Influence of temperature and pH on survival of Escherichia coli O157:H7 in dry foods and growth in reconstituted infant rice cereal

Factors affecting the ability of Escherichia coli O157:H7 to survive in foods with aw less than required for growth have not been fully defined. This study was undertaken to determine the ability of E. coli O157:H7 to survive in a commercial dry infant rice cereal as affected by aw (0.35±0.04, 0.52±0.03 and 0.73±0.03), pH (4.0 and 6.8), and temperature (5, 25, 35 and 45°C), and in nine other reduced-aw foods. Death of E. coli O157:H7 in cereal was enhanced with increased temperature and decreased pH during a 16- to 24-week storage period. Survival was enhanced at pH 6.8 compared to pH 4.0 in cereal at aw 0.34±0.04 during initial storage at 5 and 25°C. The effect of temperature (8, 15, 21 and 30°C) on survival and growth of acid-adapted cells of E. coli O157:H7 inoculated into cereal reconstituted with milk or apple juice at two inoculum levels (8.2–12.3 cfu/ml and 82–123 cfu/ml of slurry) was also studied. Growth occurred in cereal reconstituted with milk at all test temperatures and in cereal reconstituted with apple juice at 15, 21 and 30°C. Populations increased by >1 log10 cfu/ml within 3–6 h at 21 and 30°C. Acid-adapted and unadapted cells had similar growth patterns. The effects of temperature and acid adaptation on survival of E. coli O157:H7 in nine commercial foods and food ingredients with pH 4.07–6.49 and aw 0.17–0.82 were determined. The pathogen survived in these foods for various lengths of time, depending the storage temperature, with an order of survival of 5°C>21°C>37°C. Survival appeared to be enhanced in foods with highest pH, and acid-adapted cells retained higher viability than unadapted cells in only two of the nine test foods. Of particular importance is the ability of E. coli O157:H7 to survive well in dry foods with a wide range in aw and pH, particularly at refrigeration temperature.