The Effects of Freezing on the Survival of Salmonella and E. coli in Pacific Oysters

SUMMARY: A mixed inoculum of Salmonella derby or S. typhimurium and Escherichia coli I was injected into the intestinal region of Pacific oysters (Crassostrea gigas) which were then frozen by four methods. Frozen oysters were stored at O°F, and survival of the inoculated bacteria was determined over a period of two weeks. In separate experiments, inoculated oysters were homogenized and then stored, unfrozen, at 32°F and ?30°F (frozen). Routinely, bacterial counts and pH readings were taken of all samples during the course of experiments.
Both species of Salmonella proved to be highly sensitive to freezing, regardless of the freezing method, and showed a survival of 1% or less after 48 hr. E. coli proved less sensitive, showing a wide and capricious variability of survival during the first week of storage, with survival ranging from 10 to 30%. Generally, however, most samples showed a decline comparable to that of salmonellae after two weeks'storage. Because of the fluctuation in E. coli counts after freezing, it is difficult to correlate the numbers of E. colt in frozen shellfish with the count in unfrozen shellfish. Therefore, it would be inappropriate to apply coliform standards for fresh oysters to the frozen product.
In separate studies using inoculated oyster homogenates held at 32° and ?30°F for 168 hr, a higher survival rate of E. coli and salmonellae was noted in samples held at ?30°F. However, since results obtained were based solely on bacterial counts, it is not possible to say with certainty that these results indicate a protective effect by oyster homogenates against the adverse effects of freezing. Significantly, the results of these experiments did not agree with results obtained with whole oysters, thus indicating the inadvisability of attempting to apply results of homogenate studies to the whole oyster.