EFFECT OF ACID ADAPTATION ON THERMAL TOLERANCE OF ESCHERICHIA COLI O157:H7 AND SALMONELLA ENTERICA IN MEAT SERUM

Escherichia coli O157:H7 and Salmonella were adapted to acidic environments by growing in tryptic soy broth with 1% glucose (TSB + 1%G). Thermal tolerance of acid-adapted pathogens was evaluated in meat serum. Five-strain cocktail of both bacteria were grown separately in TSB and TSB + 1%G for 24 h at 37C. Meat serum was prepared from irradiated ground beef, inoculated with either acid-adapted or nonadapted E. coli O157:H7 and Salmonella, and heated at 58, 62 and 65C to determine D values. D values of acid-adapted E. coli O157:H7 were higher (P < 0.05) than nonadapted cells at 58, 62 and 65C. Significantly higher (P < 0.05) D values of acid-adapted Salmonella were observed at 58 and 62C, but not at 65C. D values were 22.46 and 10.59 min at 58C, 3.58 and 1.38 min at 62C, 1.02 and 0.75 min at 65C for acid-adapted and nonadapted E. coli O157:H7, respectively. D values of acid-adapted Salmonella were 9.36 min at 58C, 1.66 min at 62C and 1.14 min at 65C, whereas the nonadapted counterparts had D values of 6.44, 0.88 and 0.95 min at 58, 62 and 65C, respectively.