Why do some grounds float and others sink when brewing by pour over

Question

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.

Answer 1

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.

Answer 2

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.