Nine strains of probiotic bacteria (Bifidobacterium longum DSM 20088, Bifidobacterium angulatum DSM 20098, Lactobacillus acidophilus DSM 20079, Lactobacillus delbrueckii subsp bulgaricus DSM 20080, Lactobacillus casei NCAIM B01137, Lactobacillus plantarum BFEL 92122, Lactobacillus gasseri ATCC 33323, Lactobacillus rhamnosus DSM 20245 and Streptococcus thermophilus DSM 20617) were tested for their tolerance to the manufacturing conditions of yoghurt and Ras cheese (i.e. heat treatments, annatto tolerance, calcium chloride resistance, salt resistance, scalding temperature resistance, effect of low and room storage temperatures and their growth characteristics in milk).
Heat treatments dramatically decreased the free bacterial cells counts of all tested strains at 70°C and 85°C, but it completely lost their viability at 90°C. Scalding temperature (40 - 46°C) decreased variably the viable counts of tested strains with Bifidobacterium longum was the most sensitive and Lactobacillus delbrueckii subsp bulgaricus was the lowest sensitive strains. The viable count of all strains decreased gradually from 47.17 to 1.02 x108 cfu/ml during storage at 5±1°C up to 15 days while at 25°C it increased during the first 3 days then decreased until the end of storage period (15 days) to nearly the initial (108 cfu/ml) count. The viable counts of tested bacterial strains were also decreased by the decrease of pH (less than 4), and increase in titratable acidity (>0.5%) and sodium chloride (4%) concentrations, respectively. Calcium chloride and annatto had no effect on the survival of the tested strains. However, the final viable counts of all strains subjected to all treatments, expect the heat treatment, were still within the recommended counts i.e. 106 – 107 cfu/ml of probiotic to exhibit their beneficial health effect.
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