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When brewing by the pour over method (in my case with a Chemex) I've noticed that some ground beans will float when I fill the filter with hot water, and other beans will not float. It's consistent with specific roasts, so I will use two roasts for illustration (leaving out the roaster for now to avoid any prejudices).

Roast A will always float when pouring hot water over, throughout the entire brewing process.

Roast B will always sink, especially towards the end of brewing.

If I leave the grounds and water undisturbed until all of the water has gone through the filter, roast A's filter will be coated with the grounds while most of the grounds in roast B's filter will be flat on the bottom (as you'd expect).

Also of note, roast A has a much more intense bloom after an initial pour of roughly 20g of water into 20g of grounds. It looks like a freshly baked brownie, whereas roast B's bloom is much less intense.

My question is, what does this say about the beans for them to act one way or the other? My first thought was maybe it had to do with freshness of the roasted bean, but A and B could have the same roast date (less than a week old) and still act as described. Or maybe it's not freshness of the roast but freshness of the bean prior to being roasted? I'm not qualified to hypothesize the chemical reasons for behavior, but hopefully someone here is.

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  • my concern (since i am not grinding my own beans)is that something other than coffee has been added to add weight to the mix aka saw dust, wood chips etc Your thoughts
    – Al Gallucci
    Oct 23, 2017 at 14:49

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Bean density varies dramatically with roast level. The more roasted a bean is, the less dense it becomes. My guess would be that less dense beans would have more of a tendency to float and more dense beans would have a tendency to sink. However, I imagine that there are other factors as well. Grind size would affect the mass vs surface size of the grounds and have some say as well. Age could also be a factor since freshly roasted beans still contain/give off some amount of gas. That degassing time can also vary with roast level and bean variety, so just time since roast may not always be a consistent measure. Overall I'd guess there are four plus factors affecting the "float" of your grounds. And that's not to even say anything about the content of the water itself.

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  • Great explanation on the bean density! Worth noting that my brewing method was exactly the same (grind size via burr grinder, beans to water ratio, and same water source and temperature). Based on your answer, I'm leaning to it having to do with the roast level or the freshness of the bean pre-roast. I tried this today with two different roasters (again, A and B), both were roasted on the same day (12/22). Same outcome as before.
    – timgcarlson
    Dec 31, 2015 at 1:06
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    This is the most helpful and accurate explanation in my view. Freshness is difficult to gauge with buoyancy unless the coffee and roasts are identical. Darker roasts float more. 🤷‍♂️
    – Angad
    Jul 20, 2020 at 10:30
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One thing to keep in mind is that coffee beans contain CO2. This causes grounds to float and foam. If the beans don't have any CO2 they won't float. This is also a sign that they have gone stale.

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  • How much CO₂? Coffee may degas significantly after roasting. Additionally, grind coarseness and may affect float as well . Not foaming and float may be a sign of staleness, but it shouldn't be considered positive without other signs as well (aroma, taste, etc). Coffee that is within a good freshness window will not always foam and float.
    – Suspended User
    Dec 30, 2015 at 20:44
  • @Chris_in_AK, I am not exactly sure how much CO2 they contain; but I agree with you, the other things are a sign also and it's not the only sign.
    – Caleb Kleveter
    Dec 30, 2015 at 21:28

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