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Coffee bloom and degassing
Please forgive me if this is too basic of a question. I am fairly new to the world of home roasting. I roast the coffee and store it in a mason jar for 24 hours before I brew using an aeropress. Often there is a loud pop opening the mason jar. I assume that is degassing of CO2. When I add the hot water to the ground coffee, I see the bubbling and assume that is what is called the coffee bloom. Now is the coffee bloom just more CO2? Or is it something else?

Still trying to figure out what is going on...
Yes, it's degassing, mainly CO2.
As usual, nothing is wrong with this, if not exceeding a certain measure, not enough or too much.
You should wait a minimal time before brew, until enough gas is gone, else you can experience uneven extraction, due to small bubbles / air pockets, that can disrupt the contact between the coffee grounds and the water.
Conversely, too much wait, the flavours are less vibrant.
The trick is to allow it to degas sufficiently, without allowing the beans to become stale.

Storing in any hermetical jar is not good, not allowing the proper degassing.
You should use any storage solution fitted with a gas escape valve, either a bag, either solid matter, but allowing gas to escape as it is released, and not allowing oxygen replacing, because will accelerate beans oxidisation.
For me, as pound weekly home roaster, the valve bags are the most effective solution.
Others mileage may vary.
Also, please notice the vacuum solutions aren't good too, because increase forcefully the flavors extraction from the beans.

An academic approach on this subject below Grin
I've been using mason jars with standard lid for years and is one of the best and easiest to use. When you screw down the lid, barely tighten it so the jar will build little pressure and can easily degas. Fortunately, the jar won't ever pull ambient atmosphere back in as there is always some generation of gas even a couple of weeks after roasting.

One way valves are unnecessary.
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
Been thinking about this for a while, so needs a reasonable person to explain to me:..

We're outgassing a large semi-permiable bean over a longish period of time, say, 24 hours. Why isn't the outgass faster after grinding the bean and exposing all that inner surface

Would perhaps the outgassing be removing aromatics faster if the bean is ground? I haven't seen a difference between grinding whole roasted beans for today and grinding 3 days worth then storing in an airtight glass container. Grinding a couple weeks worth, yes the taste degrades noticeably
Edited by Piotrkurak on 07/19/2023 6:49 PM
Third rule of "fifteens": 15 minutes from grind to brew... otherwise stale coffee
Just quoting the collective wisdom... not a man of rules, me either.
Just a rules collector, to know how many of them I am breaking Grin
Edited by renatoa on 08/07/2023 1:01 AM
I may be off track here, but I would think, considering the lid on a mason jar has a rubber seal, basically the weight of the lid would allow gas to escape and then gravity would reseat (reseal) the contents until enough gas is produced to ever so slightly raise the lid again and continue the off-gas process. So I think one should screw the ring on and leave it somewhat loose. Does this make sense?
Home made 1 way valve jar lids: https://www.home-...t2699.html
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