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!

  • 9
    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? Apr 18, 2013 at 11:38

5 Answers 5


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.

  • 1
    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, 2013 at 12:49
  • @Fraser glad you found it. this question on milk might also be helpful.
    – MandoMando
    Apr 18, 2013 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.

  • Thanks. I've checked with everyone and no-one has cleaned the kettle
    – Fraser
    Apr 18, 2013 at 12:50
  • Couldn't such residue be combated effectively by rinsing with baking soda solution, then clean water? Dec 13, 2015 at 22:47

Maybe your milk is about to go bad? According to the website below, when the milk is almost bad the acidity in coffee or tea is just enough to make the proteins combine and make the milk curdle.

This is from foodreference.about.com:

Milk and Coffee or Tea

On occasion, cold milk added to coffee or tea will curdle. This can be alarming as curdled milk is often seen as the same as spoiled milk. In this case, it can be half true. Coffee and tea are both slightly acidic, although usually not enough to curdle fresh milk. When milk is just on the brink of spoilage and bacteria have produced some, but not enough acid to curdle the cold milk, a little bit of extra acid from the coffee or tea, along with their heat can tip the scale and cause the milk to curdle. The milk may not be spoiled enough to cause an off odor or flavor, but just enough acid and heat in addition to its own can cause curdling.

  • It's certainly been my experience. A day or two before the milk actually becomes bad, the first stage is that it curdles when added to hot tea or coffee. You can drink it without ill effect, but replace it pronto
    – user57361
    Nov 19, 2018 at 17:57

It may be your water! We recently moved into a new house that has a well. And the same thing happened when I made coffee, the milk curdled. I then made a pot with bottled water and the same milk did not curdle. Needless to say that I am having my water tested


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

  • 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
    Dec 16, 2013 at 14:58

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