Influence of growth media composition on cell stability during freeze drying

For the application of bacteria as starter cultures and/or probiotics, they are cultivated in fermenters and subsequently formulated as concentrated liquids or dried powders. Preservation of a high number of cells, their viability and activity are important criteria for their application.

Typically freeze-drying is used for drying cultures. During freeze-drying cells are exposed to harsh conditions, which can have negative impact on the cells. The extent of this impact can be influenced by factors like which species is used, cell state during harvesting, growth medium, drying medium, freeze-drying parameters and rehydration conditions.

This work investigates the impact of yeast extracts used in the growth medium on the survival and activity of S. thermophilus and L. helveticus following freeze-drying.

Main observations

  • The type of nitrogen source used in the fermentation media can not only influence growth of L. helveticus and S. thermophilus starter cultures, but can also significantly impact cell survival during subsequent freeze-drying.
  • Cellular ability to withstand freeze-drying depends on the LAB strain, and the combination of yeast extract and peptones used.
  • Consequently, optimization of the right concentration and combination of complex media compounds like yeast extracts, should be made based on performance throughout the whole production process, and not based on growth performance only.

Differences in growth and stability (left) and activity (right) of S. thermophilus cultures grown in media containing four different complex nutrient blends. The composition of the complex nutrients were different regarding source, degree of hydrolysis and content of free nucleotides.

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