A previous version of this story mistakenly referred to the American Frozen Food Institute as the American Frozen Fruit Institute.

When North Carolina State University microbiologist Benjamin Chapman makes a smoothie, he, like many of us, uses frozen berries. But first, Chapman microwaves his frozen berries to boiling and then refreezes them again before tossing them into the blender.

Why the extra steps? Chapman wants to be sure that there are no pathogens in the frozen berries that could cause a foodborne illness for him or his family.

“What I am doing may be overkill, but it makes me feel good,” says Chapman, who has been making smoothies this way for the past eight to 10 years, since his children were infants and toddlers. “I don’t have any thoughts that the berries are super high-risk to making us sick. But to me, it is something that is quick and easy to do. It’s my own risk management.”

Nor is he alone in taking extra precautions with frozen berries. In May, the Food and Drug Administration announced that it began sampling frozen berries last fall to look for hepatitis A and norovirus, two of the most common foodborne illnesses. The federal agency said that it plans to test 2,000 samples over the next 18 months from both domestic and imported sources, including food processors, distribution centers, warehouses and retailers, such as grocery stores.

As a result of this effort, there have already been two recent recalls of frozen berries announced by the FDA. They involved frozen blackberries and blended berries that were found to contain parts of the hepatitis A virus. These products were made by Townsend Farms and sold by Kroger and Costco. No outbreaks of hepatitis A have been linked to the frozen berries.

Nor does the recall mean that any of the berries contained whole, live hepatitis A virus.

“These are tests for the DNA of these organisms,” said microbiologist Donald Schaffner, distinguished professor and extension specialist at Rutgers University. “It is not an indication that these berries contain the living or intact virus that can make people sick. That said, a recall is the right thing to do.”

Schaffner predicts that there will be more recalls of frozen berries but doesn’t expect that there will be a lot of foodborne illness outbreaks linked to berries “based on what we have seen historically in the United States.”

He would hate for consumers to think that frozen berries are not safe.

“We want people to eat more fruit and vegetables,” said Schaffner, who co-hosts the Food Safety Talk podcast with Chapman and is an editor of the journal of Applied and Environmental Microbiology. “We want people to eat berries. I am not going to change my berry-eating consumption because of this, because we know that these are safe, healthy foods.”

“Frozen berries have a long history of safe use in the United States,” said Alison Bodor, president and CEO of the American Frozen Food Institute (AFFI), a national trade association for the frozen food and beverage industry. “AFFI supports the U.S. Food and Drug Administration’s sampling assignment to increase the understanding of the risk of hepatitis A and norovirus in frozen berries.”

Bodor adds that “most frozen foods are intended to be cooked, and the safest way to consume any product is to carefully follow the package cooking or preparation instructions.” However, frozen fruits do not require cooking and “are intended to be thawed and consumed right out of the bag.”

When it comes to foodborne illnesses, most people think of the culprits being undercooked hamburgers or fresh produce, such as spinach, romaine and cantaloupe. All have been fingered as the sources of recent foodborne illness outbreaks, including salmonella and E. coli 0157:H7.

Lesser known and lower in number — but still important — are a handful of foodborne illness outbreaks that have also been tied to frozen berries. According to the FDA, frozen berries caused three hepatitis A outbreaks and one norovirus outbreak in the United States from 1997 to 2016. Nearly 550 people were sickened and 53 had to be hospitalized. There were no reported deaths.

What worries microbiologists is that in Europe, frozen berries have caused a much greater foodborne illness problem. Between 2013 and 2014, mixed frozen berries caused a hepatitis A outbreak that sickened more than 1,500 people.

“As a scientific community, I don’t think that we understand the connection between viruses and frozen berries,” Schaffner said. “Why is there that hotspot connection? The FDA has now embarked on this sampling trial to try to give us more information to try to figure out what the risk actually is.”

It might seem that freezing should kill these sometimes deadly diseases. But cold temperatures are actually how microbiologists preserve hepatitis, norovirus and other foodborne microorganisms to study in the lab. “Freezing doesn’t kill them, and they die very, very slowly in the freezer,” Schaffner said. He added: “So basically, there are lots of things that can make you sick in the freezer.”

This is why Chapman always cooks frozen vegetables for his smoothies or any other dish, even if the recipe calls for just thawing in the refrigerator before adding them uncooked to dishes.

“Easily accessible recipes and food suggestions online say that if you want to make a smoothie even more healthy, take these frozen vegetables and include them without cooking them,” said Chapman, who writes the Barf Blog about food safety and serves as a scientific adviser to AFFI. “That is a much riskier situation than with frozen berries.”

Chapman recalls how his children’s pediatrician recommended that uncooked frozen peas could be used to help soothe them while teething. “As a food safety professional, I don’t expect that frozen peas are ready-to-eat,” he said. “Having a health professional tell me and presumably many other people that this is a ready-to-eat product is really concerning.”

It is so much riskier because the food processing industry doesn’t treat frozen vegetables as ready-to-eat foods. As Chapman said, “They are expecting us, the consumers, to cook them.”

Said the AFFI’s Bodor: “The bottom line is frozen foods vary and are specifically labeled in accordance with food safety standards and regulations.”

So what can you do to reduce your risk of contracting a foodborne illnesses from frozen food? Here are five tips that food safety experts not only recommend but also say they follow in their own kitchens.

Five tips for frozen food

1. Use a probe digital thermometer to check the internal temperature of all foods to ensure they reach the recommended internal temperatures.

2. Follow the cooking instructions on all frozen food and always cook frozen vegetables, which have been linked to some foodborne illness recalls.

3. Keep your freezer packed and don’t open the door except when needed to help ensure a constant temperature and less air flow, which can warm foods.

4. To take extra precautions, as N.C. State microbiologist Benjamin Chapman does: Microwave frozen fruit, stir and check that the temperature eclipses 200 degrees before using in recipes or refreezing, especially when using these ingredients in foods served to the very young, elderly, pregnant women or anyone with a compromised immune system.

5. Download the free USDA Foodkeeper app available for Apple and Android devices.

Sally Squires, is a former Washington Post reporter, who now writes the Lean Plate Club blog.

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