When you roast coffee, there's a bunch of chemistry going on, a bunch of browning reactions, and almost all of those produce carbon dioxide as a byproduct.
The darker you roast, the more of these kind of browning reactions you have, right up until the point of combustion, which I don't recommend reaching, but it does produce an enormous amount of CO2.
It allows for perfect color and browning of your pie while, at the same time, giving you enough time at a lower temperature to really bake those apples and allow them to become tender.
Gravy browning does not look good as stockings Ah, I see, I see It's an interesting colour and it's a little bit sticky so we tried it, it didn't work brilliantly.