Preparation Of Chemically Modified Chitosan And Their Antimicrobial Activities

Chitosan is inexpensive, biodegradable, and nontoxic for mammals. This makes it suitable for use as an additive in the food industry. The antimicrobial activity of chitosan against different groups of microorganisms has received considerable attention in recent years. But there still are several limitations for chitosan to use as food preservatives in food industries. For example, the antimicrobial activity is still lower than the antimicrobial agents currently used. Antimicrobial activity is limited to acidic conditions due to its poor solubility in neutral environment. The relationship of structure and the antimicrobial activity of chitosan were not elucidated yet. In this study, a series of chitosan derivatives were synthesized by chemical modification to improve the antimicrobial activity and to extent the application scope of chitosan, which will be of great significance for the use of chitosan as food preservatives.Using MeSO3H as catalyst and protecting agent, benzoyl-chitsan was synthesized by esterification between chitosan and benzoyl chloride. The structure of the product was analyzed by FITR and 1H-NMR technology. The result showed esterification mainly occured on–OH group rather than–NH2 group of chitosan. The antimicrobial activities of benzoyl chitosan derivatives were stronger than that of chitosan. The derivatives with a DS of 1.28 showed best antimicrobial activity among them.An efficient procedure to prepare p-hydroxybenzoyl-chitosan was established by using one three-step reaction. Firstly, p-acetoxybenzoyl chloride was prepared by p-acetoxybenzoic acid reflux in thionyl chloride. Secondly, chitosan was selectively esterified with the p-acetoxybenzoyl chloride in MeSO3H. Finally, the p-hydroxybenzoyl-chitosan was obtained by removal of the aromatic acetate group from the p-acetoxybenzoyl-chitosan. The chemical structure of the chitosan derivative was characterized by FT-IR and NMR analysis. Antimicrobial activities of the chitosan derivative were higher than that of chitosan.A novel water-soluble chitosan derivative, fumaryl-chitosan was prepared by using the selective partial acylation of chitosan and fumaric acid in the presence of H2SO4. The chemical structure and physical properties of fumaryl-chitosan were characterized by FTIR, NMR, and TG techniques. The results showed that derivatives had good solubility in a wide pH range, for example, the solubility of fumaryl chitosan with DS of 0.48 was 52.6g/L. The antimicrobial assays indicated that the antimicrobial activity of fumaryl chitosan was much stronger than that of chitosan and it increased with the increasing of DS.An efficient procedure to prepare sorbyl chitosan was established by using one two-step reaction. Firstly, sorbyl chloride was prepared by reflux of sorbic acid in thionyl chloride. Secondly, chitosan was selectively esterified with sorbyl chloride in MeSO3H. The chemical structure of the chitosan derivative was characterized by FT-IR, NMR and XRD analysis. The results showed sorbyl chloride mainly substituted with the hydroxyl group rather than amino group of chitsoan. The introduction of sobyl group destructed the crystalline structure of chitosan. The sorbyl chitosan with a DS of 1.93 had a good solubility in organic solvents such as ethanol. The sorbyl chitosan with a DS of 1.21 showed best antimicrobial activity among the chitoan and other derivatives.The chitosan derivatives with a moderate molecular weigh and DS showed higher antimicrobial activity than chitosan and other chitosan derivatives. The antimicrobial activityof chitosan derivative was higher than that of the corresponding mixture of chitosan and acidic preservatives. The MIC values of benzoyl chitosan(DS=1.28,164kDa) against Escherichia coil, Staphylococcus aureus, Aspergillus niger and Saccharomyces cerevisiae were 0.025,0.025,0.05,0.05%(w/v), respectively. These of p-acetoxybenzoyl-chitosan(DS=1.27,164kDa) were 0.0125,0.0125,0.025,0.025%(w/v) respectively. These of sorbyl chitosan(DS=1.26,164kDa) were 0.0125,0.0125,0.025,0.025%(w/v),respectively.These of fumaryl chitosan (DS=0.48,164kDa)were 0.05,0.05,0.05,0.05%(w/v), respectively. Compared with chitosan the derivatives showed improved antimicrobial activity, solubility and antimicrobial spectrum.