QUESTION: I use a burr grinder that has automatic settings (so it grinds for a predetermined amount of time, depending on the setting). I measure my beans by weight, though. I have noticed that there are, at times, significant differences (like 15%) in the volume of bean that can be ground in a set time. Clearly, different coffees have different weights by volume, why is this? -Nathan D.
ANSWER: Oh man, Nathan, you made my day with this question. Sometimes I feel all alone on the Island of Nerd when I am making observations about variations in coffees. Now my deserted island has a second member. I’m hoping that there are some other, underground inhabitants who will find this Q&A interesting. Keep the great thoughts & questions coming.
First, good on you for using a burr grinder. You’ve taken the first step toward great coffee. Second, brewing by weight instead of volume—be still my heart! This actually gets to the heart of your question. The reason we encourage brewing by weight rather than volume is that different coffees have different densities. As such, equal volumes of ground coffee will not translate to equal brew potency. Equal weights, however, will.
The variance in volume/density is owing to a couple major factors: coffee varietal and roast depth. Arabica coffee is akin to roses in many respects, bushes need the proper elevation, soil, climate, and pruning. They’re prone to a variety of insects, molds, mildews, and diseases. Also, just like roses, coffee has dozens of varietals or subspecies. Several of these factors have an impact on bean density—not the least of which are elevation & varietal. So I get green (unroasted) coffee at variant densities, and roasting doesn’t make them equally dense. In fact, it exacerbates the differences. The second major factor in roasted bean density is roast depth or darkness. The simplest way to present it is that the darker a coffee is roasted, the less dense it is. Next time you have a chance to see a lightly roasted coffee next a dark roast, grab one of the light beans and pop it in your mouth & chomp it. You’ll notice a sharp crack as you crush the bean with your teeth. You’ll also notice significant resistance; the bean is dense! Then grab one of the dark beans and do likewise. You’ll find a much more dull crunch and an obviously less dense bean.
Therefore, when you purchase different coffees week to week, you get roasted coffee at varying densities. Different bean sizes & densities will travel through your grinder at different rates. Therefore, I wouldn’t trust the timer dial. Weighing your coffee is the best way to ensure equal brew potency among different coffees.