Wait 24 Hours After Rain to Harvest

Small changes in farm and field management practices can have a large impact on food safety, according to research conducted in New York State spinach fields. Waiting 24 hours to harvest crops after rain and not irrigating within 24 hours of harvest are two changes that can reduce the risk of Listeria monocytogenes contamination.

The research, posted online in June in Applied and Environmental Microbiology, looked at the effect of the timing of harvest, rain, and irrigation on two spinach fields that had high and low predicted risk of L. monocytogenes isolation. The predicted risk of a field was based on its proximity to water and roads. The fields were sampled 24, 48, 72, and 144 to 192 hours after irrigation and rain events.

Previous research has found that foodborne outbreaks linked to fresh produce cause, on average, more illnesses per outbreak than any other food. Most listeriosis outbreaks traced to fresh produce are usually attributed to processing. However, low-level sporadic contamination of produce in production can result in the proliferation of pathogens and widespread contamination throughout the supply chain, the researchers say.

Researcher Laura K. Strawn, PhD, assistant professor and extension specialist at Virginia Tech—Eastern Shore AREC, says that she and her colleagues found that the prevalence of Listeria spp. was higher in soil samples collected 24 hours after irrigation and rain events compared to soil samples collected 48 hours or more after the rain. The prevalence of Listeria spp. was higher in soil samples collected after irrigation events compared with the level after rain events.

“This research suggests that growers should treat field-applied irrigation water [e.g., by chlorine tabs] within 24 hours of harvest or employ an application interval of more than 24 hours before harvest to reduce the likelihood of produce contamination,” Dr. Strawn says.

Other potential strategies could include constructing buffer zones or conserving wetlands around fields near water or roads; altering cropping schemes by, for example, planting high-risk crops in low-risk fields; and monitoring pathogen levels in irrigation water, the researchers say. More research will be needed to determine the impact of buffer zones and wetlands on the risk of contamination and to understand how buffer zones and wetlands can be more effectively used to reduce contamination, they report.

“We believe that this research can be applied to other types of produce similar to spinach or other produce grown close to the ground,” Dr. Strawn says, adding that the researchers hope to perform similar research looking at other foodborne pathogens and expand their research outside New York State.