SURVIVAL AND GROWTH OF ESCHERICHIA-COLI O157-H7 IN-GROUND, ROASTED BEEF AS AFFECTED BY PH, ACIDULANTS, AND TEMPERATURE

A study was undertaken to determine the fate of Escherichia coli O157:H7 in ground, roasted beef as influenced by the combined effects of pH, acidulants, temperature, and time. There was essentially no change in the viable population of E. coli O157:H7 when beef salads (pH 5.40 to 6.07) containing up to 40% mayonnaise were incubated at 5-degrees-C for up to 72 h. At 21 and 30-degrees-C, significant (P less-than-or-equal-to 0.05) increases in populations of the organism occurred in salads containing 16 to 32% mayonnaise (pH 5.94 to 5.55) between 10 and 24 h of incubation. Death was more rapid as the pH of acidified beef slurries incubated at 5-degrees-C was decreased from 5.98 to 4.70. E. coli O157:H7 grew in control slurries (pH 5.98) and in slurries containing citric and lactic acids (pHs 5.00 and 5.40) incubated at 21-degrees-C for 24 h; decreases occurred in slurries acidified to pHs 4.70, 5.00, and 5.40 with acetic acid or pH 4.70 with citric or lactic acid. At 30-degrees-C, populations decreased in slurries acidified to pHs 4.70 and 5.00 with acetic acid. Citric and lactic acids failed to prevent significant increases in populations in slurries at pH 4.70 to 5.40 between 10 and 24 h of incubation. The order of effectiveness of acidulants in inhibiting growth was acetic acid > lactic acid greater-than-or-equal-to citric acid. The same order was observed for inactivation of E. coli O157:H7 in acidified (pH 5.00) beef slurry heated at 54-degrees-C. Sorbitol MacConkey agar supplemented with 4-methylumbelliferyl-beta-D-glucuronide was unable to resuscitate some heat-stressed E. coli O157:H7 cells, indicating its limited value as a recovery medium for these organisms. Results indicate that E. coli O157:H7 can grow at pH values characteristic of beef salads.