The Effect of pH and Temperature on Chlorine Inactivation of Escherichia coli O157:H7

Introduction: Wash-water chlorination is a critical control step to prevent microbial cross-contamination during post-harvest processing of fresh produce. Post-harvest washing may operate at varying temperatures and pH levels for different produce.  Therefore, inactivation kinetics of chlorine at different pH and temperatures needs to be determined to optimize washing operations. 

Purpose: Evaluate the chlorine inactivation of E. coli O157:H7 as affected by pH and temperature variation.

Methods: Phosphate buffer was prepared and adjusted to two pH levels (6.5 and 8.0).  Inactivation trials were performed at 0, 0.25, 0.5, and 1 ppm free chlorine, at ambient temperature and 4°C.  Chlorinated samples (30 ml) were inoculated with 6 log CFU/ml of E. coli O157:H7 and mixed for 3, 10, 20, or 30 seconds, then neutralized using sodium thiosulfate.  Levels of E. coli O157:H7 in the samples were determined by plate counts.

Results: Reduction in E. coli O157:H7 counts increased with increasing free chlorine levels, decreasing pH, and increasing temperature.  At 4°C and 1 ppm free chlorine, after 30 second exposure, the extent of pathogen reduction increased from 0.45 ± 0.19 at pH 8.0 to 7.36 ± 0.34 log CFU/ml at pH 6.5. At pH 6.5, the difference in log reduction between 4°C and ambient temperature was approximately 2 log after 30-second exposure to 0.25 ppm of chlorine. At pH 8.0, the greatest difference occurred at 1 ppm chlorine, where treatment for 30 seconds at 4°C showed a log reduction of 0.45 ± 0.19 log CFU/ml while treatment at the ambient temperature resulted in a log reduction of 5.70 ± 0.38 log CFU/ml. 

Significance: Both pH and temperature affect chlorine inactivation of E. coli O157:H7.  Small pH variation at near-neutral pH range significantly impacts chlorine inactivation kinetics and should be tightly controlled for optimal wash water management.