Heat inactivation of Salmonella typhimurium DT104 in beef as affected by fat content

The heat resistance of an eight-strain cocktail of Salmonella typhimurium D T104 was determined at 58-65 degrees C in beef containing 7, 12, 18 or 24% fat. Inoculated beef was packaged in bags completely immersed in a circulat ing water bath and held at 58, 60, 62 . 5 and 65 degrees C for a predetermi ned length of time. The surviving cell population was enumerated by spiral plating heat-treated samples onto tryptic soy agar supplemented with 0 . 6% yeast extract and 1% sodium pyruvate. Preliminary studies on thermal inact ivation of the Salmonellae isolates in chicken broth indicated no correlati on between heat resistance and origin of the isolates. While linear surviva l curves were observed in chicken broth, inactivation kinetics in beef show ed deviations from the first order kinetics, represented by an initial lag period or shoulder before any death occurred with time. Overall, increased fat levels in beef resulted in longer lag periods and lower D-values, sugge sting that the lag periods must be taken into account and added to the D-va lues for calculating the time required at a specific temperature for achiev ing a specific lethality for Salm. typhimurium DT104 in beef. Thermal death times from this study will assist the retail food industry to design cooki ng regimes that ensure safety of beef contaminated with Salm. typhimurium D T104.