Study of optimal conditions in freezing and thawing for thermophilic lactic cultures

0 strains of Streptococcus thermophilus and 6 strains of Lactobacillus delbrueckii subsp. bulgaricus were subjected, as concentrated cultures in skim-milk, to different treatments of freezing (high rate in liquid nitrogen at -196 degree C, slow rate in cold air at -30 degree C), storing temperature (at -80 degree C, at -30 degree C) and thawing (at +5 degree C, at + 25 degree C). The effect of freezing and thawing on single strains was studied throughout a year of conservation evaluating surviving microorganisms by plate counting and residual fermentative activity as ratio between produced acidity after and before freezing, in standard conditions of time and temperature. Higher survival and satisfying residual activity occurred to all the examined strains using liquid nitrogen for freezing and storing at -80 degree C. Behaviour of L. delbrueckii subsp. bulgaricus strains risulted homogeneous showing continuous decrement on viability, very clear for frozen cultures at -30 degree C, which lowered in plate counts of two logarithmic cycles and in residual activity under 10%; for frozen cultures stored at -80 degree C counts lowered less than one logarithmic cycle in a year and residual activities reached values about 40%-70%. S. thermophilus strains showed not homogeneous behaviours with decreases within one logarithmic cycle but variations between 10% and 60% in residual activity for frozen cultures at -30 degree C, high survival with residual activity between 50%-100% for frozen cultures at -80 degree C. Thawing did not seem to have any effect on cellular viability while it caused breaking of streptococci chains