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.
Proteases in grass shrimp (Penaeus monodon) digestive tract were extracted. Four fractions, A, B, C and D, demonstrated caseinolytic activity and were purified to electrophoretic homogeneity. A, C and D were trypsin-like, while B was a chymotrypsin-like protease. Optimal temperature for proteases A, B and C were 65"C, and that for D was 55°C for hydrolysis of casein. Optimal pH of proteases A and C was 8.0, and that of D was 7.0 for hydrolysis of p-toluenesulfonyl-L-arginine methyl ester. Optimal pH of protease B for hydrolysis of N-b&zoyl-L-tyrosine ethyl ester &as 8.b. Inactivation of SO-% enzyme activity in 5 min occurred at 67°C for protease B and 50°C for proteases A, C and D.
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