Production of Concentrated Suspensions of Thermophilic Lactic Acid Bacteria in Calcium-alginate Beads

Mixed cultures of Streptococcus salivarius subsp, thermophilus and Lactobacillus delbrueckii subsp, bulgaricus were grown under free and immobilized cell conditions. Response surface methodology was used to determine the optimum pH (within the 5.2-6.1 range) and fermentation conditions for biomass production in three media: milk, Thermogold (Nordica) and whey supplemented with 0.5% meat extract. In classical free cell fermentations, the population of streptococci reached 1.5 X 10(9) CFU/ml of milk while that of the lactobacilli attained 6 X 10(8) CFU/ml. It was possible to immobilize the cultures in calcium-alginate beads and produce, under optimum fermentation conditions, cell densities 5 times higher than under free cell conditions. The overall biomass produced was 25% lower with the immobilized cell system. Cell release from the beads occurred and, at 46 degrees C, only 40% of the population was in the immobilized state. Incubating the beads at 24 degrees C increased the immobilized population in the system to 60%, but favoured the development of the streptococci.