Anyone who does any reading on pour-overs quickly comes across blooming. Most (but not all) pour over instructions include blooming in their steps. Intelligentsia, in their chemex guide, call it an “essential” step. On the other hand of the spectrum is George Howell brew guide which does not prewet at all in their chemex instructions. This got me curious so I did some research on blooming.
Blooming or prewetting is pouring small amounts of water into the coffee at the very beginning of brewing. The goal for most pour over guides in the blooming phase is to pour as much water as possible into the beans with out any water exiting the coffee. According the Blue Bottle, coffee can hold twice its weight worth of water. Reaching the ratio 2:1 water to coffee beans is generally the ideal amount of water poured in during the blooming phase.
According to Scott Rao in Everything but Espresso, blooming serves two functions. First blooming allows water to flow from wetter areas of the coffee bed to the drier areas “via capillary action” which allows for more even extraction. Second blooming allows CO2 to escape early in the process. CO2 can disrupt the process in two ways. Scott Rao is primarily concerned that during the blooming phase when coffee expands it sometimes gets pushed up onto the walls of the filter. This creates an uneven bed which leads to uneven extraction. Others, are concerned that the act of CO2 exiting the beans can prevent water from extracting particles from the grounds. Bloomin, allows much of the CO2 allowing for even extraction in the process.
Thus there are three reasons to bloom; to evenly wet the beans, to prevent an uneven bed, and to prevent CO2 from getting in the way of extraction. I have issues with all three of these reasons.
First, in theory, the capillary action Scott Rao speaks of will happen regardless of how much water is poured in. If I am doing a chemex with 25 grams of coffee grounds, and I pour 100 grams of water over the grounds, water should flow from the wet parts to the dry parts just the same as if I poured 50 grams. This is a purely theoretical critique so if others can prove that the capillary action works better with less water in the bed, be my guest.
Second in many of the pour overs I have done I have seen coffee being pushed up onto the filter creating uneven extraction. Blooming is an effective way to prevent this from occurring. However this only occurs when the beans are still very fresh. Once beans get old and CO2 naturally leaves them the beans may no longer get pushed up onto the wall. In addition one can also push those beans back into the bed with one swift pour along the edges of the filter. Some may fear that this will cause water to channel along the sides of the filter and not properly extract coffee. However one quick pour in my experience has not ruined a cup of coffee. In conclusion with fresh beans a bloom is important. If the bag was opened a few days ago, it is less so.
Last it is difficult to find credible sources to support the phenomenon that CO2 exiting beans will prevent extraction. This argument appears sporadically on miscellaneous websites. Intuitively I think it makes sense but due to the lack of good sources I am skeptical. Anyone who takes this argument seriously ought to bring forth more credible sources for the benefit of the community as a whole.
One ought to bloom with fresh beans in order to prevent an uneven coffee bed. But once beans are older it may be safe to skip the blooming process. I do not think one needs to worry about capillary action although I encourage someone to prove me wrong. Last until more evidence is brought forth indicating that CO2 prevents extraction I will be a little skeptical of the phenomenon.
I have definitely made quality pour overs with out blooming. But if I had to bet money I would bet that blooming can help create a more flavorful and aromatic coffee. I am definitely going to play around with blooming more.
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