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Came in to work this morning and made a round of coffees, the same as yesterday.

To freshly washed and fully rinsed mugs, I added instant coffee, and the milk then filled with boiling water and the milk instantly curdled and all the bits floated to the surface.

Thinking it was just bad milk (the milk didn't smell off at all but I remember seeing it sat on the side at some point yesterday) I bought fresh milk and repeated. The same thing happened.

We are using the same kettle as yesterday, the same mugs, the same instant coffee.

To be sure it was the milk, I made a coffee with just coffee and water, it was fine, I then added the milk and it curdled.

To eliminate bad luck with 2 different jugs of milk, we've poured the milk into a glass to inspect and drank it and it is fine from both milk jugs.

Please help, I had a few beers last night and need my coffee!

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Try letting the water cool down for 30 seconds before you pour it in the mug. Also, does your washing liquid happen to be lemon? –  ElendilTheTall Apr 18 '13 at 11:38

3 Answers 3

up vote 12 down vote accepted

Boil the milk on its own in a clean/rinsed container (microwave). If it curdles, it's the milk.

Otherwise either there is a decalcifying agent in the kettle or something in the coffeee is making it too acidic. It takes very little, after decalcification, we usually have to pass about a gallon of water through the coffee machine before the steam wand stops curdling milk.

If you're in a jam, boil your water in a cup in the microwave and avoid the kettle.

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Hi, we followed your advice and boiled the milk in the microwave and sure enough it curdled. Seems it's likely to be a bad batch of milk. Thanks –  Fraser Apr 18 '13 at 12:49
@Fraser glad you found it. this question on milk might also be helpful. –  MandoMando Apr 18 '13 at 15:37

This happened to me once and it turns out somebody was cleaning the kettle with lemon juice the day before and they didn't rinse it properly. The coffee w/o milk tasted fine - I couldn't taste the lemon juice, but when I poured the milk my coffee looked like I poured yoghurt into it. (From then on, I drink coffee sans milk.) My advice would be to try another brand of milk because it might be that this particular series has a problem.

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Thanks. I've checked with everyone and no-one has cleaned the kettle –  Fraser Apr 18 '13 at 12:50

problem is the full fat milk.....use low-fat milk or non-fat milk

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Hi @ali! Welcome to Seasoned Advice. It looks like the original poster was previously successful with this process with the type of milk he is currently using. Unless he has changed the type of milk he is using, your answer is probably not the culprit in this case. –  Preston Fitzgerald Dec 16 '13 at 14:58

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