How much green coffee should you load in the drum?
Every drum roaster has a stated capacity. It's tempting to treat it as the amount to roast. In our experience it's closer to the most you can physically cram in, and the best batch size usually sits below it.
Our own story
For the first months we loaded 2 kilos of green coffee for every batch. The roasts came out fine. Then, after a lot of experimenting and some reading (ChatGPT included), we tried less. We found that 1.7 kilos was the most we should charge, and that the difference showed up in the cup.
The reason, as we understand it: at first crack the beans swell and release a lot of gas and steam. If the drum is packed, the beans can't move or expand freely, and the gas has nowhere to go. Development is uneven. With a bit of empty room the beans tumble, heat evenly and let the gas out.
This is one machine and one roastery's experience. Yours may behave differently, and we can't tell you that 85% of capacity is right for you. We can tell you how to find out.
How to test it
Pick one coffee and one roast level, and keep everything else the same: same charge temperature, same profile, same drop point. Roast it at your usual load, then at about 10% less, then at 20% less.
Cup the batches together. Along with the taste, compare:
- the total time (a lighter load should run a little faster)
- the rate of rise curve, which gets smoother with less in the drum
- the weight loss, so you know you're comparing like with like
If the smaller batch tastes sweeter or cleaner, you've found something. If you can't tell the difference, the bigger load is fine and you've saved yourself nothing but time.
What smaller loads cost you
Less coffee per roast means more roasts per day for the same output, and more roasting hours. If you are selling everything you make, that's a real cost. Do the sums: sometimes a slightly smaller batch with a better cup is worth it, sometimes the better answer is to roast a bit more at the old load and sell it as a cheaper blend.
Things that change the answer
The coffee. A dense, small-beaned Ethiopian behaves differently from a big, soft Brazil. The bean's bulk density changes how much volume it takes up.
Your airflow. More airflow can compensate for a heavier load, up to a point.
Your heat source. A weaker burner struggles harder with a full drum. That's closely related to roasts that take too long.
Whatever you decide, write the charge weight down on every roast. It's the first thing you'll want when two batches of the same coffee taste different a month apart.