INJURY OF SALMONELLA SPECIES HEATED BY MICROWAVE-ENERGY

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