Chlorine demand in fresh-cut vegetable washing water

For the fresh-cut produce industry, a critical area of concern is potential pathogen cross-contamination during wash operations when wash water is reused and re-circulated in wash systems continuously imputed with fresh-cut produce. During washing of fresh-cut produce, large amounts of organic compounds from soil debris and from exudates of produce are released to wash water.

As these compounds react with chlorine, there is a rapid decrease in the level of free available chlorine as well as the potential generation of disinfection byproducts. Thus, organic input from residual soil and vegetable material 1) deteriorates water quality in terms of depletion of chlorine and increase of turbidity and chemical oxygen demand and 2) creates increasing chlorine demand within wash water.

Maintaining proper residual chlorine levels in the washing tank is challenging in the commercial produce wash processes, and the resulting low chlorine level often increases the risk of cross-contamination. Currently, minimal information is available regarding quantitative assessment of the organic fraction of wash water responsible for chlorine demand.

Scientists evaluated the organic input per unit of processed produce during a washing process by using chemical properties (COD: chemical oxygen demand or TOC: total organic carbon) of wash water from four different vegetables (romaine lettuce, iceberg lettuce, carrots, and baby spinach). The data generated is useful in terms of estimating the quantity of chlorine demand input per mass of vegetable following different produce processing techniques. Moreover, scientists assessed the chlorination kinetics of these vegetable wash waters to characterize the behavior of chlorine demand in terms of reaction time and chloramine formation.

Using a model system, vegetable juice released per kg of processed produce for shredded romaine lettuce, shredded iceberg lettuce, shredded carrot and baby spinach was 82.1 mL/kg, 94.5 mL/kg, 158 mL/kg, and 2.26 mL/kg, respectively. Batch water analysis revealed a rapid reaction between constituents in the wash water and chlorine where over a 90 minute observation period, 50% of chlorine demand occurred within the first 5 minutes, underscoring the challenge for any water treatment process to reduce chlorine demand once vegetables are deposited into washing systems.

Moreover, the results also showed sustained chlorine demand over 90-minute periods, indicating an accumulative effect on chlorine consumption with continuous organic input. TOC and COD showed good correlations to chlorine demand regardless of the type of vegetables, resulting important parameters to estimate the amount of chlorine required during the washing process. Additionally, HPLC-SEC analysis showed that the constituents contributing to chlorine demand are predominantly dissolved small molecules (<3400 Da), which will challenge water reuse treatment approaches.

Scientists conclude that the study provides information for engineering a design of water treatment for water reuse and which has implications for future development of treatment strategies for the fresh-cut produce industry.

The full study is available online since August 2015 at:

Source: ShihChi Weng, Yaguang Luo, Jie Li, Bin Zhou, Joseph G. Jacangelo, Kellogg J. Schwab, 'Assessment and speciation of chlorine demand in fresh-cut produce wash water', February 2016, Food Control, Vol. 60, pages 543-551.

Kellogg J. Schwab
Department of Environmental Health Sciences, Johns Hopkins University Bloomberg School of Public Health
615 N. Wolfe St., Room
E6620, Baltimore, MD, 21205 USA.

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