Lactobacillus helveticus Collection Cultures: Survival Rate during Lyophilization and Storage

Lyophilization is a common drying method. It preserves microorganisms in bioresource collections, producing stable dry starter cultures and probiotics with a long shelf life. However, lyophilization always damages bacterial cells, killing a certain share of the population. Cryoprotectants and freezing can protect cells from damage and increase their survival rate. This article investigates the most effective pre-freezing conditions and cryoprotectants that provide the highest survival rates while retaining the metabolic activity of the thermophilic lactic acid bacterium Lactobacillus helveticus during freezedrying and subsequent storage. The research featured three pre-freezing temperatures (–20, –30, and –55°C) and two types of cryoprotectants (sucrose and trehalose at a 4% concentration). The lowest survival rates resulted from a pre-freezing temperature of –55°C, both with and without cryoprotectants. The highest survival rate of L. helveticus strain 158-16 during freeze-drying (32–33%) occurred after pre-freezing at –20°C with sucrose. The lowest survival rate (15%) belonged to the sample with trehalose. The survival rates for L. helveticus strain 305-10 were virtually identical for the samples with sucrose (26.6%) and trehalose (27.7%). Sucrose and trehalose as cryoprotectants reduced bacterial cell loss during 12 months of storage by a factor of 3 to 11 compared to the control without cryoprotection. Overall, the freeze-drying protocol with pre-freezing at –20°C with sucrose was able to increase the survival rate of the L. helveticus strains during freeze-drying and storage.