Purpose: This study aims to determine the effect of varying pH conditions on the thermal inactivation of stationary-phase and acid-adapted Escherichia coli O157:H7 (serogroup C7927).
Methods: D- and z-values were determined for stationary-phase E. coli in an apple-carrot juice blend (80:20) adjusted to three pHs (3.3, 3.5, and 3.7) with lactic, malic, and acetic acid, and at a pH of 4.5 adjusted with NaOH. Thermal parameters were also determined for acid-adapted cells in juices acidified with malic acid. Soluble solids content effect on the thermal resistance of E. coli was studied in samples with 9.4-11.5 °Brix range adjusted to pH of 3.7. D-values were determined at 54, 56, and 58°C, and thermal treatments were conducted in triplicate.
Results: E. coli exhibited the highest thermal resistance at pH 4.5, and resistance increased significantly (P < 0.05) with acid adapted cells. In acidified juices, samples adjusted to pH 3.7 with malic acid showed the highest resistance (D54°C = 6 ± 1 min, z-value of 9.1 °C), and the lowest was observed in samples adjusted to pH 3.3 with acetic acid (D58°C = 0.03 ± 0.01 min, z-value of 10.4 °C). For juices acidified to the same endpoint pH with different acids, E. coli was found more resistant in juices acidified with malic acid, followed by lactic, and acetic acid. Increasing the °Brix from 9.4 to 11.5 did not significantly alter the thermal resistance of E. coli (P > 0.05).
Significance: Data from this study will be useful for establishing the critical limits for safe thermal processing of pH controlled juices and similar products.