抄録
Factors affecting the survival on freezing process of Lactobacillus delbrueckii subsp. bulgaricus suspension were investigated. At first, the survival rate as a function of cooling speed (from -1℃/min to -40℃/min) was examined. The results showed the least survival rate at -7℃/min, however, the differences in the all cooling rates were not significant. Subsequently, adding NaCl in medium, the effect of the final temperature (Tfa) from -15℃ to -50℃ on the damage of L. delbrueckii subsp. bulgaricus was investigated at a cooling rate of -7℃/min. It was found that the survival rate in the range Tfa> -21.1℃ ( eutectic temperature of NaCl solution) decreased significantly with Tfa, but it was constant and independent of Tfa below -21.1℃ These results suggested that the freeze concentration effect during freezing process damages L. delbrueckii subsp. bulgaricus significantly. Finally, we studied the change of survival rate when L. delbrueckii subsp. bulgaricus was immersed at room temperature in solution of NaCl and glucose with the same osmotic pressure as that observed during freezing. The results showed that there was no difference among solutions but the survival rate decreased by 70% in a short time even at room temperature. These results support that the main factor of damaging L. delbrueckii subsp. bulgaricus during freezing is the freeze concentration effect.
