Has this ever happened to you? You bought an enticing hunk and had yourself a lovely cheese board with it. You stash it back in the fridge, time passes, and the next time you lay eyes on that once-lovely wedge, it’s covered in mold. 

Hey, we’ve been there—and it’s part of the reason we designed the Cheese Grotto, which can extend the life of your cheeses to two to three weeks. But where exactly does that mold come from? Is cheese made of mold? Which molds are harmless, and which ones should you worry about?

Why Does Cheese Get Moldy?

To answer this question, we need to zoom out a bit to talk about what cheese is. It’s 99.99% milk—specifically, the fats and proteins in milk, with most of the liquid drained away. (Check out our Cheesemaking 101 post or our French fresh cheesemaking tutorial for more on how this works.)

To help transform that fat and protein into the flavorful, luscious wheels we know and love, the maker adds cultures—microscopic molds, bacteria, and/or yeasts. Along with naturally occurring microbes in the environment, these cultures help coagulate the milk into curd and release enzymes while the wheels are aging. Those enzymes break down fat and protein to create supple textures and lively flavors and aromas over time. 

The molds in this microbial mix are what we’re focusing on today.

Blue Velvet from Yellow Springs Farm in PA

What Kind of Mold Grows on Cheese?

There are two kinds of mold-ripened cheeses: Internally ripened, which grow mold inside the cheese, and externally ripened varieties, which grow mold on their exterior. 

Blue varieties with colorful veins running through the paste are the main group of internal mold-ripened cheeses. The maker adds Penicillium roqueforti mold to the milk during cheesemaking. Those microbes need oxygen to grow, so she also pierces each wheel with a long, thick needle during the aging process. That lets in oxygen and allows the mold to grow throughout the interior. 

Bloomy rinds like Brie, Camembert, Trillium, and Little Lucy Brie are examples of external mold-ripened cheeses. The maker inoculates the milk with Penicillium candidum—the signature white mold we know from our favorite bloomies. 

This mold (along with yeasts like Geotrichum candidum) forms that white bloomy rind early in the aging process. As it grows in, the maker pats it down to form the smooth, snowy coating we recognize. Meanwhile, those microbes are ripening the wheel from the outside in, creating lush cream lines and developing buttery, mushroomy flavors we know and love. 

These are just two of the many, many, molds that hang out in and on cheeses. When it comes to natural rinds—which the maker allows to grow mostly wild with different molds—there are many, many more yeasts, and bacteria on rinds that range in color from brown, yellow, red, orange, grey, white, and blue. These are all edible, but will have a strong flavor and aroma of the cheese cave in which the cheese has ripened.

The microbes in mold-ripened cheeses need oxygen to live, which is why they should never be stored in plastic. It’s also important to note that while we often think about this topic in terms of individual mold varieties, there’s typically a thriving, shifting community of microbes making magic on each wheel.

Both Little Lucy Brie and Mountaineer have bloomy white mold on their rinds, at varying degrees.  Photography by Grace Wilkey.

What to Do About Mold on Cheese

So now we know how mold grows on some of our favorite cheeses when it’s supposed to be there. But what about unwanted mold—the kind you find growing on that forgotten, half-eaten wedge you found in the back of the fridge?

Some of those same blue and white molds mentioned above (in addition to other harmless molds) will keep doing their thing—reproducing, breaking down fats and proteins—after you’ve brought the product home. For whole wheels, like rounds of Nettle Meadow’s Kunik or Petite Camembert from Marin French Cheese, this works in your favor: you can actually age young, uncut wheels in your fridge for a few weeks after purchase. (Incidentally, the Grotto is great for this.)

But for wedges that have already been portioned, that continued microbial activity shows itself as mold growth on the cut surface as well as on natural rinds. This is the white, gray, or blue mold we’ve all seen grow on a neglected wedge. 

The best way to deal with this cheese storage challenge is to prevent it in the first place. Plan to eat a bit of cheese every day (so hard, right?) and know the approximate shelf lives for the varieties you buy: 3-7 days for fresh styles, 1-2 weeks for bloomy rinds, 2 weeks for aged types. You can extend that shelf life by keeping cheeses unwrapped in your Grotto and putting it in the fridge. That way, they’ll last up to 3 weeks before the mold starts to develop. 

Generally, if you do encounter small amounts of mold growing on the cut surface of firm cheeses, you can trim it away and enjoy the rest of the wedge. When it comes to soft styles, though, the mold can penetrate much deeper than what you see on the surface, so you should trim at least an inch around the mold when removing it. Mold on fresh selections like chèvre, fromage blanc, and mozzarella, unfortunately, means that cheese should be composted. Keep an eye out for black mold—it rarely grows on cheese, but you'll want to avoid it if you see it—and monitor for pink bacteria, which can grow on cheese that's been suffocating in tight plastic wrapping for too long.

There you have it—the best ways to store cheese to keep it from getting moldy—and how you can (sometimes) still enjoy it if it does. 

Got questions about mold, microbes, or other cheese science-y stuff? Let us know!