Food scientists debunk a dangerous myth about moldy food

Mold has all the hallmarks of a cute microorganism: it’s small, colorful, and fuzzy. But when you open your bag of sandwich bread to find that small splotches of the stuff have preemptively colonized your lunch, it’s anything but endearing.

These microscopic fungi are not only figuratively stomach-turning — they can also be literally dangerous if consumed. But where do you draw the line? Is it really that bad to cut off the mold and eat the rest?

When it comes to passed-down wisdom on whether or not it’s safe to go full-steam ahead with your sandwich construction or fumigate your entire kitchen, there’s much confusion around how to safely eat — or discard — moldy food.

Inverse spoke with three food scientists and microbiologists — Betty Feng, Arun Bhunia, and Jeffrey Farber — to get to the bottom of this moldy mystery.

What is mold?

Mold comes in many forms, ranging from the dangerous black mold that might haunt the damp corners of your house to small fuzzy patches of periwinkle blue that bloom on slices of bread. But at its core mold is part of the same biological kingdom as mushrooms, says Betty Feng, an associate professor of food science at Purdue University.

“Molds are microscopic fungi,” Feng tells Inverse. “They are small but they can be found in virtually every environment. It can be detected both indoors and outdoors, year-round.”

Physically, you can think of the mold as having similar characteristics to a dandelion, Feng says. This means nearly invisible roots, a visible stalk that rises above the surface, and spores floating through the air; spreading their DNA.

“If bread has mold growing on it I’d say discard it — the whole piece.”

Mold spores are too small to see with the naked eye. But nonetheless, these spores are always floating through the air, Feng says.

What causes mold to grow on food?

Like most organisms, mold needs oxygen and nutrients to grow. That’s partially why they, like us, are attracted to food.

“[Mold] growth is usually encouraged by warm and humid conditions,” says Feng. These mold spores “float through the air and they find suitable conditions — which food is very suitable with a lot of nutrients — where they can start and start their growth circle.”

Jeffrey Farber is an adjunct professor of food science at the University of Guelph and has a Ph.D. in food microbiology. He tells Inverse emerging evidence suggests components of food like salt, sugar, and protein can be attractive building blocks for young mold spores looking to settle down. However, more research is needed to confirm exactly what the molds’ relationship is to these tasty chemicals, Faber says.

“You can think of [mold] mycotoxins in comparison to poisonous snake venom.”

Feng also says that moisture and humidity are prime conditions for mold growth, making our home kitchens a potential breeding ground.

So, can you cut the mold off food and eat it?

Mold spots on food can sometimes be so small that it may seem easier to just cut them off and carry on eating the food than to discard an otherwise perfectly good treat.

But Feng and Farber say this logic doesn’t hold across the board.

When it comes to soft or wet foods — like sandwich bread, fruit, or cream cheese — Feng says that simply cutting off the moldy spot isn’t enough to prevent accidentally ingesting the mold. This is due in part to the fact that the mold’s invisible body may be burrowing its way through these types of foods.

“If bread has mold growing on it I’d say discard it — the whole piece,” advises Feng. She even says that throwing out the entire bag of bread (which may have become a mold spore breeding ground) isn’t necessarily a bad choice either.

However, you may be able to get away with this amputation-like approach with comparatively harder foods says Farber, like hard cheese or carrots. For these types of products, the USDA recommends, “Cut[ing] off at least 1 inch around and below the mold spot.”

Not quite sure if your food is hard or soft? The USDA guide on mold has a chart to break it down.

What happens if you eat moldy food?

But say you didn’t notice your apple or bread was moldy before taking a big bite — what’s the worst that can happen?

The main thing to be concerned about when it comes to ingesting mold is the potential toxins that may be hidden inside, explains Arun Bhunia, a professor of food microbiology at Purdue University

“The mold itself may not be problematic,” Bhunia tells Inverse. “But the toxin it makes, that’s what’s problematic.

These mold-grown toxins are called mycotoxins and encompass a family of toxins called aflatoxins that have been linked to liver cancer. Feng says that you can think of mycotoxins in comparison to poisonous snake venom, except its effects tend to have a longer timeline than most deadly venoms.

The negative effects of mycotoxins don’t occur through a single ingestion. Instead, those ill effects build up in the body over time, Bhunia says. So even if you feel fine after eating a moldy piece of food, the mycotoxins may still be building up in your body.

“When in doubt, throw it out.”

And while not all molds have mycotoxins, Farber says that’s it’s impossible to tell with the naked eye which do and which don’t.

“You couldn't tell unless you actually isolated that mold on a plate and then did a number of techniques to determine what that mycotoxin may be,” explains Farber.

That said, not every mold is toxic or bad for you to eat — take the mold growing on blue cheese for example. But Feng warns that even if you don’t get sick from a mold, you may still have an allergic reaction to the spores — especially if you try to give it a good whiff.

How can you protect food against mold?

Being surrounded by invisible mold spores at all times might sound daunting, but Feng, Bhunia, and Farber all agree that there are some simple and useful ways to mitigate this risk at home. These include:

• Wrapping up perishable food (even in the fridge) to deprive mold of oxygen
• Keeping perishable foods in cold environments
• Trying to keep a clean kitchen environment as much as possible

Feng and Farber also say carefully inspecting your produce or bakery goods at the grocery store can go a long way toward not introducing new mold sources into your kitchen.

And when mold does inevitably strike, Farber says to remember the old adage: “When in doubt, throw it out.”

CHECK, PLEASE is an Inverse series that uses biology, chemistry, and physics to debunk the biggest food myths and assumptions.

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