Troubleshooting Common Roast Defects

Defects from the start of the roast

Scorching appears as dark, burned patches on the flat face of the bean. It comes from charging into a drum that is too hot, so the surface burns before the interior has begun to heat. Drop your charge temperature and give the machine time to stabilise before loading.

Tipping shows up as small dark craters at the ends of the bean, where the tip scorches first. It is also a charge temperature problem, made worse by very fast early drum speeds. A gentler start fixes both.

Defects during the middle

Baking is the hardest to see and the easiest to taste. The bean looks fine but the cup is flat, papery and lifeless. It comes from a prolonged, too-gentle middle phase - often from cutting heat too aggressively after the turning point and stalling the roast.

A stalled or flicked rate of rise shows as a sudden reversal in the curve. A stall flattens the development of sweetness; a flick - a sharp late spike in rate of rise - scorches the bean surface at the worst possible moment. Both usually trace back to an over-aggressive heat change.

Uneven roast colour across beans usually means the batch is too large for the airflow available, or drum speed is too low for the charge size.

Defects at the end

Underdevelopment is the most common defect in home roasting. The cup tastes grassy, bready, or sharply sour, and the bean interior is visibly lighter than the surface when you break one open. Extend development time or raise your drop temperature.

Charring at the drop appears on the bean surface as an oily, acrid coating after a runaway finish. The rate of rise was too high in the last minute or two. Cut heat earlier and coast to your drop temperature.

Faded or hollow flavour with plenty of roast character usually means overdevelopment - past the point where origin character survives. Shorten the development phase.

A diagnostic habit worth building

Break a bean open after every roast and compare the surface colour to the interior. An even colour throughout means even development. A pale centre in a dark bean means underdevelopment regardless of what the timer said.

Taste at least a small cup of every batch, even the ones you expect to be bad. Defects are much easier to recognise on the palate once you have deliberately tasted one.

Change one thing. If you adjust charge temperature, airflow and drop time together, you will not know which change produced the improvement - and you will not be able to reproduce it.