Antimicrobial action of hydrolyzed chitosan against spoilage yeasts and lactic acid bacteria of fermented vegetables

Abstract

The antimicrobial properties of various chitosan-lactate polymers (ranging from 0.5 to 1.2 MDa in molecular weight) against two yeasts isolated from fermented vegetables and against three lactic acid bacteria from a mixed starter for sauerkraut on methylene blue agar (MBA) and in vegetable juice medium (VJM) were investigated. Chitosan-lactate reduced the growth of all microorganisms in solid (MBA) as well as in liquid (VJM) medium. In MBA, a concentration of 5 g/liter was needed to inhibit the growth of Saccharomyces bayanus, while 1 g/liter was sufficient to inhibit the growth of Saccharomyces unisporus. Lactic acid bacteria were also inhibited in this range of concentrations. The low-molecular-weight chitosan-lactate DP3 (0.5 kDa) was most efficient in solid medium (MBA), and inhibitory activities decreased with increasing hydrolysate lengths. In liquid medium (VJM), 0.5 g of chitosan-lactate per liter reduced the growth rates for both yeasts, but 10 g/liter was insufficient to prevent yeast growth. Intermediate-molecular-weight chitosan-lactate (5 kDa) was more efficient than chitosan of low molecular weight. Native chitosan (1.2 MDa) showed no inhibition in either medium. Microscopic examination of S. unisporus Y-42 after treatment with chitosan-lactate DP25 showed agglutination of a refractive substance on the entire cell wall, suggesting an interaction between chitosan and the cell wall. When chitosanase was added to the culture media containing chitosan-lactate, refractive substances could not be observed.