2024 Volume 25 Issue 2 Pages 25-33
This study aimed to investigate the effects of soymilk ingredients on the inactivation of Lactobacillus plantarum subsp. plantarum and Lactobacillus pentosus. Pretreated bacteria were inoculated in MRS broth or soymilk and incubated at 30°C for 48 h. The cells were suspended in 0.85% NaCl or soymilk and incubated at 65°C for 60-120 s at high-pressure in the 300–450 MPa range. The logarithmic survival ratios of the lactobacilli cultured in MRS or soymilk decreased with heating time. The inactivation rate constant, k, did not change between MRS and soymilk medium by heat treatment. The difference in composition of cell membranes was also small. The survival behavior of each sample subjected to high-pressure was described by first-order kinetics. A gradual increase in k was observed at 300–450 MPa. It is found that pre-exponential factor and activation volume of the lactobacilli subjected to high-pressure treatment were related linearly. These indices did not have clear relationship with membrane composition of the lactobacilli. Thus, the different effects of heat and high-pressure stresses on cell resistance are possibly due to the incubation conditions. The results of this study indicate that the mechanism of inactivation may be essentially similar to numerous chemical processes in many microorganisms though factors for the acquisition of heat and pressure tolerance are diverse.
