Survival of Salmonella species heated by microwave energy in a liquid menstruum containing food components

Common food constituents were examined to determine the protective influence they may exert on a mixture of Salmonella species heated by microwave energy. A model system was developed, wherein combinations of sucrose, sodium chloride, caseinate and corn oil, all at 1.0% (w/v) concentrations, were added to 0.3 mM phosphate buffer (pH 6.8), producing a total volume of 100 ml. Salmonella-inoculated solutions were heated for 47 sec in a 700 watt microwave oven. Temperatures at localized areas were monitored by fluorescent fiberoptic (fluoroptic) thermometry as the solutions heated, and by mercury thermometer after heating. The mean final temperature achieved for the various combinations of solutes was not significantly different and varied only by 4 degrees C as measured by a mercury thermometer. However, solutions containing NaCl afforded the Salmonella spp. up to 170 times the protection of the phosphate buffer control. This protective effect occurred although the mean final temperature of NaCl-containing solutions would be as high as those solutions lacking salt. Fluoroptic thermometry temperature profiles revealed that surface temperatures were higher when NaCl was present in solution, due to decreased depth of penetration of the microwaves. This resulted in decreased temperatures at greater depths. Of the solutes evaluated, only NaCl appears to confer a significant protective effect.