Purpose: The objective of this study was to evaluate the heat resistance of stationary phase and acid adapted Shiga toxin-producing Escherichia coli (STEC) in orange juice by evaluating the thermal inactivation responses of individual strains.
Methods: STEC strains of O157:H7 and O111 were evaluated. Strains were grown in TSB, supplemented with 1% glucose for acid adaption, and inoculated into single-strength pasteurized orange juice without pulp. Inoculated juice was sealed into microcapillary tubes. Tubes were immersed into water baths at 56, 58, and 60°C, removed at predetermined time intervals, and placed immediately onto ice. Thermally treated and sterilized tubes were crushed in 0.1% peptone using sterile glass rod for microbiological analysis. STEC populations were enumerated on a tryptic soy agar supplemented with 0.1% sodium pyruvate.
Results: D-values for acid adapted and non-adapted E. coli O157:H7 at 56, 58, and 60°C are 2.78 ± 0.01 and 2.72 ± 0.01 min, 1.38 ± 0.02 and 1.27 ± 0.03 min, and 0.60 ± 0.05 and 0.52 ± 0.05 min, respectively. A similar trend was seen for E. coli O111, where D-values for acid adapted and non-adapted cells at 56, 58, and 60°C are 3.13 ± 0.06 and 2.90 ± 0.01 min, 1.49 ± 0.03 and 1.37 ± 0.04 min, and 0.62 ± 0.04 and 0.54 ± 0.05 min, respectively. D-values for E. coli O111 are higher at these temperatures than those of E. coli O157:H7. Acid adapted STEC have higher D-values than non adapted cells at all temperatures. D-value differences between acid adapted and non-adapted E. coli O111 cells were greater than in E. coli O157:H7 at 56 and 58°C.
Significance: Currently, all available D-values for STEC in orange juice were obtained using strain cocktails. Evaluation of individual strains using the microcapillary tubes will allow us to understand strain to strain variability.