Freeze-Drying Effects on Viability and Cellular Stability in a Subset of Sourdough Lactic Acid Bacteria Strains

Microbial resources are essential for food biotechnology, where they contribute to product quality, process efficiency, and safety. Their long-term preservation is critical for culture collections (CCs) and microbial biological resource centers (mBRCs). This study investigates the effects of freeze-drying on five lactic acid bacteria (LAB) strains from the Unimore Microbial Culture Collection (UMCC), comparing non-lyophilized and lyophilized cultures under two pre-freezing protocols: overnight incubation at -20 °C (PF-20) and 2-hour incubation at -80 °C (PF-80). Viability, fermentative performance, metabolite production, and cell integrity were assessed using microbial counts, HPLC profiling, and SEM imaging, respectively. Lactiplantibacillus plantarum UMCC 2996 maintained over 95% viability and stable fermentative traits across all conditions. In contrast, Fructilactobacillus sanfranciscensis UMCC 2990 and Leuconostoc citreum UMCC 3011 showed significant viability losses (down to 54.45% and 73.89%, respectively) and altered metabolic profiles, particularly under PF-80. SEM analysis confirmed structural damage in sensitive strains, with visible cellular debris and membrane wrinkling. This study provides novel perspectives on the customized assessment of freeze-drying protocols for sourdough-derived LAB strains with industrial potential, confirming the need for tailored preservation strategies to ensure the long-term functionality of LAB strains in CCs and mBRCs.