Inhibitory Effects of Chitosan Coating Combined with Organic Acids on Listeria monocytogenes in Refrigerated Ready-to-Eat Shrimps

Chitosan and organic acids are natural preservatives with proven antimicrobial efficacies. This study investigated the effects of chitosan coating combined with organic acids on inhibiting Listeria monocytogenes in ready-to-eat (RTE) shrimps during storage at 4 degrees C. Cooked shrimps inoculated with L. monocytogenes were coated with 0.5 and 1% chitosan solutions in combination with 2% organic acids (acetic, citric, lactic, and malic acids) for 5 min and then stored at 4 degrees C for 16 days. Bacterial counts, pH, and sensory properties were analyzed every 4 days. The results indicated that the antimicrobial effects of chitosan were significantly enhanced (P < 0.05) when it was combined with 2% acetic, citric, lactic, or malic acid. Chitosan at 1% combined with 2% acetic acid was the most effective treatment, which caused a 5.38-log CFU/g bacterial reduction after 16 days in comparison with the controls. Transmission electron microscopy further confirmed the enhanced antimicrobial effects of the combination of chitosan and organic acids. Such combined treatments also maintained the sensory properties of RTE shrimps. The use of chitosan coating combined with organic acids significantly lowered (P < 0.05) the pH values of RTE shrimps compared with the use of chitosan alone. Therefore, we conclude that chitosan coating combined with acetic acid could be a promising antimicrobial method to prevent the proliferation of L. monocytogenes in RTE shrimps with extended shelf life.