The researchers have collected data about Listeria growth on 11 of 14 commodities selected for the project. Among them are whole or fresh-cut fruits and a few vegetables, which include avocados, blueberries, grapes, mangoes, cantaloupes, pineapples, fresh-cut carrots, and cauliflowers.
"Our purpose is not simply testing the growth of Listeria on a bunch of random products. We want to learn why Listeria tends to grow or not grow on these types of fresh produce," Nou said.
So far, the results show commodity pH plays a critical role in Listeria survival and growth, as all products with low pH show rapid declines in Listeria counts.
However, the outcomes are much more complicated. For example, while both have near-neutral pH, fresh-cut cantaloupe supported Listeria growth at all temperatures tested, but fresh-cut carrots completely prevented Listeria recovery from the cut surface.
In addition, both cantaloupe and carrot juices supported
growth extremely well. The researchers are examining fresh-cut commodities that significantly promote or inhibit pathogen growth to try to identify intrinsic properties, such as nutrient availability and anti-Listerial compound production, responsible for the variations.
"We want to see which ones we might be able to manipulate for Listeria control," he said.
On some items,
populations decreased while populations of background microorganisms increased. But Nou said it's too early to know how to interpret that, although it is something they want to research further with microbiome analyses.
"When we see the decline of Listeria along with an increase in background microflora, we tend to think that is due to competition," he said. "On some other commodities you see Listeria grows well, along with big increases in background microflora."
Input from industry partners has been invaluable, Nou said.
"We might be familiar with one, two, maybe three produce items but not all we proposed to look at," he said. "So we rely on our industry partners to provide the information on how they handle these different commodities at the production and distribution processes. That information is essential for our study design."
Industry partners have shipped fresh produce directly to the researchers, so they can simulate the entire commercial storage and distribution processes. Although laboratory trial scales are small due to the use of food-borne pathogens, the researchers nonetheless try to simulate real-world conditions as closely as possible so that the results are relevant to the industry.