HEAT-SHOCK AND THERMOTOLERANCE OF ESCHERICHIA-COLI O157-H7 IN A MODELBEEF GRAVY SYSTEM AND GROUND-BEEF

Duplicate beef gravy or ground beef samples inoculated with a suspensi on of a four-strain cocktail of Escherichia coli O157:H7 were subjecte d to sublethal heating at 46 degrees C for 15-30 min, and then heated to a final internal temperature of 60 degrees C. Survivor curves were fitted using a linear model that incorporated a lag period (T-L), and D-values and 'time to a 4D inactivation' (T-4D) were calculated. Heat- shocking allowed the organism to survive longer than non-heat-shocked cells; the T-4D values at 60 degrees C increased 1.56- and 1.50-fold i n beef gravy and ground beef, respectively. In ground beef stored at 4 degrees C, thermotolerance was lost after storage for 14 h. However, heat-shocked cells appeared to maintain their thermotolerance for at l east 24 h in ground beef held at 15 or 28 degrees C. A 25 min heat sho ck at 46 degrees C in beef gravy resulted in an increase in the levels of two proteins with apparent molecular masses of 60 and 69 kDa. Thes e two proteins were shown to be immunologically related to GroEL and D naK, respectively. Increased heat resistance due to heat shock must be considered while designing thermal processes to assure the microbiolo gical safety of thermally processed foods.