Antimicrobial activity of hydrophobically modified chitosan.

Chitosan is a polysaccharide with many food applications. Chitosan has been found to have antimicrobial activity against a wide range of target organisms. Its activity varies considerably with the type of chitosan, the target organism and the environment in which it is applied. Consequently, literature reports vary somewhat and are, occasionally, contradictory. But generally speaking, yeasts and molds are the most sensitive group, followed by Gram-positive bacteria and Gram-negative bacteria.

Some chemical modifications to chitosan that give the compound advanced functionality have drawn increased interest. Among these various modifications, it's the hydrophobic modification that is able to enhance the rheological performance of chitosan. However, data on the effects of hydrophobic modifications on the antimicrobial activity of chitosan are still lacking.

Scientists at the University of Tennessee investigated the antimicrobial activity of chitosan after hydrophobic modifications were made to the compound. Their results indicate that the antimicrobial activity of chitosan is not affected with the introduction of hydrophobic groups.

The researchers prepared hydrophobically modified chitosan by reacting 500 ml of 1% chitosan in 0.5% aq. acetic acid with 5 ml palmitoyl chloride at room temperature for four hours. The modified chitosan was precipitated from the post-reaction mixture by sodium hydroxide neutralization. It was then collected by centrifugation, washed with water and acetone, and lyophilized. It was characterized using Fourier transform infrared spectroscopy, nuclear magnetic resonance, thermogravimetric analysis and differential scanning calorimetry.

Then the scientists investigated the antimicrobial activity of both non-modified and modified chitosan against L. innocua ATCC 51742 and E. coli K12 using the pour plate method. It appears that the hydrophobic modification by direct reaction with acid chloride was non-selective. The researchers obtained a mixture of N-substituted and O-substituted chitosan. At a concentration of 0.1%, both chitosan and the hydrophobically modified chitosan effectively inhibited the growth of the tested bacteria.

After 24 hours, a 3-log reduction occurred in L. innocua. A 2-log reduction took place in E. coli. On the other hand, the researchers saw that 3-log and 4-log increases occurred in the control for L. innocua and E. coli, respectively.

Further information. Svetlana Zivanovic, Food Biopolymers Research Group, Department of Food Science and Technology, University of Tennessee, 2509 River Dr., Knoxville, TN 37996; phone: 865-974-0844; fax: 865-974-7332; email: lanaz@tennessee.edu.

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