. In order to select optimal conditions for the production of dry and active starter cultures it is important to determine the influence of growth conditions on the residual activity of dried bacteria. The influence of medium composition, pH during growth, continuous vs batch reactor and growth phase was studied on the residual activity of Lactobacillus plantarum after drying. The effect of high sodium chloride concentrations during growth on the residual activity of Lact. plantarum after drying was measured. The samples were dried by convection and in a fluidized bed. Bacteria with the highest residual activity after drying were produced in batch or chemostat with pH-controlled growth using enriched or diluted MRS medium. The presence of 1 or 1·25 mol l −1 NaCl during growth resulted in a decreased residual activity after drying. Variations in growth conditions (application of stress) generally did not result in higher residual activities after drying.
The production of active dried starter cultures can be influenced at several levels in the production process. In this paper the following process factors are discussed: osmotic stress during growth and cell density prior to drying. Contradicting results are reported in the literature on the influence of osmotic stress during growth on the residual activity after drying. The combined approach in which two process factors were studied at a time resulted in an explanation for the discrepancy in earlier work. The cell density prior to drying had an important influence on the glucose fermenting activity after drying. Residual activities ranging from 0.10 to 0.83 were achieved using initial cell densities between 0.025 and 0.23 g of cell/g of sample, respectively. The drying tolerance of cells grown with osmotic stress of 1 M NaCl was low (residual activity = 0. 06) and was not related to the cell density prior to drying. The influence of osmotic stress during growth on the drying tolerance of Lactobacillus plantarum was dependent on the cell density prior to drying.
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