Antibacterial activity of high molecular weight water-soluble (HMWWS) chitosan (800 kDa) was investigated against four Gram-negative (Escherichia coli, Salmonella typhimurium, Vibrio cholerae and Vibrio parahaemolyticus) and two Gram-positive (Staphylococcus aureus and Listeria monocytogenes) bacteria. Catfish fillets were surface-inoculated with these food-borne pathogens and coated with chitosan dissolved in aspartic acid (AS) or acetic acid (AC) solution at different concentrations (1% or 3%). Samples were stored at 4°C for 8 days, except for those inoculated with Vibrio species (10°C for 6 days). Overall, the most effective coating treatment was the 3% HMWWS chitosan in AS solution (800AS3%). Compared with the control, significant (P < 0.05) reductions caused by 800AS3% were observed for all tested pathogens at the end of storage. The growth of V. parahaemolyticus was completely suppressed by 800AS3%. This study demonstrated that HMWWS chitosan in AS solution could be used as an alternative antimicrobial coating for catfish fillets.

Chitosan with higher molecular weight exhibited higher antimicrobial efficacy against foodborne pathogens. However, the poor water solubility of higher or medium molecular weight chitosan limits its applications. To overcome the challenge, our research team searched for simple preparation procedure for fast‐dissolving medium molecular weight chitosan in water. Throughout the process, we were able to obtain a higher concentration of medium molecular weight water‐soluble (MMWWS) chitosan (400 kDa). The MMWWS chitosan showed physicochemical properties that are suitable for edible coating. Antibacterial activities of 400‐kDa chitosan coating prepared in acetic acid (1% v/v) or aspartic acid (1% or 3% w/v) were examined. The surface of catfish cubes was inoculated with six foodborne pathogens and then coated with chitosan solutions. The survival of each pathogen was evaluated during shelf life storage. Compared with the control, 3% w/v chitosan coating in aspartic acid solution exhibited the most effective antibacterial activities among other coating treatments, completely inhibiting Vibrio parahaemolyticus on the surface of catfish. The study suggested that chitosan dissolved in aspartic acid has the potential for use as an alternative antimicrobial coating for catfish fillet.

Summary Antimicrobial activities of high molecular weight water‐soluble chitosans (HMWWS) against selected Gram‐negative and Gram‐positive foodborne pathogens (initial inoculation of ca. 6.5 Log CFU mL−1) were evaluated. Chitosans with 789 kDa and/or 1017 kDa were dissolved in aspartic acid (AS) to obtain 1–4% w/v solutions. Among HMWWS, only 4% 789 kDa AS chitosan reduced E. coli counts by 2 Log CFU mL−1 from 7.33 at 0 h to 5.16 Log CFU mL−1 at 96 h, and they were not effective against S. Typhimurium. Depending on the concentrations, HMWWS completely inhibited V. cholerae, V. vulnificus and V. parahaemolyticus as well as B. cereus and L. monocytogenes after 48 h or 96 h of incubation. Compared with the control (no HMWWS), 2% or 3% 1017 kDa AS chitosans showed about 3 Log CFU mL−1 lower (4.72–4.86 vs. 7.71) for S. aureus at 96 h of incubation.