Mold on Food, Explained

Mold is a key “ingredient” in many of this world’s greatest foods: Cheese, soy sauce, dry-aged steak, and sake to name just a few. But knowing that mold has graced some of the foods I love rarely results in the sort of confidence I need to casually cut some fuzz from a loaf of bread and get on with my life. To get a better sense for how relaxed we can all be about a little mold on a hunk of cheese — or a whole lot of mold in a tub of jam — I reached out to a few experts.

Those patches of mold you find on an old tomato, or clinging to the lid of a deeply cursed storage container, are clusters of microscopic fungi, of which there are thousands and thousands of species. Under a microscope, they look similar to mushrooms. According to the USDA, the roots of each mushroom-like particle “invade” a surface. At the end of each stalk, which shoots up from the roots, are spores. These spores form the colorful patches we can spot without a microscope. In other words, much like roots belonging to a grove of trees, the structure of the mold often stretches deep below the surface, and isn’t visible to the naked eye. That means that even if you skim a layer of mold spores off a jar of preserves, or scrape it off your toast, there’s a chance the mold goes deeper — particularly in foodstuffs that allow for airflow in their structures, like a spongy loaf of bread or a day-old pastry.

While home cooks have devised methods for distinguishing between the good and the bad — the green fuzz is fine, the black stuff is not, for instance — there isn’t a lot to back these theories up. Johnny Drain, a scientist whose work focuses on fermentation, explains that the world of mold is too vast for the spores to be easily color coded, or identified by a home or even restaurant-trained cook. “There aren’t really hard and fast rules. Some people say if it’s green, then it’s okay, and if it’s black, never eat it,” he says. “But those ideas aren’t really founded on rigorous science. It’s not like the green guys are good, and the ones that produce black spores or black fuzz are bad for you.” He points out that there are black molds used in food production, but there are also black molds that are lethal to humans. “It’s kind of a spectrum, where on the left-hand side, you have something that is disgusting or toxic, and then on the right-hand side you have something that’s producing something that’s delicious and tasty.”

Oftentimes, fermentation experts won’t worry about a bit of mold, even if it’s not an intended component of their fermenting process. “There are certain processes where mold is part of the story — to point to a very obvious and no-brainer example: cheese,” says Kevin West, a cookbook author and a Master Food Preserver, certified by the University of California Cooperative Extension. “In other instances though, we consider mold a form of spoilage. If you have mold on bread, you probably don’t want that.” West notes that when he’s making a batch of sauerkraut, or any number of other fermented foods, it’s not uncommon for spots of mold to show up in his crock. “You will almost inevitably find a little speck of mold floating on the surface of the brine, and you can throw it away, and it’s fine. You’re creating a microbial ecology and mold is part of that ecology.”

Mold, however, is not part of the ecology when you’re making something like jam or jelly, and there’s really no reason it should make an appearance. “In sweet preserves, what you’re really doing is trying to stabilize fruit with sugar,” West explains. He likens the process to salting and drying meat, pulling out moisture and regulating what’s called the “water activity” to create an environment that’s inhospitable to mold and other forms of spoilage. With meat, preservation is achieved through some mixture of salting, aging, and smoking. In jam-making, the process is obviously quite different. “Sugar is to fruit what salt is to ham,” he tells me. “If you take a peach and you set it on the counter, it’s going to rot very quickly… However, if you take the peach and cut it up, combine it with sugar, and then cook it, you’re boiling off water; you’re dehydrating it. You’re lowering the water activity. So you wind up with something that is more or less shelf stable. The more water you remove, the more shelf stable it is.”

But West has noticed that increasingly, people want fruit-forward jams that aren’t overpowered by mountains of added sugar. That can present some issues. “The contemporary taste for jam is to have less sugar,” he says. “A lot of lower-sugar jams are more ephemeral than more traditional jams, which have a higher sugar content.” That means that, in contrast with the virtually indestructible Smucker’s of the world, low-sugar jams are prone to go bad — if they aren’t properly jarred and stored.

Neither West nor Drain are as squeamish about mold in their own kitchens as I am. West has been known to slice a moldy nibben off the country ham that hangs in his kitchen. Drain will often skim a bit of mold off the top of a jam jar, and after we got off the phone, he texted me a picture of a jet-black piece of bread he’d eaten. It took me a minute to realize the thin fuzz wasn’t olive tapenade. It was mold.

Sometimes, though, it’s better to be safe than sorry, and there are certain foods Drain suggests throwing out when they develop mold, including rice and fresh meat. “Some of the well-known pathogenic molds and bacterias are more likely to grow on those high protein environments, or, rice starch, where the molds break down into sugar,” he says.

When it comes to deciding whether or not you should toss out that slightly moldy treat, it’s often a matter of intuition and common sense. It’s one thing to skim a bit of mold from a tub of yogurt (even knowing that its root structure is invisible below the surface). It’s another to open an ancient jar and encounter a cloud of mold spores billowing out. “If you get a piece of bread with a little teensy speck of mold on it, would you cut it off and go ahead and make your toast for breakfast?” West asks. “For myself at home, I would cut off that little speck and make my toast. I’m not at all alarmed by finding specks of mold,” he says. On the other hand, “Anything that is covered with mold, anything that’s smelly, anything that is slimy, anything that is putrid… Anything that evokes an aesthetic repugnance, obviously I’m not going to eat.”

Scraping a bit of mold off of a leftover sandwich, or cutting the corner off of a sad strawberry might not be for everybody, but it generally comes at a low risk. Use your best judgement, and if your lunch is freaking you out, don’t eat it. West emphasizes, however, that this isn’t the same judgement call he’d make if he was cooking dinner for friends (when that was still a thing we did) or if he was serving food at a restaurant.

“If I’m doing anything that involves feeding other people, I’m really quite rigorous when it comes to food safety,” he says. “I think there’s a really important distinction between home use and home practice, and public health and public safety.”