Purpose: The objective of this study was to evaluate the heat resistance of acid adapted and non-adapted stationary phase Salmonella spp. and Listeria monocytogenes in single strength orange juice by evaluating the thermal inactivation response of each strain/serotypes.
Methods: Three Salmonella and L. monocytogenes strains/serotypes were evaluated. Salmonella isolates were grown in TSB and L. monocytogenes strains were grown in BHI, supplemented with 1% glucose for acid adaption, and inoculated into single-strength pasteurized orange juice without pulp. Sealed microcapillary tubes with inoculated juice were immersed into water baths at 55, 58, and 60°C for Salmonella serotypes and 56, 58, and 60°C for L. monocytogenes strains, removed at predetermined time intervals, and placed immediately onto ice. Populations of Salmonella were enumerated on TSA supplemented with 0.1% sodium pyruvate; BHI agar supplemented with 0.1% sodium pyruvate was used for L. monocytogenes strains.
Results: Different strains in the same species responded to heat differently. Thermal tolerance did not increase significantly (P < 0.05) for acid adapted Salmonella spp. and L. monocytogenes strains at most temperatures tested. Salmonella serotypes are less heat resistant at all temperatures than L. monocytogenes strains. Combining individual results of Salmonella spp. and L. monocytogenes strains and three strains of Shiga toxin-producing E. coli previously tested, the formula of log D = 8.2 – 0.14T (°C) was used to calculate a general process for orange juice at 71.1°C. Using this equation, a 5-log reduction of all three pathogens in single strength orange juice requires 5.29 s at 71.1°C, with a z-value of 7.1°C.
Significance: All available D-values for pathogens in orange juice were obtained using strain cocktails. Evaluation of individual strains using the microcapillary tubes allows understanding of strain to strain variability that may impact public health.