Why is it safe to eat moldy cheese?

photo shows two large wedges of blue cheese on a cutting board next to a small cheese knife and surrounded by grapes and other fruits
How come the blue mold that gives blue cheese its name is safe for us to eat? (Image credit: bhofack2 via Getty Images)

You're at a dinner party and have just locked eyes with the cheese board. As the first tantalizing bite of Gorgonzola passes your lips, a question might cross your mind: Why is it okay to eat this moldy food, when a lot of mold makes us sick?

Molds are thread-like fungi that grow on plants and animal products. There are millions of species of mold: some are dangerous to humans, but most are harmless. That's the case for the mold in cheeses.

There are two main varieties of moldy cheese: the blue-mold cheeses, such as Stilton, Roquefort and Gorgonzola, and the white-mold cheeses, including brie or Camembert. 

Blue cheeses contain a species of mold called Penicillium roqueforti. During production, the mold is mixed in with the clumps of coagulated milk, called curds, that are used to make cheese. The mold then develops inside the cheese and turns blue, providing blue cheese its characteristic, sharp, strong flavor profile, Heather Hallen-Adams, a food microbiologist at the University of Nebraska-Lincoln, told Live Science. 

Related: Scientists identify the microbes that make buffalo mozzarella so yummy 

White-mold cheeses, on the other hand, contain a different species of mold called Penicillium camemberti. In this case, the cheese is made and then the mold is introduced on the outside of the final product, which is what you notice as the hard outer layer or rind of Camembert, for instance. 

"Both molds are doing what all fungi do," Hallen-Adams said. When we eat food, our guts secrete enzymes that allow us to digest the food, and fungi do something similar. 

"Fungi secrete digestive enzymes into their environment, break things down into simple fatty acids, amino acids [the building blocks of proteins] and carbohydrates and then absorb them," Hallen-Adams said. 

In the fungi in cheese, these enzymes are usually proteases, which break down proteins, and lipases, which digest fats. A wheel of Camembert, for example, has a thin, flat, large surface area, which allows these digestive enzymes to get all the way into the middle of the cheese, giving it its creamy texture, Hallen-Adams said. 

"With the blue cheeses where the fungus is throughout the cheese, that doesn't matter as much and due to differences in the fungus, it's more of a crumbly texture than a creamy texture," Hallen-Adams said. 

Humans figured out that it was safe to eat these Penicillium molds largely through trial and error, Hallen-Adams said. Legend has it that blue cheeses were discovered hundreds of years ago when a shepherd forgot about some cheese that he'd left in a cave for a few months. When he came back, he noticed that it contained mold that had been growing in the cave — what we now know to be P. roqueforti.

The discovery of the white-mold cheese Camembert was a little bit more deliberate, but still involved a sense of figuring out that, "okay, we can eat this," she said.  

What about mold on other types of cheese? 

"I always tell people, 'Roquefort is meant to be a blue cheese, cheddar generally isn't,'" Hallen-Adams said. "If you have blue mold on your cheddar, and that will happen sometimes, that's probably a Penicillium and you probably don't want to eat that as it could well be a different species," which could make you sick, she said. 

Other species of mold, such as Aspergillus flavus, can also grow on cheeses and produce toxins that are harmful to humans. Pathogenic species of bacteria — for example, Staphylococcus aureus or Escherichia coli — can also grow on cheese alongside mold. 

However, you can safely remove suspect mold without having to throw the whole lump of cheese in the trash. "Generally, you're safe cutting back maybe half a centimeter [0.2 inches] or a centimeter [0.4 inches] behind the growing front of the mold and the rest of your cheese is safe and fine," Hallen-Adams said.

It should be noted, however, that harmless bacteria and yeast also play a role in the cheese production process.

"Cheese is actually a pretty complex ecosystem. You've got the molds that you put in, some molds you don't, some yeasts [such as Debaryomyces hansenii or Geotrichum candidum], and almost any cheese has lactic acid bacteria that are the main culture organisms to make the cheese to begin with," Hallens-Adam said. 

"It's a whole microbial party there," she said. 

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Emily Cooke
Staff Writer

Emily is a health news writer based in London, United Kingdom. She holds a bachelor's degree in biology from Durham University and a master's degree in clinical and therapeutic neuroscience from Oxford University. She has worked in science communication, medical writing and as a local news reporter while undertaking journalism training. In 2018, she was named one of MHP Communications' 30 journalists to watch under 30. (emily.cooke@futurenet.com