Why did my coffee foam over when I added milk?

[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

 

[fresh_42]

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.

 

[Greg Bernhardt]

was the almond milk still good?

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

 

[phinds]

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

walnuts?

 

[fresh_42]

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

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.

 

[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.

 

[Greg Bernhardt]

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

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

 

[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!

 

[fresh_42]

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

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.

 

[Greg Bernhardt]

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.

Very neat!

 

[Merlin3189]

Yes, superheating in microwave ovens is a well known and dangerous phenomenon.
http://www.animations.physics.unsw.edu.au/jw/superheating.htm

 

[anorlunda]

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.

Normally, a microwave is capable of superheating water. It is then above 100°C, but still liquid, because it lacks nucleation sites. Crystals like sugar provide such sites, so this would have been my first guess. But "stays there for the duration of the drinking" is strange, I hope you don't drink your coffe while it's above 100°C. Do you think this might be it? en.wikipedia.org/wiki/Superheating – rumtscho♦ Jul 12 '11 at 14:53

Freeze dried crystals might act like sugar crystals.

yes, water can “explode” in the fashion described above. However, it takes near perfect conditions to bring this about, thus “exploding water” is not something the average hot beverage drinker who would otherwise now be eyeing his microwave with trepidation need fear. Odds are, you’ll go through life without ever viewing this phenomenon first-hand, and if you’re one of the rare few who does get to see it, you will likely not be harmed by the experience (that would take your standing right over the cup at the instant it happened, and the liquid’s bolting up and hitting your skin).

The Food and Drug Administration (FDA) has advised consumers:

This type of phenomenon occurs if water is heated in a clean cup. If foreign materials such as instant coffee or sugar are added before heating, the risk is greatly reduced. If superheating has occurred, a slight disturbance or movement such as picking up the cup, or pouring in a spoon full of instant coffee, may result in a violent eruption with the boiling water exploding out of the cup.

 

[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.

 

[anorlunda]

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.