The Effect of Water Hardness on the Efficacy of Sodium Hypochlorite Inactivation of Eschericha coli O157:H7 in Water

Introduction: Fresh produce grows in natural environments and is susceptible to microbial contamination. Sodium hypochlorite is often added to wash water by the fresh produce industry to prevent cross-contamination during postharvest washing.  Many of the fresh produce processing facilities in the U.S. are located in regions with hard water.  The effect of water hardness on the efficacy of sodium hypochlorite in inactivating Escherichia coli O157:H7 remains to be determined.

Purpose: This study examines how the hardness of water affects the efficacy of sodium hypochlorite in inactivating E. coli O157:H7 in water.

Methods: Water was prepared at different degrees of total hardness (0, 50, 100, 200, 500, 1000, 2000, and 5000 mg/l CaCO₃).  Trials were performed at different levels of sodium hypochlorite (0, 0.2, 0.5, and 1 ppm).  Thirty ml of chlorinated water was inoculated with 6 log CFU/ml of E. coli O157:H7 and allowed to mix for 3, 10, 20, or 30 seconds.  After the specified exposure time, sodium thiosulfate was added to neutralize the sodium hypochlorite. The level of E. coli O157:H7 in the treated water was determined by plate counts.  

Results: In the absence of sodium hypochlorite, no reduction in E. coli O157:H7 counts were seen regardless of degree of water hardness.  After a 10 second exposure to 0.5 ppm sodium hypochlorite, E. coli O157:H7 counts were reduced by 5.3 ± 0.5 log CFU/ml in water containing 0 mg/l CaCO₃.  In contrast, when the water contained 100, 1000, 2000, or 5000 mg/l CaCO₃, E. coli O157:H7 counts were only reduced by 1.6 ± 0.4, 0.9 ± 0.6, 0.1 ± 0.0, or 0.1 ± 0.1 log CFU/ml, respectively, after 10 seconds exposure to 0.5 ppm sodium hypochlorite.

Significance: Water hardness may need to be taken into account when determining the effective level of sodium hypochlorite needed to inactivate E. coli O157:H7 in wash water.