Leaf through the studies on roasting and the stages are all there: light, medium, dark. A small table on paper looks far more certain than the words on a packet. Put the two together, and it seems possible to say what temperature dark roast means.
This piece looks at whether that single table can be made, starting from how each study built its stages.
Only the names line up
Lay several reports side by side and each has its light, medium and dark. Even so, no table can match those stages to roast levels across shops.
It is like measuring a mountain from a different valley each time. What can be built is the order of light and dark within one shop's range. On the research side, too, each study defines its stages differently.
The sources go as far as this: what lines up is the names of the stages. After the names comes the question of where the heat was measured.
Measuring heat in different places
Around a roasting machine the heat gathers unevenly, and the reading depends on where the probe sits. Temperatures cannot be lined up from one study to the next.
One prefectural station measured the air leaving the machine. A national institute measured the surface of the leaf, and another prefecture's study the inside of the drum. The same number of degrees points to a different place each time.
The sources show that the figures cannot simply be compared as higher or lower. After where the heat was read comes how the stages were made.