Fate of Escherichia coli O157:H7 and Listeria monocytogenes in strawberry juice and acidified media at different pH values and temperatures

Abstract

Survival and growth of Escherichia coli O157:H7 and Listeria monocytogenes in strawberry juice and acidified media at different pH levels (pH 3.4 to 6.8) and temperatures were studied. Sterile strawberry juice (pH 3.6) and acidified trypticase soy broth (TSB) media (pH 3.4 to 6.8) were inoculated with approximately 6.7 log CFU/ml E. coli O157:H7 or 7.3 log CFU/ ml L. monocytogenes, incubated for 3 days at 4 and 37°C. Bacterial levels were determined after 2 h, 1 day, and 3 days using surface plating nonselectively on tryptic soy agar and selectively on sorbitol MacConkey agar for E. coli O157:H7 or modified Oxford agar for L. monocytogenes. A spectrophotometer (660 nm) was also used to study growth inhibition of L. monocytogenes in different TSB and strawberry juice media (pH 3.4 to 7.3). E. coli O157:H7 survived well at pH values of 3.4 to 6.8 at 4°C, but the number of injured cells increased as pH decreased and incubation time increased. At 37°C. E. coli O157:H7 was inactivated at pH of ≤3.6 but could grow at pH 4.7. L. monocytogenes was quickly injured at pH of ≤4.7 within 2 h of storage at 4°C and then was slightly and gradually inactivated as storage time increased. L. monocytogenes survived well at pH 6.8 at 4°C and grew well at 37°C. Growth of L. monocytogenes at 37°C was inhibited in TSB by 1% citric acid and 0.5% malic acids at pH 3.4 or by 50% strawberry juice at pH 4.7. Bacterial injury and inactivation appeared to be induced by the acids in strawberry juice. The acids, pH value, temperature, and time were important factors for bacterial survival, inactivation, and growth in the media tested.