Water Testing for Produce Food Safety

Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu

We recently had a round of trainings on produce food safety for fresh produce growers and packers. One of the questions that has come up from these trainings is how water testing should be done and what types of tests are required.

Water is tested for certain bacterial indicator species, which show the potential for produce to become contaminated with microbes coming from animal or human feces in that water.

The following are some guidelines for growers and packers to follow.

● All water sources must be tested, including: irrigation wells, packing house wells, wells used for spray water, and surface irrigation water sources. In other words, all irrigation, spray, and wash water sources must be tested.

● Surface water is not acceptable in the packing shed; only well water should be used. Potable water is required for all employee uses (drinking, hand washing, sanitation). Well water is the preferred source for all other uses (irrigation and spray water).

● The schedule for testing should be once a year for municipal water and twice a year for well water. Surface (pond, ditch, stream or river) irrigation water sources should be tested three times a year; at the beginning of the irrigation season, at peak irrigation, and near harvest.

● For packing house water, use the following procedures to collect the water:

          ○ Run the water through the plumbing system for at least 5 minutes to flush the lines.

          ○ Obtain the samples from a spigot or faucet and ensure that water in the bottle has not contacted other surfaces that may contaminate the water such as bare hands and sleeves. Use sterile bottles supplied by the water testing laboratory you are going to use. Samples need to get to the laboratory within 24 hours.

● For irrigation water, use the following procedures to collect the water:

          ○ Run the water through the irrigation system for at least 20 min to flush the lines.

          ○ Obtain at least two samples, as microbial levels can fluctuate.

          ○ Obtain the samples from the sprinkler system (not the intake area) and ensure that water in the bottle has not contacted other surfaces that may contaminate the water such as bare hands and sleeves. Use sterile bottles supplied by the water testing laboratory you are going to use. You will need to put the samples on ice and deliver them to the laboratory within 6 hours.

The microbiological tests that are routinely done as indictors of water quality are: total coliform bacteria, fecal coliform bacteria, and E. coli. Total coliform is a measure of those bacteria that are found in the soil, in water that has been influenced by surface water, and in human or animal waste including some that may cause food borne illness. Fecal coliforms are the group of the total coliforms that are considered to be present specifically in the gut and feces of warm-blooded animals. Because the origins of fecal coliforms are more specific than the origins of the more general total coliform group of bacteria, fecal coliforms are considered a more accurate indication of animal or human waste than the total coliforms. Escherichia coli (E. coli) is the major species of concern in the fecal coliform group. Of the five general groups of bacteria that comprise the total coliforms, only E. coli does not grow and reproduce in the environment. Consequently, E. coli is considered to be the species of coliform bacteria that is the best indicator of fecal pollution and the possible presence of human pathogens.

It is important to note that these are just indicator tests and are not specific for a pathogen such as salmonella.

For well water used in the packing shed, worker drinking, washing, or worker sanitation it is best to run tests for total coliform and fecal coliform and/or E. coli at the same time when the sample is sent off. For irrigation water from surface sources, you should have water tested for total coliform and fecal coliform or E. coli. Most commonly, a combination test is done for total coliform and E. coli bacteria.

There are no clear cut levels for contamination with fecal coliforms or E. coli in irrigation water and produce safety. It depends on many factors. However, research has shown that when irrigating strawberries, irrigation water which exceeded the guideline of 100 colony-forming units (CFU) of E. coli per 100 mL of water, transferred E. coli to the berries. In research with other produce, the numbers were higher.

Based on the best currently available information, the following are some standards to apply when you receive the results of you tests:

● For overhead irrigation water source and the fecal coliform test, any samples above 200 CFU/100 ml should be viewed as contaminated and for the E. coli test, any samples above 126 CFU/100 ml should be viewed as contaminated.

● For drip irrigation water sources (non foliar contact) using the E. coli test, any samples above 576 CFU/100 ml should be viewed as contaminated.

When water samples are above these levels, efforts need to be made to find the source of contamination. Irrigation water use from these sources may need to be stopped or a water treatment system to reduce bacterial loads may need to be installed.

Some water testing laboratories that may be used are given below (other laboratories may also be available to do the tests – the list is not an endorsement of any laboratory).

630 Churchmans Road
Newark, DE 19702
Phone: 302-266-9121
Fax: 302-454-8720

51 Clark Street
Harrington, DE 19952
Phone: 302-398-4313
Fax: 302-398-4312

113 High Street
Salisbury, MD 21810
MD Phone: (410) 546-1318
DE Phone: (302) 628-8876

The Delaware Health Department does water testing for private wells. The cost for private well testing is $4.00 each for a bacterial and chemical test. Private well owners may pick up a sample form and bottle from the State Service Centers in each county or the Delaware Public Health Laboratory. Follow the directions given and return in a timely manner to the office where you received the bottle.  The Delaware Public Health Laboratory can also do water quality testing for commercial produce farms.  Contact the laboratory at (302) 223-1520 for sampling kits and procedures.   For more information go to: http://www.dhss.delaware.gov/dph/lab/watertesting.html

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