Impact of protectants on drying kinetics and viability of microwave freeze‐dried Lactobacillus paracasei ssp. paracasei F19

The three carbohydrates sorbitol, trehalose, and maltodextrin, differing in molecular weight, were investigated as excipients to enhance survival of the probiotic strain Lactobacillus paracasei ssp. paracasei F19 during a microwave‐assisted freeze‐drying process. This technique was further compared to conventional freeze‐drying, where thermal energy is supplied by heat conduction instead of radiant energy. Survival rates revealed similar values for both techniques after drying without protectants (~10%). While addition of maltodextrin and trehalose led to a two‐ up to threefold increase in survival for both dryings, sorbitol containing samples showed the greatest difference between the two techniques: 65% after freeze‐drying versus 12% after microwave freeze‐drying. Though protection mechanisms were generally found to be equal to conventional lyophilization, microwaves are likely to induce vibrations in the hydrophilic sugars. Thus, not all protectants which are applicable in freeze‐drying, can be applied in microwave freeze‐drying to enhance bacterial viability. Practical applications A new drying process for the efficient preservation of bacterial cultures, such as lactic acid bacteria, was investigated. Microwave freeze‐drying is a much faster and less costly process than conventional freeze‐drying leading to equal product qualities and reducing the problem that drying often is the bottleneck in industrial biotechnological processes. However, no work on microwave freeze‐drying of bacterial cultures has been published so far. Knowledge on the impact of protectants on the survival of the cultures is a crucial aspect to make microwave freeze‐drying applicable in industrial processes. The selection of proper protectants is important to achieve high survival after drying and during storage.