Assessing water fluorescence as a rapid predictor for chlorine disinfection of Escherichia coli O157:H7 and Listeria monocytogenes in fresh produce wash water

Monitoring chlorine demand (CLD) during fresh produce washing could improve the dosing of free chlorine (FC) to cope with FC losses due to the reaction with produce exudate and control foodborne pathogens in the wash water. Previous research has shown a correlation between CLD and UV absorbance measurements of the wash water at selected wavelengths. The method suffered from interferences, and the wavelength selection was based on the crop. This study utilized fluorescence spectroscopy to correlate the CLD of standardized vegetable wash water prepared from fresh cabbage, carrot and onion with fluorescence intensity. Excitation-emission matrices were compared for filtered and unfiltered wash water. The influence of the vegetable type, organic load concentration, pH and temperature on fluorescence intensity was investigated. A cocktail of Escherichia coli O157:H7 ( E. coli O157:H7) and Listeria monocytogenes ( L. monocytogenes ) was used for reconditioning and cabbage washing simulation. The results indicated that emission intensity measurements were not significantly different for filtered and unfiltered wash water, and high residual FC did not affect the emission measurements. A single protein-like peak at excitation/emission 280/354 (nm) was used to monitor fluorescence in the wash waters. Fluorescence correlated linearly with CLD, this relationship depended on vegetable type and pH. When the wash water CLD reduced to zero, fluorescence intensities converged below 3000 CPS/mA, irrespective of vegetable type and pH. During wash water reconditioning, pathogens were not detected at fluorescence intensities below 4500 CPS/mA. In the dynamic washing, determining microbial inactivation by fluorescence intensity was pH-independent; below 2000 CPS/mA, E. coli O157:H7 was not detected, and 2/92 samples tested positive for L. monocytogenes . This study demonstrated the potential of fluorescence spectroscopy to monitor CLD and to indicate the presence of adequate residual FC for pathogen control in the wash water. • Fluorescence intensity and chlorine demand linearly proportionate at 280/354 nm. • Sample filtration is not needed for fluorescence emission measurement. • High free chlorine residuals do not interfere with fluorescence emission. • E. coli O157:H7 and L. monocytogenes are below detection limit at low fluorescence. • Fluorescence intensity is pH independent in the absence of chlorine demand.