Survival of Salmonella typhimurium and Escherichia coli O157:H7 in cheese brines

Abstract

Survival of Salmonella typhimurium and Escherichia coli O157:H7 was studied in model brines and brine from three cheese plants. Three strain mixtures of S. typhimurium and E. coli O157:H7 (10 6 CFU/ml) were inoculated separately into 23% model brine with or without added pasteurized whey (2%) and as a combined inoculum into the commercial brines. The model brines were incubated at 8 and 15°C for 28 days, and the commercial brines at 4 and 13°C for 35 days. Populations of both pathogens in the model brine+whey decreased slowly over 28 days (1.0–2.0 log CFU/ml) with greater survival at 8°C than at 15°C. Corresponding decreases in model brine without whey were 1.9–3.0 log CFU/ml, with greater survival at 8°C than at 15°C. Both S. typhimurium and E. coli O157:H7 survived significantly better ( P<0.05) at 4°C than at 13°C in two of the commercial brines. The survival of each pathogen in the commercial brines at 13°C was significantly influenced by brine pH. Both pathogen populations decreased most rapidly in commercial brines during the first week of storage (2.5–4.0 and 2.3–2.8 log CFU/ml for S. typhimurium and E. coli O157:H7, respectively) with significant recovery (ca. 0.5 log CFU/ml increase) often occurring in the second week of storage. Counts changed little thereafter. Overall, E. coli O157:H7 survived better than S. typhimurium, with differences of 0.1–1.2 log CFU/ml between the two pathogens. Results of this study show that cheese brine could support the survival of contaminating S. typhimurium and E. coli O157:H7 for several weeks under typical brining conditions.