COLONIAL VARIATION AND FREEZE-THAW RESISTANCE OF STREPTOCOCCUS-THERMOPHILUS

Two colonial variants (D12 and D31) of Streptococcus thermophilus CNRZ 368 differing in cell morphologies and chain lengths were investigated for their freeze-thawing resistance. After growth in a semi-synthetic medium, their survivabilities were significantly different, and cells frozen in stationary growth phase resisted freezing better than cells frozen in exponential growth phase. After growth in milk, the survival percentages of these variants were almost the same, and cell recovery was better, than in the semi-synthetic medium. In milk, no significant difference has been found between exponential and stationary growth phases. Survival rates of the variants increased with the freezing rate; survivabilities were lower for D12. The variant D12 was insensitive to variation in the warming rate, but survival of D31 increased with the warming rate. Both variants showed better resistance in the presence of cryoprotectants (dimethylsulfoxide, glycerol, adonitol, xylitol, and raffinose), and D31 was more labile to cryoprotection than D12. Prolongation of warming after ice melting altered survivability of D12. These differing susceptibilities to the freeze-thawing parameters might be useful for the preparation of Streptococcus thermophilus frozen starters.