Studies on the effects of low temperatures on lactic acid bacteria

Studies were conducted on different strains of L. bulgaricus, L. casei, S. thermophilus, S. lactis, and S. cremoris isolated in Bulgaria and applied as pure cultures and in combinations as starters. All the strains under investigation were found to preserve, on “freezing-thawing” their characteristic morphological and biochemical properties, regardless of the temperature and rate of cooling, but the optimum freezing temperature of the strains studied is ?196 °C (in liquid nitrogen). High cooling rates provide higher viability and activity of lactic acid bacterial cells. Lactic acid streptococci, S. lactis and S. thermophilus, are considerably more resistant than lactic acid rods, L. casei and L. bulgaricus, at all the freezing regimens tested.