Food-borne illnesses make headlines. Although E. coli and Salmonella are the most common causes of illness related to contaminated food, Listeria has started to appear more frequently in the news.
Martin Bucknavage, senior food safety extension associate at Penn State Department of Food Science, says that a lot of Listeria research has been focused in dairy, meat and poultry. “In produce, because of some of the issues we’ve had, we want to understand what the organism is,” he said. “Listeria monocytogenes is a facultative organism, which means it can grow with or without oxygen. If you take a product and vacuum package it, it can still grow. If it’s out in the air, it can grow.”
Bucknavage says Listeria usually doesn’t come directly from the field. “In a lot of the cases, issues start at the processing step, whether it’s in the packing house or during blanching,” said Bucknavage. “There are some factors that influence how much contamination is picked up in the field, but the real issue is moisture, different soils, food juices ‑ that’s where Listeria becomes an issue.”
The contamination of caramel apples with Listeria in 2015 quickly became headline news. Bucknavage explained that the packinghouse was highly contaminated, and that bacteria was transferred to the apples, then when those apples went through processing for caramel apples, bacteria numbers increased. “The same thing happened with the cantaloupes,” he said. “In going through the processing, contamination was building up in the equipment and was transferred to the cantaloupes and increased in number.”
Listeria is a concern because it’s a food-borne pathogen that can cause serious illness and death. “It gets onto the food, and it’s either in high numbers or grows to a high number,” said Bucknavage. “High enough to cause illness in someone.”
Those who are at highest risk are in previously designated high-risk groups including elderly, infants, and people with compromised immune systems. “When the organism is consumed, it affects the white blood cells,” said Bucknavage. “Initially, someone might have a stiff neck, a headache and confusion. Then it gets worse—meningitis and encephalitis. There is a high mortality rate associated with this organism.” The other group that Listeria is especially dangerous to is pregnant women because of the immunosuppression that occurs during pregnancy.
As is the case with most bacterial illnesses, the higher the contamination rate, the higher the risk. “Consumption of less than 1,000 organisms can cause disease, especially in those who are immune-compromised. At higher levels, Listeria can infect healthy people. We want to keep it out of the food system and prevent growth.”
Foods that are highest risk of harboring Listeria are those that allow the organism to grow, including unpasteurized milk, unpasteurized milk cheeses, ready-to-eat meats (hot dogs, lunchmeat), and seafood.
“One of the keys is that it can grow at refrigeration temperatures,” said Bucknavage. “However, the optimal condition for growth is room temperature. As the temperature drops, growth slows. Some people don’t keep refrigerators cold enough, especially older people who might keep the temperature higher. So that means an already high risk group is at even higher risk.”
Does freezing kill Listeria? Unfortunately, no. There might be a slight reduction in the level of organisms, but the organism survives freezing. In early May, CRF Frozen Foods recalled numerous products, both organic and conventionally produced, due to possible Listeria contamination. Although cooking the product would kill the bacteria, some of the items such as blueberries and kale, would possibly be used raw in smoothies or other dishes that are not cooked.
If the Listeria organism survives the blanching process due to inadequate temperature or too high volume of product through the process, there’s potential for problems. “The organism makes it through the blancher and can contaminate the post-process side of the operation,” said Bucknavage. “It isn’t the organism making it through the blancher that becomes the bigger issue, it’s the fact that it’s contaminated the post-process side of the operation. Once it’s on the other side of the heating cycle, and before the freezer and packaging equipment, it grows at high numbers and re-contaminates the product.”
Lee Showalter, grower services and food safety manager at Rice Fruit Company in Biglerville, PA, talks about Listeria concerns in the packinghouse. “We became involved in the conversation about Listeria a few years ago,” he said, adding that the company receives 2.4 million bushels of apples for packing annually. “Through the efforts of the U.S Apple Association, we organized a meeting that related to fresh fruit packers.”
Showalter says the FDA had no current plans to go into the marketplace and target apples for testing. “That was good news for us,” he said. “They’re providing guidance for industry to go along with FSMA and the produce act that will be more commodity-specific. They indicated that if industry could come up with good practices for our industry, they would look at them. We viewed that as an opportunity. There are a lot of gray areas, and it would be an opportunity to make some things clear as far as best practices in a packing house.”
Current guidance for apple packers is ‘take what you have in your facility and make it as clean as possible’.
“That’s a wide-sweeping statement,” said Showalter. “They recognize that we don’t have a sterile environment for packing fruit, and our equipment is not designed under sanitary principles. They understand that Listeria is widespread in the environment.” Showalter added that he was quoted the figure of potentially two percent of apples that come in from the field might be contaminated with Listeria, but no reference was provided for that figure.
Showalter says that the FDA intends to work with state agencies to enforce the produce rule and preventative controls rule, but they lack resources to do this.
“They’ll concentrate on higher risk produce items first,” he said. “They indicated that whole, fresh tree fruit is lower risk. We tried to make the distinction that apples are a raw agricultural commodity as opposed to ‘ready to eat’. This is important because the ‘ready to eat category’ becomes involved with produce testing and holding programs.”
As with any other agriculture practice program, a good traceability program will help identify the scope of fruit involved in a recall and limit the amount of fruit involved in a recall.