FATE OF ESCHERICHIA-COLI O157-H7 AND OTHER COLIFORMS IN COMMERCIAL MAYONNAISE AND REFRIGERATED SALAD DRESSING

Commercial mayonnaise and refrigerated ranch salad dressing were inocu lated at two levels with two strains of Escherichia coli O157:H7, a no n-pathogenic E. coli, and the non-fecal coliform Enterobacter aerogene s. Results showed that at the high inoculation level (>10(6) colony fo rming units [CFU]/g) in mayonnaise stored at room temperature (ca. 22 degrees C) both strains of O157:H7 were undetected at 96 h. At the hig h inoculation level, all strains of coliform bacteria tested survived longer in salad dressing stored at 4 degrees C than in mayonnaise stor ed at 22 degrees C. The O157:H7 strains were still present at low leve ls after 17 days. The survival time in the low-level inoculum (10(4) C FU/g) study decreased, but the survival pattern in the two products wa s similar to that observed in the high-level inoculum study. Slight di fferences in survival among strains were observed. The greater antimic robial effect of mayonnaise may be attributable to differences in pH, water activity (a(w)), nutrients, storage temperature, and the presenc e of lysozyme in the whole eggs used in the production of commercial m ayonnaise. Coliform bacteria survived longer in refrigerated salad dre ssing than in mayonnaise particularly at the high-level inoculum. Both mayonnaise (pH 3.91) and salad dressing (pH 4.51) did not support the growth of any of the microorganisms even though survival was observed .