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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?

Paul Jackson
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  • All I can say is... >. – ElendilTheTall Oct 19 '11 at 14:35
  • Read http://cooking.stackexchange.com/questions/16102/sugar-forming-tiny-bubbles-in-microwaved-coffee, 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 --- ex-moderator kitten 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 00:51
  • How much time is the coffee spending between 140F and 40F while exposed to air? Could it be spontaneously fermenting? – baka Oct 20 '11 at 01:12
  • That's a great question. I have not found the time leading up to chilling to be a factor. I will pay closer attention now that you mention it. – Paul Jackson Oct 20 '11 at 02:08
  • I suggest getting a French press so it's easier to make smaller batches of fresh coffee :) But then you say you like the flavor of your aged coffee... so maybe that wouldn't actually be a benefit for you. :) – Flimzy Oct 20 '11 at 11:58
  • OK. I performed two tests today and both discredit the super-heating idea. For the first cup I snapped off the end of a bamboo skewer and stuck it in the coffee - the wood is supposed to provide nucleation sites that prevent super-heating. After heating, I stirred the coffee with the skewer and then added sugar - still got the fine foam. For the second test I warmed the coffee for 2/3 the normal time. I then measured the temp of the coffee with a lab thermometer (that I use for home-brewing beer) and it read 135F. Still produced the foam. IMO, the gas in the foam can't be steam. – Paul Jackson Oct 21 '11 at 03:09
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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

3 Answers3

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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.

Eric Hu
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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.

Graham T
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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.

apaderno
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fuz
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