everyday chemistry - Why does milk flake?

While drinking milk (or better 'while seeing the milk I'm gonna drink') a question came up to my mind:

Why does the milk sometimes flake, even if not in contact with some other substances?

I guess the correct term to use is flocculation, at least this is the word that raises up when talking about cheese production. I just learned the word casein. However, my question is not about cheese production.

My question is also not about flaking milk when poured into a cup of coffee which seems to be dependent on the acidity of the coffee. But casein is again the keyword.

I think the answer to my actual question will also contain the word casein, but here's the scenario:

I open a milk carton for the first time. This package doesn't have any hole and the shelf life isn't reached yet. I pour the milk into a clean cup and then observing some flakes at top of the milk.

What I was told as a child, when curiously asking, flaking is an indicator that the milk is getting acid. And in that case the keyword casein would fit again, but there's still one thing why I'm not persuaded: the milk was quite tasty and my stomach also didn't complain about it.

So, what exactly has happened to my milk when I see some flakes on top of it? Is the milk indeed spoiled and I actually shouldn't have drunk it (and just was lucky) or is there anything else which can lead to flakes?

Answer

I think the right word for it is curdle. Have you read, for example, the Wikipedia article on curd? It says:

“Milk that has been left to sour (raw milk alone or pasteurized milk with added lactic acid bacteria or yeast) will also naturally produce curds, and sour milk cheese is produced this way”

or this excerpt of the “Milk” page:

When raw milk is left standing for a while, it turns "sour". This is the result of fermentation, where lactic acid bacteria ferment the lactose in the milk into lactic acid.

So, milk naturally contains bacteria which turn it acid, and hence it curdles spontaneously. You see the beginning of this process.

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An invaluable source of information on this topic is McGee’s On food and cooking. Great reading if you like chemistry and food.