We have developed a method to quantify the resistance to freezing and
frozen 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 step and storage. The loss
of acidification activity during freezing was quantified by the difference (Δtm)
between the tm values of the concentrated cell suspension before and after freezing.
During storage at −20 °C, linear relationships between tm and the storage time were
established. Their slope, k, allowed the quantitation of the decrease in acidification
activity during 9–14 weeks of frozen storage. The method was applied to determine
the resistance to freezing and frozen storage of four strains of lactic acid bacteria
and to quantify the cryoprotective effect of glycerol.