By Harold McGee
Coffee beans are roasted to temperatures between 375 and 425°F/190–220°C; the process usually takes between 90 seconds and 15 minutes. As the bean temperature approaches the boiling point of water, the small amounts of moisture inside the cells turn into steam and puff the bean up to half again its original volume. Then at 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. At around 320°F/160°C, 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 400°F/200°C. If the roasting continues, oil begins to escape from the damaged cells to the bean surface, where it provides a visible gloss.