Control of lactobacillus fructivorans in low-acid and -salt dressing

Lactobacillus fructivorans has been identified as a spoilage bacterium in dressing, since it is strongly resistant to acetic acid. In recent years, the spoilage of dressing has increased in frequency because of the reduced concentrations of both acetic acid and NaCl in dressings. To develop new control methods for L. fructivorans in lowacetic acid dressing, the properties of L. fructivorans were investigated. Doubling time of the bacterium was longer than that of other lactic acid bacteria. In addition, the incorporation rate of fluorescent dye or esterase activity of the bacterium was also lower than observed with Lactobacillus brevis. These results suggest the importance of increasing the incorporation of acetic acid into the bacterial cells. For this purpose, chitosan was employed. Cells treated with chitosan showed a nearly 10-fold increase in the incorporation of SYTO green compared to control cells. Chitosan adsorbed onto the surface of L. fructivorans and increased the surface hydrophobicity of the cells. The chitosan-treated cells localized at the water-oil interface of the dressing, resulting in decreased combined antibacterial action of chitosan and acetic acid. The addition of 0.04% thiamine dilaurylsulfate, an amphipathic antibacterial compound, completely inhibited the growth of L. fructivorans in the dressing with 0.04% chitosan and 3.5% NaCl at pH 4.1, for 60 days at 30°C.