What you see: A smeary white crust on your aged cheddar. You may be paranoid that it's mold.
What it is: Calcium lactate crystals!
Eat or toss: Eat! Like the crystals that can appear in aged Italian-style cheeses, this white crusting is harmless.
Here we have another kind of white crystal that might fool you into a moldy panic. But, similar to the white "tyrosine" dots in parmesan and other aged Dutch and Italian cheese, the smear of crystals atop some aged cheddar and other varieties is simply evidence of some cheesy chemistry.
During an early step in cheesemaking, bacteria turn milk sugar (lactose), into lactic acid. Then:
Lactic acid + the calcium that naturally occurs in milk = calcium lactate.
Calcium lactate can form crystals, which, when they're clustered densely enough to be visible, look like the crust in the image above.
Unlike tyrosine, an amino acid that forms white crystalline dots on certain types of aged cheese, calcium lactate tends to be visible in powdery smudges on the exterior of the cheese. You'll likely see it on surfaces where moisture may have collected.
Why calcium lactate crystalizes
Calcium lactate starts out dissolved in water in the cheese. So when a cheese loses water during aging, the calcium lactate stays behind, becoming concentrated in the body of the hardening cheese. As long as it's dissolved in water or spread out, you probably won't see it.
But, if moisture forms on the surface of the cheese, the water-loving calcium lactate will be drawn to it. And when that water evaporates away, the calcium lactate will be left behind. This time in a smeary layer that's visible to the naked eye.
Loose-fitting packaging and temperature fluctuations can encourage moisture to form on the surface. However, if you're looking at a nicely aged piece of cheese, the calcium lactate is more likely to be evidence of a long, flavor-nurturing process than anything problematic in its handling.
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