Microbial die-off: A complicated path to mitigate water contamination
April 7 is the first compliance date for the new provisions of the preharvest water rules announced in 2024 under the (FSMA) Food Safety Modernization Act’s Produce Safety Rule. That date affects farms that sold more than $500,000 of fresh produce in the previous three years.
”Compliance with the preharvest water rules will be within the inspectors’ scope of work, and they can ask large growers about it,” said Don Stoeckel, Produce Safety Alliance Extension educator in Cornell University’s Department of Food Science.
Compliance for growers with more than $250,000 of sales in the previous three years will come in 2026 and for all other growers in 2027. Growers with $32,000 or less in sales in the previous three years are not covered by the rule. This exemption started at $25,000 and increases every year with inflation.
Operations using groundwater for preharvest uses are exempt if the water meets testing and quality criteria. Operations using a public water system or supply are also exempt if the water meets rule requirements, as is water treated with an EPA-registered sanitizer.
Non-exempt growers must conduct an Agricultural Water Assessment (AgWA), a written assessment that subjectively identifies risk factors and determines whether corrective or mitigation measures are needed to reduce the potential for contamination.
MICROBIAL DIE-OFF AS MITIGATION
Microbial die-off is one of the allowable mitigation options.
“E. coli is from the GI (gastrointestinal) tract and likes warm, moist conditions and abundant nutrients,” said Annalisa Hultberg, University of Minnesota Extension educator in food safety.
The E. coli die when exposed to the outside elements.
“Most plant surfaces already have active microbial populations on them that can outcompete the pathogens,” Stoeckel said. “We can expect fecal pathogens to die off outside the body, but the rate is complex.”
The new water rules recognize increasing the time interval between the last direct application of agricultural water and the harvest of the covered produce can mitigate risk by allowing for more microbial die-off. But how much more time is needed?
“There is no set answer,” Stoeckel said. “It’s situational, but longer is better.” Delaying harvest for six days is better than four days, if the delay won’t impact quality or harvest, he said.
“But is six long enough?” Stoeckel said. “There are so many uncertainties. Microbial die-off is good, but when is it good enough?”
Location is also a factor.
“Is it a dry climate, or is it a moist climate?” Hultberg said.
A high amount of ultraviolet (UV) light increases die-off. However, a denser crop canopy gives the microbes more protection from UV light.
“Survival will vary from crop to crop and setting to setting,” Stoeckel said.
Temperature and humidity both affect the die-off rate, said Phil Tocco, Michigan State University Extension food safety educator. Generally, higher temperatures and lower humidities produce faster die-off rates.
“An environment that has a more diverse microbial population, like an organic farm with lots of plant diversity, will suppress the pathogens faster than an environment with less diversity,” Tocco said.
Types of crops being grown also affect rates. A lettuce leaf has a lot of surface area and many nooks and crannies where microbes can shelter, Hultberg said, while die-off will be faster on a smooth melon.
WATER TESTING PLAYS KEY ROLE
A sticking point is the new water rules say the die-off must have supporting scientific data and information, and that is not available for all covered crops. A study on the die-off rate in low, medium and high canopy watermelons, for example, doesn’t apply to peppers.
“Scientific data doesn’t cover all situations, and it’s difficult to unlock knowledge from scientific data that you can directly apply,” Stoeckel said.
Water quality is the key determiner of the time needed for an adequate die-off. Testing is not required by the new rules but is a realistic way to assess water quality, experts say. Testing five random samples throughout the season will provide the most defensible data.
“Those numbers will give you the data to justify your decision to your reviewer,” Hultberg said. “Understand the quality of your water.”
Microbial die-off can be used as a routine part of regular procedures. Growers may establish a policy to always wait two days between the last water application and harvest. The practice could also be used to mitigate an unforeseen produce safety event.
“It’s on the growers to know how much die-off will accomplish,” Hultberg said.
FOCUS ON PREVENTION
The new preharvest water rules also list other corrective or mitigation measures. These include making necessary repairs and using safer methods of applying water, such as drip irrigation instead of overhead application.
The rules also recognize increasing the time interval between harvest and the end of storage to allow for microbial die-off in storage. The same factors affecting the die-off rate in the field also apply in storage. However, humidity and temperature are known, and there isn’t any UV light.
Thorough washing before putting produce in storage and proper cleaning and sanitizing in the storage area are sound practices, but die-off becomes a consideration when something goes wrong.
“Focus on preventing a problem,” Stoeckel said, instead of using die-off as problem mitigation.
Parts of the new rules are open to interpretation and require clear communication.
“Some inspectors have their own ideas about what’s safe,” Tocco said. “Always talk to your inspectors about what they think is safe enough.”
