Effects of cold shocking on the survival and injury of Escherichia coli O157:H7 under freezing conditions
The survival and injury of 'Escherichia coli' O157:H7 strains in both frozen foods and trypticase soy broth (TSB) was investigated following cold shocking at 10C for 1.5 h. Using both trypticase soy agar (TSA) and violet red bile agar (VRBA) as recovery media, it was demonstrated that survival levels between cold shocked (CS) and non-cold shocked (NS) ' E. coli' in ground beef or ground pork were not significantly different (p <= 0.05). In contrast cold shocking 'E. coli' in either milk, whole egg or sausage resulted in a significant (p <= 0.05) enhancement in survival. For milk, survival levels of CS 'E. coli', after 28 days of frozen storage, were 1.89 and 1.66 log10 CFU/ml higher on TSA and VRBA respectively, when compared to NS cells. In egg, these values were 0.64 and 1.31, while in sausage, values of 0.74 and 1.19 were obtained. In TSB (pH 7) survival of CS 'E. coli' for some strains was about 3 log10 CFU/ml higher when compared to NS cells. Acidification of TSB (pH 5), however, appeared to negate the protective effects of the cold shock treatment. In milk, increasing the differential between the growth and the cold shock temperatures resulted in higher numbers of survivors. In this regard, the growth-cold shock temperature protocol giving optimum survival was 37-10C. In contrast, growth of 'E. coli' at 20C followed by cold shocking at 10C did not result in any significant increase in survival. In addition, increased survival due to cold shocking was correlated with the appearance of a novel protein with a pl of 4.8 using isoelectric focusing analysis. Preliminary studies also indicated no clear survival pattern between cold and non-cold shocked cells following heat treatment at 48 and 51C.