Depletion of Free Chlorine and Generation of Trichloromethane in the Presence of pH Control Agents in Chlorinated Water at pH 6.5

Chlorine is commonly used by the fresh produce industry to sanitize water and minimize pathogen cross-contamination during handling. The pH of chlorinated water is often reduced to values of pH 6-7, most commonly with citric acid to stabilize the active antimicrobial, hypochlorous acid (a form of free chlorine). Previous studies have demonstrated that citric acid reacts with chlorine to form trichloromethane, a major chlorine by-product in water and a potential human carcinogen. However, it is unclear if other pH control agents could be used in the place of citric acid to minimize the formation of trichloromethane. The objective of the present study was to determine the reactivity of organic and inorganic pH control agents, with chlorine, to generate trichloromethane. Free chlorine (∼100 mg/L) was mixed with 10 mM of each of twelve organic acids and two inorganic pH control agents (i.e., sodium acid sulfate and phosphoric acid) to effect a pH level of 6.5. Free chlorine and trichloromethane levels were measured over 3 h at 3 and 22°C. Results demonstrated that ascorbic acid, dehydroascorbic acid, citric acid, and malic acid rapidly depleted free chlorine concentrations at both 22°C and 3°C, while tartaric acid and lactic acid decreased chlorine concentrations more slowly. Other pH control agents did not significantly reduce free chlorine either at 22 or 3°C. Citric acid led to the generation of significantly higher concentrations of trichloromethane than did other acids. Chloroacetone was also found in chlorinated water in the presence of citric acid and ascorbic acid. Taking buffering capacity and pKa values into account, phosphoric acid and some organic acids may be used to replace citric acid as pH control agents in chlorinated water for washing fresh produce, to stabilize free chlorine level and reduce the generation of trichloromethane.

Pre-oxidation of spent lettuce wash water by continuous Advanced Oxidation Process to reduce chlorine demand and cross-contamination of pathogens during post-harvest washing

A continuous Photo-Fenton Advanced-Oxidation-Process (AOP) for reducing the chlorine-demand of spent lettuce wash water was developed based on the generation of hydroxyl-radicals from the UV-C degradation of hydrogen peroxide in the presence of ferric-catalyst. It was found that an interaction between UV-C and hydrogen peroxide or ferric-catalyst concentration was associated with high hydroxyl-radical generation as determined from the oxidation of methylene blue. The optimal AOP treatment was identified as 320 mJ/cm2 UV-C dose, 9.6 mg/L H2O2, and 9 mg/L ferric-catalyst. When the treatment was applied to simulated lettuce spent wash water (6.6 g romaine lettuce per liter of distilled water containing 100 mg bentonite; pH 6.9) the chlorine demand was reduced from 150 ppm to 130 ppm. The chlorination of AOP treated water did not result in a greater log reduction of pathogens (Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella) on lettuce but did reduce cross-contamination between batches during washing. The chlorinated byproducts formed in AOP treated water exhibited higher antimicrobial activity compared to untreated controls. Although the treatment was successful in reducing cross-contamination of lettuce batches the cytotoxicity of disinfection byproducts requires to be assessed.

Application of an innovative water-assisted ultraviolet C light technology for the inactivation of microorganisms in tomato processing industries

We aimed to study the efficacy of a water-assisted UVC light device (WUVC) as an innovative clean technology for the disinfection of fresh sound tomatoes and processing wash water and water turbidity was evaluated as a critical parameter. First, wash waters with different turbidities (from 0.4 to 828 NTU) were inoculated with Listeria innocua and treated in the WUVC device at different dosages. Secondly, fresh tomatoes, inoculated with L. innocua and non-inoculated ones, were treated using the WUVC device containing wash water of different turbidities for different times. The reduction of L. innocua populations on wash water and on the surface of tomato was influenced by turbidity; lower reduction values were observed at higher turbidities. Washing tomatoes with tap water with UVC lamps off (control treatment, TW) decreased L. innocua population on the surface of tomatoes but did not eliminate those bacteria that went into the water. Contrarily, when UVC lights were on, L. innocua population in wash water after treatment significantly decreased, those in clean water being the lowest populations. Reductions of native microbiota on the clean water treated with the highest UV-C radiation dose were lower than those obtained when tomatoes were artificially inoculated. We demonstrated that high reductions of L. innocua population on fresh tomatoes could be achieved using the WUVC system but some drawbacks related to the increase of turbidity should be solved for its implementation in real conditions.