Defrosted milk looks weird, chunky

What you see: The milk you had frozen is now…weird. Maybe it’s chunky or separated or grainy. You might notice this after defrosting or while it’s defrosting. 
What it is: Coagulation and/or the fat globules have collapsed, creating butter grains and pools of oil if the milk is heated. 
Eat, drink or toss?: If the milk still tastes and smells OK, or OK enough, you could cook with it. We can’t imagine anyone wanting to drink it, though!

Freezing can nudge along milk coagulation

Whenever milk starts to smell off, I do one of two things: I make pancakes on the spot, or pour the milk into a tall plastic deli container and pop it in the freezer for future pancakes (like these!). At that point, it tastes not quite good enough for drinking, but it still looks normal and will be fine for cooking. Here’s more on why slightly spoiled milk is still usable, but very chunky, offensive smelling milk is not worth it. 

But, sometimes, when the clock strikes pancake and I defrost my frozen milk I discover that it’s now, somehow, clumpy. Was I blind to these white blobs when I first put the milk in the freezer? Did it spoil more while it was frozen? Have I discovered a new form of cheese, Fromage ala Freezer, perhaps?

And, most importantly, can I still make those pancakes?

I checked in with Nicole Martin, a dairy food scientist at Cornell University. The short answer is that the milk was probably coagulating, or the proteins were clumping together. This can happen while milk already past its prime defrosts, without any additional microbial growth. In older milk, the proteins are already a little destabilized and the shock of freezing and defrosting is enough to cause them to clump. So the defrosted milk is still usable, and perfectly fine for cooking. (I just can’t handle any big chunks, though, and they usually end up in the compost.) 

As long as the milk was passable when you put it in the freezer, “I wouldn’t consider it a safety issue,” said Martin, who is associate director of the Milk Quality Improvement Program at Cornell. “I would just consider it a modification of where it was at when you put it into the freezer.”

Indeed, I also freeze any milk in our fridge shortly before we go out of town. This milk is often totally fine and smells normal when it goes in the freezer, but it’s not, of course, fresh from the store. That milk, too, is often clumpy when I defrost it. 

What if the milk is grainy?

Freezing also messes with the fat in milk. The fat in milk is packaged into globules surrounded by membranes that protect them from fat-digesting enzymes that are also present in milk, according to On Food and Cooking by Harold McGee. Without the protection of those special membranes, the fat droplets would pool together and the enzymes would break them down into rancid-smelling, bitter fatty acids. 

The membrane, McGee writes, is only a few molecules thick and easily pierced by ice and fat crystals when frozen. When you freeze milk, the punctured membranes end up floating in the milk and the fat globules get stuck to each other, leading to grains of what is essentially butter within the milk. If you heat thawed milk or cream, you’ll see puddles of oil, according to McGee.

Why does defrosting milk look yellowish?

In normal circumstances, milk looks white because its fat globules and protein bundles deflect light. 

When casein proteins in milk separate out, it leaves behind the more liquidy whey, which has a yellowish cast, just like the puddle of whey you might see atop yogurt. The color comes from riboflavin, also known as vitamin B12.

Here’s a more in-depth explanation for why milk can be chunkier after you defrost it

Much of the behavior of milk comes down to what its proteins are up to. Broadly speaking, those proteins can be divided between caseins, which clump together in acidic conditions (and produce everything from yogurt to cheese, depending on how they’re handled), and whey proteins, which stay suspended in the liquid that caseins are separating from. Bacterial growth, which strikes just about all pasteurized milk after a certain amount of time, can shift the pH to be slightly more acidic, which makes the caseins more likely to clump together. 

“By the time you’re observing some sort of a sensory attribute that would lead you to believe this milk is on its way out, the microbial load in that milk is probably pretty high,” Martin said.  “And they’re producing these metabolites that are leading you to be able to observe that, maybe it’s a little fruity or um a little bitter or something like that.”

The moment when those odd flavors and smells show up is exactly when I freeze the milk for future pancakes. 

At that point, Martin told me, the milk probably isn’t forming clumps, but the slight pH change destabilizes its proteins enough that they’ll more easily clump if nudged. One way to nudge them? Freezing the milk. 

“And then you throw it in the freezer and because milk is mostly water, the crystallization of the water in that milk is just going to put those proteins right over the edge and they’re going to coagulate,” she said. 

Heat will have the same effect–if you’ve ever poured milk that looked perfectly fine into a hot drink and saw little curdles form, you were probably dealing with milk with destabilized proteins (most likely because the milk was older).

Back to Martin to wrap things up for us:

“Milk is so incredible because it is such a complex matrix, but somehow stays in solution through a lot of different conditions. But at some point, those proteins have had enough stressors on them that they can no longer stay in solution. And that’s exactly what happens when you have a milk product that looks great going in the freezer or looks great going into your milk frother and then doesn’t look so great on the other side. Those proteins have just had enough. They destabilized and coagulated.”