Method of quantifying the loss of acidification activity of lactic acid starters during freezing and frozen storage

We have developed a method to quantify the resistance to freezing and froze n storage of lactic acid starters. based on measuring the time necessary to reach the maximum acidification rate in milk (tm) using the Cinac system. Depending on the operating conditions, tm increased during the freezing ste p and storage. The loss of acidification activity during freezing was quant ified by the difference (Delta tm) between the tm values of the concentrate d cell suspension before and after freezing. During storage at - 20 degrees C, linear relationships between tm and the storage time were established. Their slope, k, allowed the quantitation of the decrease in acidification a ctivity during 9-14 weeks of frozen storage. The method was applied to dete rmine tl-le resistance to freezing and frozen storage of four strains of la ctic acid bacteria and to quantify the cryoprotective effect of glycerol.