Heat Resistance of Escherichia coli O157:H7 in a Nutrient Medium and in Ground Beef Patties as Influenced by Storage and Holding Temperatures

Abstract

Stationary-phase cultures of Escherichia coli O157:H7 were inoculated into tryptic soy broth, sealed in vials, and stored at −18°C for 1, 8, and 15 days, or 3 or 15°C for 3, 6, and 9 h. Thermal resistance was determined at 55°C. Each storage treatment was repeated with additional holding at 23 or 30°C for 1, 2, 3, or 4 h prior to heating to simulate potential temperature abuse during handling. Cultures under treatments enabling the growth of E. coli O157:H7 were generally more heat sensitive than those held at temperatures which restricted growth or enabled growth to stationary phase. Cultures stored frozen (−18°C) without holding at elevated temperatures had greater heat resistance than those stored under refrigeration (3°C) or at 15°C. Subsequent holding of frozen cultures at 23 or 30°C resulted in a decrease in heat resistance. To determine whether these responses would be observed under typical commercial preparation procedures, ground beef patties were inoculated with E. coli O157:H7 and stored at 3 or 15°C for 9 h or at −18°C for 8 d and then held at 21 or 30°C for 0 or 4 h. Patties were grilled to an internal temperature of 54.4°C (130°F), 62.8°C (145°F), or 68.3°C (155°F). Cultures were most resistant in frozen patties, while cultures in patties stored at 15°C were the most heat sensitive. Holding patties at 21 or 30°C prior to grilling resulted in increased sensitivity. Storage and holding temperatures similar to those encountered in food service may influence the ability of E. coli O157:H7 to survive heat treatments.