If coffee could talk when it comes out of the roaster it would say, Im feeling a little gassy.
The difference between green unroasted coffee beans and brown roasted coffee beans is significant, not only in appearance and size but in density and chemical composition. Like cooking in general, the roasting process is transformative, turning a hard raw coffee bean into a brittle cooked coffee bean that can be ground up, making its soluble material fully available to extraction by hot water. Its sort of remarkable, actually. One of the results is that a roasted coffee bean is bloated and gassy. Fresh roasted coffee sheds gas like your uncle on Thanksgiving, and once the coffee is ground the process accelerates dramatically. It is this CO2 degassing that causes fresh roasted, fresh ground coffee to bloom, like a flower in fast motion, when hot water hits it. Hot water essentially is the end of the degassing game, or speeds the end, which is what youre seeing when the coffee blooms.
Generally, this is what is meant by coffee bloom, though coffee bloom could be used in reference to the flower that blooms on a coffee plant, or the new film from India by that name. You can go all Bill Nye on this if you want and set up a little experiment. Buy some fresh roasted whole bean coffee, just days out of the roasters. Most specialty coffee roasters put roast dates on their coffee and if you buy coffee from a roaster retailer or a coffee retailer that uses a local roaster you should be able to get your hands on some coffee that is only a few days, no more than a week, out of the roaster. Then go to the grocery store and buy some coffee that was roasted and ground nobody knows when, but not recently. HINT: It often comes in a can. Youll need two drink glasses that can handle high temperature (about 200 degrees), preferably 12 ounce glasses, at least. Scoop 3 tablespoons of stale coffee (thats the coffee you bought at the grocery store) into one glass, then grind and scoop 3 tablespoons of your fresh roasted coffee into the other glass. In a moment, youll pour 9 ounces of water, just off the boil, into both glasses, but before you do, smell both of them. Take a good healthy whiff. Smell that? Not only is that the olfactory equivalent of the taste difference between fresh roasted coffee and dead coffee and what you smell is the degassing.
Pour 9 ounces of hot water, which should be no cooler than 195 degrees, over the grounds in both glasses and watch. The fresh roasted, fresh ground coffee will expand like a just-add-water-sponge. It might even overflow the glass (you've been warned). The dead coffee will not expand very much. Compared to the fresh roasted coffee, it will look, well, dead, which is how it will taste. Understanding coffee bloom won't change your life but it does help explain why you like our coffee so much.