Chitosan is obtained by deacetylating chitin, which is naturally abundant, nontoxic，and biodegradable. Chitosan has several useful physicochemical and bioactive properties such as antimicrobial activity. The objective of this study is to investigate the practical application of the antimicrobial activity of chitosan for food preservation. The inhibitory mode of action and the effects of food components on the antibacterial activity of chitosan were investigated. In addition, the selective control of yeast using chitosan was studied in a mixed culture with lactic acid bacteria.
The antimicrobial activity of chitosan was investigated against bacteria and yeast. Chitosan of MW 3 kDa showed higher inhibitory effect than chitosan of MW 10 kDa, 50 kDa and glucosamine. Transmission electron microscopic observation showed chitosan disrupted cell envelopes, released cellular components, resulting in loss of cell viability.
The binding of chitosan on the cell surface was investigated using FITC-chitosan with non-growing cells. The binding was accomplished within 5 min and the maximum amount of bound FITC-chitosan was proportional to the concentration up to 200 ppm. The pretreatment of Ca2+, Mg2+ to the cells decreased the inhibitory activity of chitosan and binding of FITC-chitosan. Bound FITC-chitosan was decreased about 20-50% by chitosanase.
To investigate the application of chitosan as natural food preservative, the effects of several food components on the inhibitory activity of chitosan was determined. The addition of NaCl and sucrose decreased the inhibitory effect of chitosan while ethanol and the organic acids enhanced the antibacterial activity. The results obtained using the non-growing cells, showed NaCl repressed the inhibitory activity of chitoasn by acting directly on the cells. In contrast, sucrose added to the cell prior to chitosan treatment showed no effect on the viability or FITC-chitosan binding of the cells, which suggested the interaction of sucrose with chitosan affected its activity.
To control yeast by chitosan in a mixed culture with lactic acid bacteria, the addition of 100 ppm of chitosan inhibited S. cerevisiae and K. lactis more than Lactobacilli spp. or P. pentosaceus. The addition of chitosan in the period of mixed fermentation reduced the population of yeast and lactic acid bacteria depending on the concentration of chitosan. This suggests the possible use of chitosan to inhibit selectively yeast in a mixed fermentation process with lactic acid bacteria and to adjust the population of yeast and lactic strains at an adequate level.
In conclusion, this study showed the practical use of chitosan in food products as biopreservative against a broad spectrum of bacteria and yeast associated with food. The possibility of chitosan to control some species among the large variety of microorganisms involved in the mixed fermentation was also demonstrated.