Evaluation of the Thermally Dried Immobilized Cells of Lactobacillus delbrueckii subsp. <i>bulgaricus</i> on Apple Pieces as a Potent Starter Culture

The aim of the present study was to evaluate the impact of thermal drying of immobilized Lactobacillus delbrueckii subsp. bulgaricus on apple pieces on the use of the derived biocatalyst in whey fermentation. The thermally dried immobilized biocatalyst was compared to wet and freeze-dried immobilized cells, in respect to maintenance of cell viability and fermentation efficiency. The thermal drying process appeared to be more efficient on survival rate as an 84% of the cells used for immobilization survived the process, while the freeze-drying process led to a 78% rate. The thermally dried immobilized biocatalyst was used in 12 repeated batch fermentations of synthetic lactose medium and whey at 37, 45, and 50 °C in order to evaluate its metabolic activity. The high number of repeated batch fermentations showed a tendency for high operational stability. Fermentations continued for up to 2 months without any significant loss of metabolic activity. SPME GC/MS analysis of aroma-related compounds revealed the distinctive character of fermented whey produced by the thermally dried immobilized bacterium cells. The effect of storage at 4–6 °C for up to 165 days of the biocatalyst, held directly after drying and after repeated batch fermentations, on fermentation activity was also studied. After storage, reactivation in whey was immediate, and the immobilized biocatalyst was able to produce up to 51.7 g/L lactic acid at 37 °C. The potential of thermally dried immobilized L. delbrueckii as a starter culture for food production was subsequently evaluated.