Purpose: The effects of strain, physiological state, and acidulant on the thermal inactivation and survival of E. coli in apple juice were studied.
Methods: D56°C was determined for E. coli O157:H7 C7927 and ATCC® 43895™, and E. coli O111 at four physiological states: unadapted, acid-shocked (two methodologies used), and acid-adapted cells. The acidulant effect was evaluated by determining D56°C for the C7927 and 43895 strains subjected to acid shock during 18h in trypticase soy broth (TSB) at pH 5 adjusted with hydrochloric, lactic, and malic acids. E. coli survival was determined in juice stored at 1 ± 1°C and 24 ± 2°C. Trials were performed in triplicate.
Results: For thermal inactivation, a significant interaction was observed between strain and physiological state (P < 0.0001). Highest thermal tolerance was observed for 43895 subjected to acid shock during 18h in TSB acidified with HCl (D56°C = 3.0 ± 0.1 min), and the lowest for acid-shocked C7927 treated for 4h in TSB containing HCl (D56°C = 0.45 ± 0.06 min). Acidulants did not alter the heat tolerance of C7927 (D56°C = 1.9 ± 0.1 min) (P > 0.05), but significantly affected strain 43895 (P < 0.05); showing the greatest tolerance with malic acid (D56°C = 3.7 ± 0.3 min). A significant interaction between strain, storage temperature, and physiological state was noted during the survival experiments (P < 0.05). E. coli O111 was the most resistant strain surviving 6 and 23 days at 24 °C and 1°C, respectively.
Significance: These findings may assist in designing challenge studies for this and similar pH-controlled products, where Shiga toxin-producing E. coli represents the pathogen of concern.