Why is my roast taking so long?

Every roaster has a window in which the machine does its best work. For a small drum roaster it's often somewhere around 10 to 17 minutes, depending on the machine and the style of roast. When batches keep running well over, the coffee usually tastes duller than it should, and it's worth figuring out why.

Our own story

On a machine like ours a roast should take 15 to 17 minutes at the very most. In practice we were nearly always over 20, somewhere between 20 and 25. We never proved the cause, but our best guess is the gas. We run on mains gas, and the pressure that reaches the burner may simply not be what the machine was designed for.

That's a guess. We'd tell you how we'd check it, rather than present it as an answer.

Why a long roast is a problem

The beans spend too long in the middle of the roast, where browning happens. That tends to produce coffee that's muted: lower acidity, less sweetness, a flat or bready note. The development you want after first crack also gets squeezed, because everything is spread thin.

Things to check, cheapest first

The charge. If you're loading near the machine's limit, the burner has too much mass to heat. Try a smaller load before anything else. See how much green coffee to load.

The charge temperature. If the drum isn't hot enough when the beans go in, the roast starts behind. Let the machine preheat longer, and note the temperature each time.

Airflow. Too much air pulls heat out of the drum. Too little and the beans don't dry well. Try reducing the damper a little in the first half and see what changes.

The gas. Check that the supply is at the pressure the machine expects. If you have a gauge on the line, look at it during a roast: the pressure can drop when you run other gas appliances. A gas fitter or the machine's manufacturer can tell you what's normal for it. Don't adjust regulators yourself without someone qualified.

The burner. Dirty jets and clogged burner holes lower the output. Clean it and compare.

Ambient conditions. A cold, damp shop in winter makes a different roast from a warm one in summer. Log the room temperature if you can.

How to know it worked

Compare your logs before and after one change. Look at the total time, but also at the rate of rise: it should start higher and glide down more smoothly. And taste it. A shorter roast that tastes worse isn't an improvement.

Change one thing at a time. If you adjust five and it gets better, you won't know which one mattered, and you'll be back in the same place next time something shifts.