I've seen these bubbles when pressing coffee that I've home roasted in two different presses with two different water sources. I've seen it with coffee from multiple roasts now. The beans are the same in all cases (Rwandan single origin). I am grinding with a burr grinder. Do these large bubbles in the bloom indicate anything about the bean or the roast?

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    The bubbles almost look like they have a sheen of dish detergent (rinse well!). Otherwise, I suspect it's a product of the particular processing process used in your beans; can you share anything else you know about the beans? Specifically, how long ago they were roasted, and whether wet, dry, or semi-washed process? Might be caused by mucilage if it's dry processed...? I think I have noticed this before but haven't pinpointed a source.
    – hoc_age
    Jul 18, 2016 at 11:50
  • This is the coffee, the description says wet washed and sun dried. This roast was done yesterday - 18hrs ago or so. I am generally consuming 24-48hrs after roasting. The sheen is what bugs me too. I have not used any detergent in a long time - I water rinse my presses. Maybe it is time to wash out all of the oils? I am not brewing with filtered water so I suppose it could be water supply related, but I work in a different city than I live in so I am seeing this with water from two different municipal supplies.
    – Nick
    Jul 18, 2016 at 12:14
  • You're roasting this yourself, then... to what roast level -- Full City-ish? Might be worth allowing the beans to rest longer (48+ hours after roast perhaps) to see the difference in bloom from the same roast batch. Does the coffee taste good? If so, is there a problem? :) Do you have another specific concern (e.g., damaged coffee, etc.) or merely asking if there is a potential problem (both okay for sure :))?
    – hoc_age
    Jul 18, 2016 at 13:40
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    By chance, is this CO2? It may be carbon dioxide produced during roasting and released by grinding just before preparing the French press. 18 hours is not quite enough for degassing IMHO. 3-4 days seems more appropriate. Could you please try after a few days with the same batch of roasted beans and observe again?
    – MTSan
    Jul 21, 2016 at 21:17
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    @MTSan I am working on finding that out. I roasted double (2 1/4lb roasts done the same way) what I normally do on Tuesday and drank it Wednesday and Today. I'll keep brewing it for the next 3 days and see if there is a change.
    – Nick
    Jul 21, 2016 at 22:58

1 Answer 1


The bubbles are the result of "degassing" — carbon dioxide escaping from the coffee. Degassing will be especially dramatic in the first 24 hours after roasting, during which 40% of the CO2 leaves the bean. See this blog post: Understanding Degassing.

  • After waiting a week to brew I do not see these large bubbles anymore. This does seem to indicate degassing is to blame.
    – Nick
    Jul 30, 2016 at 16:44

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