# B Why did my coffee foam over when I added milk?

1. Dec 7, 2018

### Greg Bernhardt

I've made coffee before hundreds of times but today I was startled when poured in a bit of cold almond milk to my very hot coffee resulting in a violent foaming over the cup. It wasn't like a slop over spill. The coffee was not boiling at that moment, but adding the cold milk seemed to create an effect like it instantly and violently boiled over. It wasn't like the pouring created a nice froth and I added too much. It was a little bit and the foaming was like chemical reaction as if it were boiling. I'm a sure there is a simple explanation, but it's weird that never has happened to me before.

Solved! See post #9 and #11

Last edited: Dec 7, 2018
2. Dec 7, 2018

### Staff: Mentor

Just a question: was the almond milk still good? I could imagine that if it had built $CO_2$ this could explain the effect or at least some resolved gas.

3. Dec 7, 2018

### Greg Bernhardt

I hope so, what does bad almond milk taste like?

4. Dec 7, 2018

walnuts?

5. Dec 7, 2018

### Staff: Mentor

I don't know. I only have a similar strange and simple question about another effect which makes no physical sense. But I thought I'd better wait for an answer to your question before I hijack it, especially as I'm at least as curious as you are. A longer test series would be nice: varying by temperatures of both, or is it repeatable at all. However, you shouldn't drink every mug of the series: caffeine overdoses are mean.

6. Dec 7, 2018

### Klystron

I noticed a similar effect adding freeze-dried coffee crystals to just-below-boiling water in a cup on mornings I don't feel like a fresh pot. I assumed since I heated the cup and water in the microwave (I know, ugh!), the crystals broke surface tension and the "slurry" of hot water and partially dissolved crystals boiled over.

Wonder how hot your cup became from the coffee before adding almond milk? If relevant.

7. Dec 7, 2018

### Greg Bernhardt

Ah maybe an interesting fact is the cup was microwaved with the coffee in it.

8. Dec 7, 2018

### Klystron

Two data points? Darn, I microwaved a cup of brewed coffee strictly in the pursuit of science to test the hot-cup hypothesis but ran out of milk!

9. Dec 7, 2018

### Staff: Mentor

This could explain it. As the temperature distribution in microwaves isn't uniform, it might have happened, that some regions had been above the boiling point and some colder regions on top. Then the almond milk worked as a catalysator and its sinking created a path for the hotter coffee to boil. Combined with the milk, it resulted in foam.

10. Dec 7, 2018

### Greg Bernhardt

Very neat!

11. Dec 7, 2018

### Merlin3189

12. Dec 7, 2018

### Staff: Mentor

Just a speculation, but some of those crystals might have been heated before they dissolved. Was there also sugar in the coffee?

A steam explosion required more than different temperatures. It needs a large surface area of contact between the hot and cold stuff; as in a vapor.

Freeze dried crystals might act like sugar crystals.

13. Dec 7, 2018

### Merlin3189

Crystals can provide nucleation centres to initiate boiling in the superheated water.
Small bubbles can have the same effect, as illustrated by shaking a pop bottle (soda bottle?) Generating a few bubbles by shaking provides the necessary low pressure surface for gas to come out of solution. Rapid growth of the bubbles causing turbulence and break up of the bubbles, is positive feedback in bubble formation. Whoosh!
Pouring milk into the superheated coffee less than very carefully almost certainly produces a few bubbles and there you go again.

14. Dec 7, 2018

### Staff: Mentor

The moral to the story is, never wash your coffee cup.

I did that in my college years. A side benefit, when I ran out of instant coffee, just hot water in the dirty cup made a good drink.