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?
Best Answer
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.