On Food and Cooking

progressively higher temperatures, the
proteins, sugars, phenolic materials, and other
constituents begin to break into molecular
fragments and react with each other, and
develop the brown pigments and roasted
aromas typical of the Maillard reactions (p.
778). At around 320ºF/160ºC, these reactions
become self-sustaining, like a candle flame,
and extreme molecular breakdown generates
more water vapor and carbon dioxide gas,
whose production rises sharply at
400ºF/200ºC. If the roasting continues, oil
begins to escape from the damaged cells to
the bean surface, where it provides a visible
gloss.
When the beans have reached the desired
degree of roast, the roaster cools the beans
immediately with cold air or a water spray to
quench the molecular breakdown. The result
is a brown, brittle, spongelike bean, with the
holes in the sponge filled with carbon dioxide.