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Since I am the only coffee drinker in the house, I started making coffee a quart at a time and saving it in a mason jar in the fridge. I discovered this interesting thing. Starting with about the 3rd opening of the jar, when I warm up the coffee in the microwave and then add a spoon of sugar, a fine lace of foam rises to the surface, giving the coffee the texture of crema coffee. I assume something in the air (nitrogen?) is being dissolved into the chilled coffee and then brought out of solution.

Further, I think the coffee tastes smoother after it has aged a couple days. I don't have a great palate, but I suspect something is going on that is analogous to the binding of tannins in wine, but I have no idea.

Can anyone shed some light on this?

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All I can say is... >.< –  ElendilTheTall Oct 19 '11 at 14:35
    
Read cooking.stackexchange.com/questions/16102/…, probably even a dupe. –  rumtscho Oct 19 '11 at 15:45
    
The answer to the previous iteration seems to focus on superheating. Alas, the comments suggest that this may not be the case (they also suggest that the question was unclear for a time). Super-heating water based liquids in any kind of bulk produces quite spectacular (and somewhat dangerous) results. Heating domains in microwaves should have a linear scale on order of a centimeter. –  dmckee Oct 19 '11 at 16:02
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This is interesting but I am dubious because it does not occur with coffee that has not been exposed to air while cold. That is, make coffee, fill jar, lid, chill, wait weeks, heat, sugar, no bubbles. (On a side not, the lid pops the first time you open it because the steam in the jar condenses leaving a vacuum.) Once air is introduced and then more chilling the phenomenon occurs. I have witnessed the explosion, though, so I will test the popsicle stick idea. –  Paul Jackson Oct 20 '11 at 0:51
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Making coffee a quart at a time, refrigerating it and reheating it in a microwave is inhumane and should be made illegal. Please, for your own sake, get one of those coffee machines that use capsules - it's quick and effortless and makes great coffee! plus, you don't have to be worried about strange chemical reactions in your beverage. –  Talbatz Nov 24 '11 at 22:46
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Your post combined with Graham T's answer leads me to believe that brewed coffee has some organic molecules that, over time, break down to react with something in white table sugar that produces a gas. The thing is there exists over 1000 different kinds of volatile organic compounds in roasted coffee that it would be hard to pin down precisely.

You mention in one of the comments that you're a beer brewer--I assume you're a drinker too. Are you familiar with Guinness? They normally can their beers with nitrogen for smaller bubbles. If you can distinguish between co2 foam and no2 foam, that may help you narrow down what's going on here.

As an aside: This is a really cool question and I hope you find a more acceptable answer. This one sounds like it's pushing the knowledge of molecular gastronamy.

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Measly insight, but I don't believe the refrigeration has any effect. I've reheated old coffee (room temperature) from the morning with a spoonful of sugar, and the similar foam you're describing develops. Pretty certain it's the sugar, as I generally drink black coffee, and have never seen plain black coffee foam when reheated in a similar fashion.

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I can't speak of the crema, but stored coffee generally gets smoother for me unless I shake it; a lot of the bitter seems to settle out as sediment.

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