Safe, Easy, Low-Cost Produce Handling: Managing Dunk Tank Water

Rinsing (aka washing) produce is often necessary to remove soil, and by cooling produce (removing field heat) it can also increase quality and shelf life. When multiple rinses are used and/or sanitizer is added to wash water, the risk of spreading human pathogens via cross-contamination can be reduced. However, improper management of produce “wash” water is a significant food safety risk.

dunk tank filled with water and leafy greens
Clean water in a dunk tank can both cool and remove grit from fresh produce. Yet that same produce can also contaminate dunk tank water, so it is important to manage dunk tank water to minimize this risk.

Design goals

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  • Change water frequently enough to reliably remove soil and debris from produce in order to reduce potential for cross-contamination.
  • A sediment trap should allow easy removal of soil and debris from bottom of dunk tank so it does not contaminate clean water when refilling the tank.
  • Water quality is acceptable. The wash water source is tested at least annually to assure potability; i.e. there is no detectable generic e. coli, and if sanitizer is used, the concentration is monitored.
  • Water temperature is kept cool to help remove field heat from produce. However, the difference between water temperature and produce temperature should not exceed about 10 degrees Fahrenheit with some crops such as peppers and cantaloupe to avoid contraction of small air spaces underneath produce skin, which can suck dunk tank water into plant tissue.

Design elements

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  • An SOP is available to employees to guide when to wash produce, or not. Avoiding unnecessary washing can reduce produce safety risks.
  • An SOP is available to guide employees on consistent changing of wash water based on volume of produce washed, appearance (turbidity) of water, and/or level of sanitizer concentration measured. Water turbidity alone is not a measure of cross-contamination risk (i.e. don’t use appearance alone as basis for changing water).
  • A thermometer is used to monitor the temperature of water in dunk tank.
  • Only labeled products are used as sanitizers in produce wash water (link to list). Sanitizers are an effective tool for reducing risk of cross-contamination.
  • A dispenser is used to safely and accurately add the proper quantity of sanitizer to the dunk tank, and protective equipment is worn by employees.
  • Test strips are used to measure sanitizer concentration in wash water initially and over time, to monitor the decline in concentration that occurs when washing produce.  
  • Multiple dunk tanks to provide sequential multiple rinses are used, versus a single rinse dunk tank.  Two or three rinses in succession helps reduce the risk of cross-contamination.

Benefits

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  • Customer satisfaction. No one likes a gritty salad mix.
  • Product quality. Rinsing (and possibly use of sanitizer) can increase product shelf life.
  • Food safety risks are reduced. No grower wants to get their customers sick.

Photos

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wash water sanitizer kit
Wash water sanitizer kit includes a sanitizer labeled for fresh produce rinsing, sanitizer concentration test strips, and protective eyewear and gloves.
dose calibration
Using test strips to calibrate the product label dosing rate with reality.  This is important because label dose has a big range, and most growers find they need to use less then the directed on the label.  In this case, label dose was followed, and test strip suggests around 100 ppm (a bit over the recommended dose).  
water use
This photo series, unsurprising for produce growers, illustrates how wash water sanitizer lowers food safety  risk.  After dunking only two heads of visibly clean lettuce, the dunk tank contained 3 jumping worms, a slug, three spiders, plenty of suspended bacteria, and a significant amount of soil on the bottom.  with dozens of heads going though one tank, the wash water will get dirtier, and dose all "washed" produce with what is suspended in the water. This dilute sanitizer will kill suspended (isolated) bacteria that can make people sick without impacting the produce itself.
a sink with sediment in it
Frequently changing the dunk tank water also significantly lowers risk of cross contamination.  But don't forget that sediment must be removed after rinsing produce to avoid contamination of the tank when refilling.

 

Authors: Hans Estrin and Vern Grubinger, UVM Extension

This work is supported by the Food Safety Outreach Program Name, project award no. 2023-70020-40688, from the U.S. Department of Agriculture’s National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and should not be construed to represent any official USDA or U.S. Government determination or policy.

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