Roast coffee beans and small cracks start to pop. Hojicha and coffee both speak of light and dark roasts. A shared name, though, does not mean the same temperature or time.

What follows uses a coffee roaster's manual as a guide to reading change during a roast: the clock, the sound, the colour, and where a temperature is measured.

Three overlapping times

Open a roaster's manual and reference times for light, medium and dark roasts are lined up. Those ranges overlap one another.

In the manual for one home coffee roaster, the time ranges for the three roast levels are staggered and overlapping. Twelve minutes, for instance, falls inside all three. The table is not a chart saying a given minute means a given roast.

Even within one machine's instructions, elapsed time alone does not settle the name of the finish. The record supports only what can be read from the table itself. After the clock comes the sound from the beans.

What the crack announces

Partway through the roast, the beans start to crack and pop. That sound announces a change happening inside the bean.

During roasting, a first crack is heard, then a second, according to one account. Gas builds up inside the bean and it pops as it swells, says another. The change in the bean is caught by ear as well as by surface colour.

The clock shows how long heating has gone on; the sound tells what is happening to the bean. They are separate clues. The record supports only what holds for coffee beans. After the beans comes the tea side.

Stems changing colour

As tea stems roast, their colour shifts from a pale green toward a reddish tone. In hojicha, the crack of a bean cannot serve as the signal to stop.

Tea uses leaves or stems; coffee uses beans. The crack is a marker of change in coffee beans, and it cannot simply be carried over as the cue to stop roasting tea. Research on the tea side has tracked stem colour moving from yellowish green to a reddish yellow as roasting goes on. Stems roasted hot and briefly were also seen to swell.

Where beans are read by sound, this research used colour and shape as clues. The record supports only what was seen in stem tea. After the colour comes where a temperature is taken.

Whose temperature is it?

A temperature glows small in the machine's display. The same number does not mean the same condition if it was taken somewhere else.

A preheating temperature, the temperature of the drum's inner wall and the surface temperature of the material belong to different places or stages. Like time, a temperature has its context. When a source gives a temperature, look with it for the machine, the point measured, and whether it is preheating or mid-roast.

What is not written down is not guessed at, and what is unknown is left uncompared. That way of reading carries over to hojicha machines and research papers. After temperature come the names light and dark themselves.

Same name, different making

Both tea packets and coffee bags carry the words dark roast. The same words do not point to the same way of making.

The times quoted above are one machine's guide, not settings for other coffee equipment or for hojicha. To compare, first follow each on its own terms: what material, what machine, how the finish is judged. Behind light and dark, separate clues then appear: the clock, the sound, where temperature is measured, and the change in the material.

The record supports only what the numbers explain and what they leave unexplained. The machine and the research are named below.

The numbers in detail

Reference times
from the instruction manual for FUJI ROYAL's Coffee Discovery. Light roast about 10–13 minutes, medium about 11–14, dark about 12–15.
Overlap
12 minutes falls inside all three ranges, light, medium and dark.
The cracks
the account of a first crack followed by a second is Iwatani's.
Why it cracks
the account of gas building inside the bean and popping as it swells is FUJI ROYAL's.
The tea study
a paper in the journal of the Japanese Society for Food Science and Technology, using stems of first-flush Yabukita tea from Kagoshima Prefecture.
Colour and shape
roasting turned the stems from yellowish green to yellowish red, and samples roasted hot and briefly showed the stems swelling.