Influences of heating temperature, pH, and soluble solids on the decimal reduction times of acid-adapted and non-adapted Escherichia coli O157:H7 (HCIPH 96055) in a defined liquid heating medium

The study characterized the influences of various combinations of process and product parameters namely, heating temperature (53, 55, 57.5, 60, 62 °C), pH (2.0, 3.0, 4.5, 6.0, 7.0), and soluble solids (SS) (1.4, 15, 35, 55, 69°Brix) on the thermal inactivation of non-adapted and acid-adapted E. coli O157:H7 (HCIPH 96055) in a defined liquid heating medium (LHM). Acid adaptation was conducted by propagating cells in a gradually acidifying nutrient broth medium, supplemented with 1% glucose. The D values of non-adapted cells ranged from 1.43 s (0.02 min) to 304.89 s (5.08 min). Acid-adapted cells had D values that ranged from 1.33 s (0.02 min) to 2628.57 s (43.81 min). Adaptation did not always result in more resistant cells as indicated by the Log (D(adapted)/D(non-adapted)) values calculated in all combinations tested, with values ranging from -1.10 to 1.40. The linear effects of temperature and pH, and the joint effects of pH and SS significantly influenced the thermal resistance of non-adapted cells. Only the linear and quadratic effects of both pH and SS significantly influenced the D values of acid-adapted cells. Generally, the D values of acid-adapted cells decreased at SS greater than 55 °Brix, suggesting the possible cancelation of thermal cross protection by acid habituation at such SS levels. The relatively wide ranges of LHM pH and SS values tested in the study allowed for better examination of the effects of these factors on the thermal death of the pathogen. The results established in this work may be used in the evaluation, control and improvement of safety of juice products; and of other liquid foods with physicochemical properties that fall within the ranges tested in this work.