Injury of Salmonella Species Heated by Microwave Energy.

A pool of five Salmonella strains were heated by microwaves in milk and beef broth, representing low and high sodium concentrations, respectively, and the amount of injury and death incurred after heating was compared. Temperatures were monitored during heating using fluoroptic thermometry, Milk was heated to mean final temperatures ranging from 66 to 74°C, while beef broth was heated to temperatures ranging from 64 to 72°C. The food systems were either immediately stirred to allow equilibration of temperature and sampled, or left unstirred and allowed a standing time of 5 or 10 min. Heated cells were allowed to recover within the heating medium over a 72 h period. Samples were surface plated on both trypticase soy agar (TSA) and MacConkey agar immediately after heating and for 24 h post-heating. When left unstirred during post-heating holding times of 5 or 10 min, viable cells were recovered from food systems heated to any temperature examined. When milk was heated to 68°C and beef broth was heated to 70°C and samples were immediately stirred and plated, no viable cells were recovered over the 72 h recovery period. Eliminating large temperature gradients that develop within microwave-heated foods proved to be an important factor from a microbial-safety standpoint.