Supplement C18:1 in Culture Medium to Improve Survival Rate and Fermentation Activity of Lyophilization of <i>Lacticaseibacillus paracasei</i> L9

This study explored the impact of C18:1 fatty acid supplementation in culture medium on the survival rate, fermentation activity, and membrane stability of <i>Lacticaseibacillus paracasei</i> L9 during freeze-drying. Cells cultured in medium supplemented with C18:1 (0.2 mg/ml) exhibited a higher survival rate (95.38%) after freeze-drying compared to the control (83.56%). The fermentation activity of the freeze-dried powder was significantly enhanced, with improvements in viable count (Δ viable counts: 1.79 Log CFU/ml), pH reduction (Δ pH: 2.03), and titratable acidity (Δ titratable acidity: 99.4 °T) in skim milk. Supplementation with C18:1 increased the activity of key enzymes, such as <i>β</i>-galactosidase (13.25 U/ml) and lactate dehydrogenase (2.16 U), while reducing cell membrane permeability. The addition of C18:1 also modulated the expression of membrane synthesis-related genes, including <i>plsX</i> and <i>YfhO</i>, and altered the membrane fatty acid composition by increasing the proportion of unsaturated fatty acids (UFA). Specifically, the proportion of UFA in the membrane increased from 53.36% in the freeze-dried control to 63.28% in the C18:1-supplemented group. These results indicate that C18:1 supplementation improves cell membrane stability and fermentation activity by modulating gene expression and membrane fatty acid composition, thereby enhancing the freeze-drying tolerance of <i>L. paracasei</i> L9.