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 foo ds with a(w) less than required for growth have not been fully defined. Thi s study was undertaken to determine the ability of E. coli O157:H7 to survi ve 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 degrees 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 com pared to pH 4.0 in cereal at a(w) 0.34+/-0.04 during initial storage at 5 a nd 25 degrees C. The effect of temperature (8, 15, 21 and 30 degrees C) on survival and growth of acid-adapted cells of E. coli O157:H7 inoculated int o 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 r econstituted with apple juice at 15, 21 and 30 degrees C. Populations incre ased by > 1 log(10) cfu/ml within 3-6 h at 21 and 30 degrees C. Acid-adapte d and unadapted cells had similar growth patterns. The effects of temperatu re and acid adaptation on survival of E. coli O157:H7 in nine commercial fo ods and food ingredients with pH 4.07-6.49 and a(w) 0.17-0.82 were determin ed. The pathogen survived in these foods for various lengths of time, depen ding the storage temperature, with an order of survival of 5 degrees C > 21 degrees C > 37 degrees C. Survival appeared to be enhanced in foods with h ighest pH, and acid-adapted cells retained higher viability than unadapted cells in only two of the nine test foods. Of particular importance is the a bility of E. coli O157:H7 to survive well in dry foods with a wide range in a(w) and pH, particularly at refrigeration temperature. (C) 1998 Elsevier Science B.V. All rights reserved.