Investigation on chlorine-based sanitization under stabilized conditions in the presence of organic load

Chlorine, the most commonly used sanitizer for fresh produce washing, has constantly shown inferior sanitizing efficacy in the presence of organic load. Conventionally this is attributed indirectly to the rapid chlorine depletion by organics leading to fluctuating free chlorine (FC) contents. However, little is known on whether organic load affects the sanitization process directly at well-maintained FC levels. Hereby, a sustained chlorine decay approach was employed to study the inactivation of Escherichia coli O157:H7 under stabilized washing conditions. Chlorine solution was first incubated with organic load for up to 4 h, modeling the chlorination in produce washing lines. The FC level was then stabilized at five targeted values for sanitization study. Our study showed decreased sanitizing efficacy as the organic load increased. At 5 s residence time and pH 6.5, a minimum of 0.5 and 7.5 mg/L FC were needed to achieve a 5 log reduction at 0 and 900 mg/L chemical oxygen demand (COD), respectively. The decrease was more pronounced at lower FC, higher COD, higher pH, and shorter residence time values. The organics-associated interference with FC measurement and disruption of chlorine/bacteria interaction, together with the chlorine demand of concentrated inoculum per se, collectively resulted in inadequate sanitization. Finally, our results were compared with existing studies conducted under dynamic conditions in the context of different experimental settings. This study provided a feasible method for studying the bacteria/sanitizer interaction while ruling out the confounding effect from fluctuating FC levels, and it indicated the direct, negative impact of organic load.