Thermal Inactivation of Shiga Toxin-Producing Escherichia coli in Ground Beef with Varying Fat Content

Decimal reduction time (D-value) was calculated for six non-O157 Shiga toxin-producing Escherichia coli (STEC) in a laboratory medium and ground beef. For the laboratory medium, an overnight culture of each strain of STEC was divided into 10-mL sample bags and heated in a water bath for a specific time on the basis of the temperatures. Survival curves were generated by plotting the surviving bacterial population against time, and a linear-log primary model was used to estimate the D-values from survival curves. The z-values (the temperature raised to reduce the D-value by one-tenth) were calculated by plotting the log D-values against temperature. Similarly, for ground beef, six fat contents, 5, 10, 15, 20, 25, and 30% of ground beef were formulated for this study. Inoculated meat was divided into 5-g pouches and submerged in a water bath set at specific temperatures (55, 60, 65, 68, and 71.1 degrees C). The average D-value for these strains in a laboratory medium was 17.96 min at 55 degrees C, which reduced significantly (P < 0.05) to 1.58 min at 60 degrees C, and then further reduced (P < 0.05) to 0.46 min at 65 degrees C. In ground beef, a negative correlation (P < 0.05) between fat content of ground beef and D-values was observed at 55 degrees C. However, at temperatures greater than 60 degrees C, there was no impact (P > 0.05) of fat content of ground beef on the thermal resistance of non-O157 STECs. Irrespective of the fat content of ground beef, the D-values ranged from 15.93 to 11.69, 1.15 to 1.12, and 0.14 to 0.09 min and 0.05 at 55, 60, 65, and 68 degrees C, respectively. The data generated from this study can be helpful for the meat industry to develop predictive models for thermal inactivation of non-O157 STECs in ground beef with varying fat content.