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This question already has an answer here:

We have a bottle of milk that is half gone. It is not sour.

If I pour hot water onto instant coffee, and then top up with this milk, the milk splits into a grainy solid.

If I repeat the process but use a newly opened bottle of milk, the milk does not split.

Why does the first lot of milk split?

Thanks in advance.

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marked as duplicate by Mithoron, airhuff, andselisk, Jon Custer, Tyberius Jan 31 at 17:37

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    $\begingroup$ The milk is starting to sour, and will curdle in hot water (coffee, tea whatever) before it will noticeably curdle in the cold fridge. $\endgroup$ – MaxW Jan 31 at 2:25