Food safety in the cooler and cold storage - keeping it cool and safe

UNIVERSITY OF MINNESOTA EXTENSION - www.extension.umn.edu

During the summer months our on-farm coolers are full of fresh produce. These cold storage areas are critical to keep our produce fresh and to improve shelf life, but they can get dirty and are often continually wet and humid. Where there is water and soil, biofilms can form and trap pathogens or spoilage microorganisms that can cause your product to rot. Read more for tips about maintaining coolers during the summer months.

What is the risks of condensate dripping in a cooler?

When condensate drips from our cooling units, this condensate could contain heavy metals and/or microbial contamination. In cool environments we are most concerned about Listeria monocytogenes (LM), a bacteria that can live and reproduce at temperatures down to 31 degrees F, which is much colder than most other bacteria can tolerate.  If a cooler fan unit is not regularly cleaned, the drip plate and other surfaces that are continually wet will form biofilms, those slimy coatings. The walls, floors, drain and pallets can also have biofilms build up unless they are regularly cleaned.

Cold-loving LM can enter a cooler from dust, dirt etc brought in from dirty produce, boots, tools or equipment, then form biofilms, where it then will persist. From a cooling fan, each drip can spread the bacteria that may be thriving inside to the produce. LM is a major concern in many large processing packing houses, as it has been tied to many recalls in the past decade related to fresh vegetables (celery, melons, apples, salad mix, mushrooms, lettuce) and can be a serious health risk to consumers if ingested.

Listeria monocytogenes under a scanning electron microscope. Listeria is the third leading cause of death from foodborne illness in the United States. Listeriosis can also cause intestinal illness, fever and other symptoms. Those most at risk include older adults, individuals with weakened immune systems, and pregnant women.

What can I do to reduce the risk of Listeria getting on my fresh produce?

You cannot adequately wash the bacteria off the produce once it is contaminated, as the bacteria might be protected in the folds or crevices of the produce. The goal is to prevent drips from the condenser fan or other walls from getting on the produce in the first place. Fixing the issue so that the drip is not dripping on the produce via a diverter or other means is the best way to control the potential issue.

This cooler was dripping, so the owners installed a pipe to divert the drip outside of the cooler so it does not pool on the floor or drip on the produce. 

Regular cleaning and sanitizing is also recommended for the cooler. Opening up the condenser fan and clean the surfaces as best you can to scrub the drip pan and other surfaces that you can access to remove and break up the biofilms. Maintaining and regularly cleaning coolers and condenser fans and components regularly can keep your produce clean, safe and long-lasting as it prevents the spread of not only pathogens like LM that can cause human illness, but it can prevent other microorganisms that cause decay from taking hold in your storage unit.

Here are some great supporting materials you can read up on/watch about cleaning and maintaining cold storage: 

Document: “Farm-Cooler Checklist” by University of Vermont Extension 

Video: “Cleaning & Sanitizing Cold Storage for Produce Safety” by University of Maryland Plant Science Food Safety Group

Webinar and blog: Managing Humidity and Condensation in Coolers to prevent Listeria monocytogenes contamination: UVM: (check out the webinar at the bottom of the page for an excellent overview of why LM is a risk in coolers, and what you can do to prevent it through design, maintenance and cleaning)