Mild acidic stress enhances freeze-drying survival in Leuconostoc mesenteroides WiKim0121 by activating carbohydrate transport and metabolism

The aim of the present study was to explore the effect of fermentation pH during the cultivation of Leuconostoc mesenteroides WiKim0121 on its viability after harvesting and survival rate after freeze-drying (FD). The viability and survival rate of Leu. mesenteroides WiKim0121 differed with fermentation pH ranging from 5 to 7 in a 5 L fermenter. The highest viable cells of 6.73 × 10⁹ CFU/mL was observed with the pH 6 controlled culture, whereas the highest survival rate of 89 % after FD was observed with the pH 5 controlled culture. An increase in the proportion of cyclopropane fatty acids (CFA) was observed with the pH 5 controlled culture. In the pH 5 controlled culture, CFA were represented exclusively by C17:0 cyclo at 7 %, whereas the proportion of CFA was below 3 % at the other fermentation pH values. To identify the genetic indicators of stress resilience, transcriptome analysis was performed using samples with the highest and lowest survival rates. An increase in membrane-stabilizing compounds was observed, accompanied by up-regulation of genes related to stress-response pathways, including the phosphotransferase system, pyruvate metabolism, and carbohydrate transport. Thus, controlling the fermentation pH is an effective means of tolerating FD stress during the FD process by activating carbohydrate transport and metabolism. These findings suggested that the robustness of starter-culture through controlling fermentation pH could contribute to the industrialization of starter culture.