KEY TAKEAWAYS:
- Maintaining WIP (work-in-process) produce at 4 degrees Celsius (39 degrees Fahrenheit) is paramount in minimizing pathogen growth.
- Some WIP items, such as carrots, have natural antimicrobial properties against Listeria.
- Washing WIP produce with peracetic acid sometimes promotes Listeria growth, likely because it alters competing microbes.
Referenced CPS Research:
Maintaining proper cold-storage temperatures appears to minimize potential foodborne pathogen growth among six ingredients of fresh-cut salads as they’re held before blending.
That is but one of the findings of a two-year project led by Xuetong Fan, Ph.D., at the U.S. Department of Agriculture, Agricultural Research Service (USDA-ARS), that examined pathogen survival on work-in-process (WIP) produce, as fresh-cut salad ingredients are called. Not only did the research gauge microbial growth at various temperatures and durations, but it also examined the effects of standard sanitizer washes on microbial activity.
The data collected was used by Bryan Vinyard, Ph.D., at USDA-ARS, to develop mathematical models to predict growth of three pathogens as a function of storage time, temperature, and sanitizer.
Joining Fan as co-investigators in the CPS-funded project were Vinyard and Joshua Gurtler, Ph.D., at USDA-ARS.
Although researchers have studied the survival of human pathogens on various produce items in the past, Fan said there was a dearth of data on their survival on WIP fresh-cut ingredients during the holding period before blending and packaging. It is during this step that he and his team hoped to fill in some of the knowledge gaps.
The project involved minimally processed carrots, red cabbage, kale, Brussels sprouts, yellow onions, and broccoli stalks. The researchers inoculated individual produce items with five-strain cocktails comprising cold-adapted Listeria monocytogenes, E. coli O157:H7, and Salmonella species.
They then stored the samples at 4, 8, and 12 degrees Celsius (39-54 degrees Fahrenheit) for up to seven days, periodically assaying them for microbial growth. At the same time, they detailed microbiota activity, including total aerobic count, yeasts, molds, and other psychrophilic, or cold-loving, bacteria.
They found very minimal growth at 4 °C during the holding period. But even a few degrees of temperature increase promoted microbial growth, particularly for Listeria but not as much for E. coli or Salmonella.
“So the information is important for the processing plant,” Fan said. “That information may be useful for them so that they can make sure that they maintain the temperatures.”
He and his team also confirmed earlier studies that identified some WIP ingredients have natural antimicrobials or microbiota that inhibit Listeria growth. Take carrots, for example.
When they inoculated carrot samples, Listeria populations rapidly decreased by 2 to 3 logs to almost undetectable levels because of natural antimicrobial properties. But the researchers didn’t see the same reductions in E. coli or Salmonella.
“Although there are some studies that have identified some of the antimicrobial compounds, I think more studies are needed to fully understand why the carrots have anti-Listeria activity,” Fan said. “They have a very limited effect on the E. coli and Salmonella, so they're only very effective against gram-positive bacteria, such as Listeria.”
Despite their findings, he said that processors should still follow recommended hazard-prevention practices for all produce, including carrots.
As part of the project, they also inoculated WIP ingredients and washed them with a minimum of 10 ppm chlorine or 80 ppm peracetic acid (PAA) before the holding period to study the effects of sanitation. Surprisingly, Gurtler said, the PAA-washed samples sometimes had a greater pathogen growth rate, especially for Listeria, during the holding period than the chlorine-washed samples even though PAA was more effective than chlorine in reducing initial Listeria populations on some WIP ingredients. The researchers theorize that this may be due to PAA killing background microbiota, allowing Listeria to grow without much competition.
In a small-scale study, the researchers also performed a quick rinse of containers in which contaminated WIP items had been stored, to gauge potential transfer to a fresh batch. They found that a much more thorough washing with sanitizers, or possibly treatment with an alternative technology, was needed to ensure that pathogens on the containers were 100% inactivated.
Using data from the CPS project, the researchers are currently working with Luyao Ma, Ph.D., at Oregon State University, who is using machine learning to develop more accurate risk predictors. These predictors will incorporate key inputs such as holding temperature, holding time, sanitizer wash conditions, type of WIP ingredient, and other relevant factors