How come milk curdles when you put acid in it, but not when you mix it with a low pH non-acid substance like espresso? Or, put another way, what is it about acid that makes milk curdle, since it doesn't appear to be the pH?

Coffee has a pH of around 5, which is very mild.

---

Are you accusing me of not having a viable magnetic field? - Masaq' Hub, Look to Windward

There's no such thing as a non-acid with low pH; that's how you define an acid in the first place.

Hey it's totally propaganda for the HealthWise Corporation, but here's the Ph for a bunch of different coffee brands.
http://www.healthwisecoffee.com/blog/?page_id=89

IIRC, the acid denatures the protein in the milk, causing the protein chains to 'unwind' and thus make odd shapes like the curds, and solidifying. Higher the PH the more reactive? Something along those lines. A substance with a PH pretty close to the milk won't do that.

7 is Ph neutral, lower than that is acidic, so lower = more reactive in this case.
It would seem that 5 is simply not acidic enough to react with milk.
Keep in mind that Ph is logarithmic, so each number below 7 is 10x as acidic.

Fair enough, I meant 'higher' as in 'more acidic'. But yes, you are correct.

Thanks all.