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. But improper management of produce “wash” water is a significant food safety risk, because dirty water can contaminate all produce dunked in it.   With frequent water changes,  multiple rinses and/or sanitizer is added to wash water, however, the risk of spreading human pathogens via cross-contamination can be minimized.  

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

Body
  • Rinse off dirt and grit. Gentle agitation dislodges dirt and grit from produce and allows it to settle to the bottom of dunk tank.
  • Minimize cross-contamination.   Reused dunk tank water can contain human pathogens that may contaminate produce. there are several practices to minimize this contamination risk
  • Lower produce temp (if needed). Relatively cool water can effectively chill produce, prevent dehydration and increase shelf life. But caution should be taken to avoid soaking warm pulpy produce (e.g. melons, cukes) in cold water.

Design Elements

Body
  • Gentle agitation can be manual (with hands or net for smaller scale), or automated (bubblers or flumes for larger scale).  Water depth should be at least 12 inches (or a long shallow “trough” or  flume o)  to allow suspended grit to settle.
  • Employees are training when to wash produce, or not. Avoiding unnecessary washing can reduce produce safety risks and save time and resources.
  • Water is changed frequently enough (and tanks/sinks rinsed of sediment) to minimize suspended solids and lower cross-contamination risk (This frequency is determined by each growers situation (e.g. crop mix and volume, soil type, tank number and volume).  A standard operating procedure (SOP) guides employees on consistent changing of dunk tank water based on volume of produce washed, appearance (turbidity) of water, and/or level of sanitizer measured. Water turbidity alone is not a measure of cross-contamination risk (i.e. don’t use appearance alone as basis for changing water).
  • If sanitizer is used in dunk tanks:
    • Only EPA-labeled products* are used as sanitizers. Sanitizers are an effective tool for reducing risk of cross-contamination in dunk tanks. (*labelled for use in produce wash-water to kill human pathogens--e.g. Tsunami 100, Germicidal bleach)
    • 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.  
  • If no sanitizer is used, multiple dunk tanks providing sequential multiple rinses are used, (versus a single rinse dunk tank).  Two or three rinses in succession helps reduce the overall risk of cross-contamination.
  • Caution is used if dunk water is used to remove field heat from pulpy produce (e.g. melons, peppers).  Warm produce submerged in cold dunk water can cause infiltration of tank water into contracting air spaces under the skin .  This risk can be minimized by reduction of water-to-pulp temperature difference (<10 F is best), or dunking time (avoid longer soaking); or risk can be avoided altogether with alternative methods of removing field heat (such as forced air cooling).

Benefits

Body
  • 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

Body
wash water sanitizer kit
This wash water sanitizer "starter" kit includes a sanitizer labeled for fresh produce rinsing, sanitizer concentration test strips, and protective eyewear and gloves. Photo by Hans Estrin.
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).  Photo by Hans Estrin.
water use
This photo series, unsurprising for experienced growers, shows why wash water sanitizer can lower food safety  risk:  After dunking only two heads of visibly clean lettuce, the dunk tank contains 3 worms, 2 slug, 3 spiders, plenty of suspended bacteria, and a significant amount of sediment on the bottom.  With dozens of heads going though one tank, the wash water will get dirtier,  "washing" additional produce with what is suspended in the water. Dilute sanitizer kills only suspended (isolated) microbes that can make people sick without impacting the produce itself. Photo by Hans Estrin.
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. Photo by Vern Grubinger.

 

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.

USDA NIFA logo