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

Abstract

Factors affecting the ability of Escherichia coli O157:H7 to survive in foods with a w 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 a w (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- a w 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 a w 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 log 10 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 a w 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 a w and pH, particularly at refrigeration temperature.