LETHALITY OF HEAT TO ESCHERICHIA-COLI O157-H7 - D-VALUE AND Z-VALUE DETERMINATIONS IN GROUND-BEEF

D-values and z-values were determined for lean (2.0% fat) and fatty (30.5% fat) ground beef inoculated with approximately 10(7) Escherichia coli O157:H7 cells per g. Inoculated ground meat was sealed in glass thermal death time tubes which were completely immersed in a circulating water bath and held at prescribed temperatures for predetermined lengths of time. Survival was determined by enumeration on plate count agar (PCA) containing 1% sodium pyruvate and by the 2-h indole test of Lee and McClain (7). D-values for fatty ground beef exceeded those for lean ground beef at the temperatures tested. D-values for lean and fatty ground beef at 125-degrees-F were 78.2 and 115.5 min, respectively, as enumerated on PCA plus pyruvate. D-values at 135-degrees-F were 4.1 and 5.3 min for lean and fatty beef. At 145-degrees-F D-values were determined to be 0.3 and 0.5 min. D-values calculated from 2-h indole test data for lean and fatty ground beef at 125-degrees-F were 80.1 and 121.0 min, respectively. D-values at 135-degrees-F were 4.0 and 7.4 min for lean and fatty beef and at 145-degrees-F a D-value of 0.2 min was calculated for lean beef only, due to insufficient survival of E. coli O157:H7 in fatty beef at this temperature. The z-values determined for lean beef and fatty beef using PCA were 8.3 and 8.4-degrees-F respectively. The z-value for lean beef using the 2-h indole data was 7.8-degrees-F. No z-value for fatty beef using 2-h indole data could be determined.