I was wondering if any of you roasters routinely take bean density readings and if you build your roast profiles around the density data you collect. The way I calculate density is by weighing out a volumetric cylinder filled with beans and dividing it into the volume of the cylinder, thereby giving me a kg/hl reading. I’ve been logging these readings along with bean descriptions (elevation, center cut depth, size, etc.) and moisture content for all the coffees I’ve been roasting and trying to coorilate my roasting approaches to that data. Density, while just a single variable, is kind of an interesting one to me because it affects both displacement in the drum and heat application during the roast, so I think about it a lot.
Recently I was thinking about how moisture content and density relate and got to reconsidering my approach of how I calculate bean density. The higher the moisture content, the higher the perceived density of the bean would be, right? So, moisture content would thow all my density data? What if I subtracted the moisture content percentage from my bean weight and then calculated the density from the adjusted weight? I feel there is a lot out there for me to learn about roasting still, but I’ve never heard anyone consider subtracting the moisture weight out. Does anyone does this? What are your thoughts?
What you are measuring is not bean density (which is the mass of a bean divided by it’s volume), but rather bulk density - which includes the void space (pore space) between beans.
Bulk density is as much dependent on the shape of the cylinder and how you’ve put the beans into it, as it is on the shape, size and density of the beans. Consider that the bulk density of small low density beans could be the same as that of larger, more dense beans. Likewise, beans of variable size / shape will typically have greater bulk density than uniformly sizes beans, assuming the same bean density.
That said, bulk density might still be a useful measurement when coupled with bean density, as both density and size will influence roast dynamics.
One way to determine the bean density (if you don’t mind wasting some beans) is to fill the void space with a liquid (oil or water - something not absorbed into the beans). This will allow you to determine the volume occupied by the beans (by using the liquid to determine the volume of the void space), and thus their mean density.
Hi Troy,
Thanks for the reply! Yes, you’re right, I am taking bulk density readings as opposed to actual density readings, which I should do! Do you think moisture content should still be considered for actual density readings?