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Permalink Reply by Daniel Williamson on May 1, 2011 at 9:39am

Read Scott Rao's book "Everything But Espresso" to deal with this question.  Basically, from what I recall, the coffee should be fresh.  If it's fresh, it's releasing CO2 which repels water.  You introduce a small amount of water (roughly 2 grams water for every gram of coffee) or just enough to get it all wet.  The water helps force the CO2 away from the grounds and out of the cells which contain most of the volatile aromatics and oils.  So the bloom allows water to displace the majority of the CO2, essentially prepping the coffee bed for extraction.  Then when you add more water (the pour), diffusion occurs between the water in the cells (that just replaced the previously present CO2) and the surrounding solution thereby imparting the useful flavor from the coffee and depositing it in the cup.  Others, please correct me if I'm wrong, but that's what I seem to recall.  Btw, the bloom doesn't just happen in pour overs.  It's present in every non-espresso extraction of fresh coffee.

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Permalink Reply by Deferio on May 1, 2011 at 9:43am

Basically this:

Assuming you are using coffee within a few-10 days off roast the coffee retains CO2 that in it's bean form outgasses slowly. Then, once ground, it is released even more, finally when hot water comes in contact with the coffee the coffee instantly purges itself of the CO2 causing the bloom. During the blooms out gassing process the coffee is exerting force outward repelling (in Scott Raos book this is called turbulence) the water that needs to get in to extract the solubles. By blooming first we are essentially waiting until the coffee makes room for water. It's as if I have a back seat that is full of junk but a couple of friends need a ride somewhere. They need to first wait until I make room for them by emptying out the contents of my back seat. Same thing with blooming.

Hope that's helpful.

-CD

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Permalink Reply by John Gibbons on May 4, 2011 at 1:40pm

There really is no need from my knowledge. I will do it forever because I enjoy it and it makes for a structurally sound formula for pour overs. It's supposed to "Pre-infuse" the coffee to release gasses, but more importantly (in my opinion) it re-settles the coffee like pre-infusion on espresso. This allows for less erratic fines migration and for espresso helps avoid channeling. Pour over is different as it will help the grounds re-settle and make for a more even extraction. 

 

You can find a link on the 3fe site where Colin Harmon (Irish champ) says he stopped pre-infusing. 

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