Freezing injury of Escherichia coli during the production of ice cream

Samples of ice cream were contaminated with Escherichia coli cells at approximately 3 x 10(5) CFU/g and analysed throughout the productive process and frozen storage. The effects of the age of culture (cells in exponential or stationary phase), operation (freezing and hardening), time of conservation (1, 2, 4 and 8 weeks at -22 degreesC) were investigated in order to evaluate the rates of lethal and sublethal injury. The concentration of E. coli during a procedure of resuscitation was monitored to determine the phenomenon of repair of damaged bacteria: TSA counts for total viable cells and VRBLA counts for uninjured cells were used. Cultures in exponential phase of growth were significantly more susceptible (P < 0.01) to freezing injury than those in stationary phase; survival rates for the former ranged from 21% to 0.001%, from the start of freezing till the end of storage, whereas those for the latter were respectively from 70% to 13%. The greatest lost in viability was observed during the freezing operation step; the hardening treatment caused only a sublethal damage to the cells. Survival decreased from the beginning to the end of the conservation while, on average, the proportion of sublethally injured population was unchanged: 73% for exponential cells and 72% for the stationary cells. The time for the recovery of the damaged bacteria resulted 60 min at 30 degreesC in TSYB broth, but a proliferation of uninjured cells occurred from the start of the resuscitation step.