Why Is the Inside of the Bean Darker While the Outside Is Lighter, Causing an Unbalanced Flavor?

A reader asked:

Reader:
“I would like to ask about coffee roasting. I bought your book and tried following it, but I have some questions.

I’m using a Kaleido Sniper M2 roaster. I tried to control the heat after the drying phase to reach first crack at my planned target. However, the coffee came out burned inside.

From my understanding, in a typical gas drum roaster, if we push too hard, the outer surface would burn first — causing chipping or scorching.

But what I experienced was different. When I increased the heat toward the end, the inside of the bean burned, while the outside looked normal with no visible surface defects.

So I would like to ask whether this could be because the heat source of this machine is infrared ceramic, which provides heat in the form of radiation. Could it be that it burns the inside before the outside is fully cooked?

And if that’s the case, how should I think about controlling the energy?

The darker color starts from the center of the bean and moves outward toward the edges.

In the cup, I get sourness from the less-developed outer layer and bitterness from the burned inner part.

But when I adjusted the profile — instead of pushing hard before first crack, I applied more energy from bean-in and then gradually eased off toward the end — the internal burning became less severe.

It just feels unfamiliar compared to what most textbooks and online sources say, which usually explain that if you push hard, the outside should burn before the inside.”

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My reply:

In general, radiation does not transfer heat to the inside more than to the outside. In any case, the heat must pass through the outer surface first.

For cases where the outer surface is lighter in color than the inner part, we often see this when there is heat loss in the roasting chamber toward the end of the roast — for example, when airflow is increased and the overall heat drops.

When the hot air temperature becomes lower than the bean temperature, it starts pulling heat out of the bean.

The bean begins losing heat from the outer surface first.

As a result, the rate of browning development slows down. The color progression on the surface moves more slowly.

Meanwhile, the inside has not yet lost heat (or loses heat later), so browning development continues at a faster rate there.

Eventually, the inside becomes darker while the outside remains lighter, and the flavor becomes unbalanced as described.

This kind of issue can also occur with manual drum roasters. Some roasters open the airflow wide near the end of the roast. Others cut the heat in the final stage and increase airflow — perhaps to clear smoke or because they have a certain flavor idea they expect to achieve.

Reader:
“Thank you very much. I will try adjusting based on this.”