There’s a lot of focus on keeping plants healthy – and keeping pests away – while they’re growing, but there are dangers post-harvest as well. One of major concern is listeria.

Tackling the topic of listeria in cold storage is Teresa Bergholtz, Ph.D., associate professor in food science at Michigan State University. While the genus Listeria includes 20 different species, only one is a human pathogen, L. monocytogenes. It can be found almost everywhere – in soil, in water, on animals – and in food processing facilities.

Bergholtz said listeria can survive for lengthy periods of time on foods and in food processing environments, even persisting for years in food processing facilities. It can even grow at refrigeration temperatures. “It has the potential to transfer from the environment to food – but many outbreaks of L. monocytogenes have been linked to contamination events during processing,” she said.

Significant costs are associated with listeria outbreaks and subsequent recalls. In the last decade, there were outbreaks and recalls in produce and value-added items in 2014 (caramel apples), 2016 (packaged salads and frozen vegetables), 2020 (enoki mushrooms), 2021 (packaged salads), 2022 (enoki mushrooms) and 2023 (peaches, nectarines, plums and leafy greens).

Bergholtz said the way to find out if listeria is present is to test the produce directly; test the soil and water; test the fields and orchards on the farm; and test surfaces and processing facilities.

The key to avoiding a major outbreak is to monitor the processing environment to prevent cross contamination from the environment to finished products. With data collected over time, processors can identify areas of concern to be addressed, identity patterns and determine if any practices need to be changed.

Bergholtz identified four zones for environmental monitoring:

  • Zone 1: Food contact surfaces (slicers, peelers, fillers, hoppers, screens, conveyor belts, air blowers, employees’ hands, knives, racks, worktables).
  • Zone 2: Non-food contact surfaces in close proximity to food and food contact surfaces (processing equipment exteriors and framework, refrigeration units).
  • Zone 3: More remote non-food contact surfaces located in or near the processing areas (forklifts, hand trucks, carts, wheels, air return covers, hoses, walls, floors, drains).
  • Zone 4: Non-food contact surfaces outside of the processing area (locker rooms, cafeterias, entry/access ways, loading bays, finished product storage areas, maintenance areas).

“Focus on samples from Zones 2 and 3 often,” she suggested. Processors can use sterile sponges to collect samples from these areas.

Looking specifically at the apple production chain, Bergholtz talked about a study the FDA conducted looking at listeria in tree fruit packinghouses over two years. The research found higher incidences of the bacteria on the floors of cold storage, on the floor directly below the spray/wash area, in the fan dry area and throughout the waxing areas. These areas would be the most important to monitor.

In a second study, Zone 3 samples were collected from four apple packinghouses over 18 months. After the initial sampling, interventions were proposed for the sites with high incidences of listeria – forklift stops, forklifts, drains and floor-wall junctions. The recommendations to reduce incidences at these spots were to maintain a forklift cleaning schedule; and for waxing areas, processors should replace and clean brushes as needed, keeping two sets of brushes so a clean set is always readily available.

Researchers also wanted to look at controlled atmosphere rooms, where long-term storage of apples takes place. They tested these rooms before and after storage, focusing on Zone 3 sites and the floors, walls and under condensers.

“We found evidence of listeria persistence over time – but the shorter the storage time, the lower the incidence,” Bergholtz reported.

So, to save your crop from listeria outbreaks, she offered these recommendations: Eliminate listeria prior to storage by monitoring the sites after cleaning/sanitation. If they’re found positive, clean/sanitize them again. And minimize the introduction of listeria via movement of materials through forklift sanitation and intelligent forklift use.

by Courtney Llewellyn