[Notso (OP)]

One of my co-workers was making coffee - when she poured the milk in, stirred, and felt something solid in the cup. When she fished it out, a large lump of brown, rubbery solid was on the teaspoon. It was malleable, easily breakable, and must have formed in a couple of seconds. The milk was fine to drink and make coffee with just a few hours ago, although the thermos flask hadn't been in the fridge because it was being defrosted (the fridge, not the flask). The coffee was far more watery than usual, which suggests the milk was "used up" in the reaction. What could have caused this?

newbielink:http://i.imgur.com/Pj9Ij3V.jpg [nonactive]
[Click for larger picture.]

I wasn't sure whether to post this in chemistry or biology, but I couldn't see one in biology that was particularly related to beverage science, and this does seem to be some sort of chemical reaction - if it's misplaced, apologies. [:

[CliffordK]

A few weeks ago I was making hot chocolate, and added a touch of lemonade mix, and ended up with lumpy milk.

I'm wondering if your coffee was more acidic than usual.  Old burnt coffee?

[Colin2B]

One of my co-workers was making coffee - when she poured the milk in, stirred, and felt something solid in the cup. When she fished it out, a large lump of brown, rubbery solid was on the teaspoon. It was malleable, easily breakable, and must have formed in a couple of seconds.

A few weeks ago I was making hot chocolate, and added a touch of lemonade mix, and ended up with lumpy milk.

I'm wondering if your coffee was more acidic than usual.  Old burnt coffee?

Takes me back to a book of home chemistry experiments I had a a kid. Clifford is right, the coffee or the milk must have been acidic and what you have got is casein plastic, the casein in the milk has formed a long chain. I suspect that although the milk was ok it might have been close to turning hence more lactic acid.
If I remember correctly they used formalin to harden it, was used for knife handles, buttons.

[Notso (OP)]

Thanks for your responses! [:

We wondered about the possibility of something being slightly acidic, but what stumped us was that the whole thing was fished out from the "coffee" in a single lump (what you see in the picture is after we broke it apart) - normally when milk curdles you get lots of little lumps, and this felt like cooked halloumi.

[Monox D. I-Fly]

Takes me back to a book of home chemistry experiments I had a a kid. Clifford is right, the coffee or the milk must have been acidic and what you have got is casein plastic, the casein in the milk has formed a long chain. I suspect that although the milk was ok it might have been close to turning hence more lactic acid.

I remember back then when I was a teenager, my parents often asked me to buy milk from the dairy farm. After I boiled it, there's something which looked like a layer of thin plastic covering the surface. I tried to eat it and immediately feeling dizzy afterwards. What was that?

[Bored chemist]

. I tried to eat it and immediately feeling dizzy afterwards. What was that?

https://en.wikipedia.org/wiki/Psychosomatic_medicine