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I’m familiar with what the bloom is and why we do it. But all of the instructions and guides about pour-over coffee I’ve ever seen recommend a specific time to let the coffee bloom before continuing with the brew. 40-45 seconds seems to be the most common consensus.

But I have to assume that every coffee setup is going to be a little bit different. Small changes in the freshness of the beans, the size and shape of the grind, the temperature of the water, maybe even things like the mineral content of the water? - all of these factors and more are going to have at least some effect on how much CO2 the coffee wants to offgas and how long it takes. So, “45 seconds” seems like a good guideline or rule of thumb, but I would imagine that some brews need less time and some need more.

So, if I want to gauge my bloom time by my senses instead of by the clock, what should I look for? I have noticed that toward the end of the bloom, the coffee starts to make a faint crackling noise. As the bloom “falls”, it becomes flat, and then cracks begin to appear into the surface, and eventually it falls past flat and becomes a little bit concave. (When it’s flat and starting to crack it looks to me a little bit like a chocolate cookie. It makes me hungry in the morning.)

That’s what I’ve noticed so far about the “phases” of the bloom. At which point in the process is it best to begin the rest of the pour?

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The bloom should be 30s for V60 and 40-45s for Chemex and Kalita and 30s for full immersion methods (like AeroPress or French Press). Blooming is not just about degassing but also to saturate the grounds (which obviously goes hand in hand) and to start dissolving solid compounds (mainly acids and caffeine, sugars come later).

If you'd want to change, just to see what happens (I doubt you will make better coffee with blooming times that diverge a lot from those "standards", since they have been proven to be optimal for the respective brewing devices used), then you could consider shortening the blooming time when the coffee is a few weeks old and has oxidized and degassed quite a bit. You notice when it stops bubbling, that would be a definite visual clue. If the same coffee that's been degassing until 30s (for V60), suddenly stops bubbling at 20s, you can try to reduce blooming time a bit. You'll run into trouble though, because your overall extraction time will likely be shorter until you figured out new grinder settings. The phases you describe really depend on the freshness of the coffee and the amount of CO2 it contains (darker roast would contain more e.g. because it's more porous). The slurry of very fresh coffee is still bubbly and has foam on top after 30s, not noticeably decreasing in volume, while a not so fresh coffee might fall flat after 15s already. I don't know what exactly you mean with cracks, I'll pay attention next time. More like holes where the CO2 exits, but I do agree it looks a little bit like chocolate cookies, when the coffee bed fell flat and the gas has mostly evaporated.

You can stir with a wooden spatula/spoon/chop stick to make sure you saturate all the grounds, but in my opinion that is not necessary and I've noticed that too much agitation somehow screws up the brewing, often over extracting. Which makes sense, more agitation, faster extraction. If you pour it nicely in concentric circles agitating the grounds slightly, you can manage to saturate all grounds without stirring and without agitating too much. That's ideal in my opinion, but the opinions on this point split widely.

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