-chitin were further chemically deacetylated to obtain various chitosan products of which their DD ranged from low (47-53%) through medium DD (74-76%) to high (95-98%). In addition, MO-chitin was deacetylated also by various proteases. The antimicrobial activities of these products were evaluated in medium with pH 6.0. Neither the CH-chitin, MO-chitin nor protease-deacetylated chitinous products showed any antimicrobial activity. For chitosan, antimicrobial activity increased with increasing DD, and was stronger against bacteria than against fungi. The minimal lethal concentrations (MLC) of chitosan with a high DD against Bacillus cereus, Escherichia coli, Listeria monocytogenes, Pseudomonas aeruginosa, Shigella dysenteriae, Staphylococcus aureus, Vibrio cholerae, and V. parahaemolyticus were all in the range of 50-200 p.p.m., whereas the MLC against Candida albicans and Fusarium oxysporum were 200 p.p.m. and 500 p.p.m., respectively. No antifungal activity was found at 2000 p.p.m. against Aspergillus fumigatus or A. parasiticus. Pretreatment of fish fillets (Oncorhynchus nereka) with 1% chitosan solution (high DD) for 3 h retarded the increase in the volatile basic nitrogen content, as well as the counts for mesophiles, psychrotrophs, coliforms, Aeromonas spp., and Vibrio spp. The shelf life was consequently extended from 5 days to 9 days.

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