If your taste in cheese has evolved beyond the individually-wrapped processed American slices and into blocks of the hard stuff—like cheddar—you’ve probably noticed that some cheeses can develop a chalky white substance on their surface. At first glance, it looks like mold. It’s usually not a good idea to eat mold.
Is it fungus? Or is your cheese harmless and just making you paranoid?
It’s probably fine. The white stuff seen on cheddar is typically calcium lactate, which is the result of lactic acid interacting with calcium. When cheese has more water content, the lactate isn’t visible. As cheese ages, some of the moisture moves to the surface, and the lactate moves with it. When that water ebbs, the lactate remains behind and can appear as powdery, crystal-like particles on the surface of the cheese.
Calcium lactate is completely harmless. It might even be a sign the cheese has matured and is therefore tastier. But it’s also remarkably similar in appearance to mold. So how can you tell the difference? Calcium lactate crystals are going to feel slightly coarse, more like sand. Mold will be soft and is more likely to stick up from the surface of the cheese than the lactate, which is flat. Fondling the cheese should give you some indication of which is which.
You can also try scraping it with a knife. If it flakes off, it’s probably calcium lactate. If it just disintegrates, it’s probably mold.
Other types of cheese can harbor similar growths. Italian, Swiss, and Dutch cheeses may have visible Lactobacillus helveticus, which is added to helped create amino acids for flavor. The bacteria spurs the development of tyrosine, which also appears as white crystals as cheese loses moisture. Unlike calcium lactate, tyrosine is usually found on the inside, not the surface, of cheese.
In the event your cheese is actually growing mold, you can never go wrong by simply throwing it out. But according to the U.S. Department of Agriculture, it should be safe to cut mold out of hard cheeses and consume the remainder. Be sure to cut at least 1 inch around the moldy area and don’t let the knife come in contact with the growth. Mold in any soft foods (bread, yogurt) means it can’t be locally contained and should be discarded.