Coffee foams when water is added

[Steve Freides]

We home roast - my wife uses our coffee in a French Press - grinds it,
puts it in the pot, adds water.

The thing foams like some kind of science experiment, most foam when
closest to fresh-roasted as far as we can tell, less after a couple of
days but still foams for as long as the batch last, which is never
more than a week.

She says store-bought coffee never does this.

Why?

Thanks!

-S-

[Maryann & Dave]

Because store-bought had lost most of it's carbon dioxide 20 minutes after they ground it.

Dave S.

[Steve Freides]

So you are saying it's the fact that it's fresh-ground, and not
recently roasted, that causes the foaming? We didn't experience this
store-bought coffee, even though we still ground it right before
brewing.

It's been long enough that perhaps I'm not remembering that correctly.

-S-

[Maryann & Dave]

You won't see the foam on coffee, even just ground, more than two weeks after roasting. Of course, grinding speeds up the process of out-gassing.

Dave S.

[scott stap]

At work here we get preground, individually packaged, coffee from Sam Club. Of course I try to make it as good as it can be, so I use my special process stolen from pour over instructionals. I "preinfuse" the grounds with hot water before letting the Bunn brewer take over. I was pleasantly surprised when the grounds show some signs of "blooming". Of course it took a bit of hot water to achieve this reaction and it did not happen instantly as is the case with fresh roast BUT, it did bloom. 

Scott

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[Yong-Gu Bae]

Hello, Steve!

During the roasting process, the cell walls of coffee beans break, and coffee beans start to give out carbon dioxide with aromatic compounds.  Blooming occurs due to carbon dioxide in coffee, so as time goes, less dramatic the blooming becomes, and the coffee loses its distinctive scents. Grinding basically breaks the cells apart and increases surface contact areas of coffee grinds, so the whole degassing process significantly accelerates. Therefore, it's important to check the roasting date when you purchasse coffee from stores and grind right before you have coffee.

Happy New Year!

Yong