During coffee roasting, at what stage does the glass transition occur?

The correct stage for the glass transition to occur during coffee roasting is when the beans reach the "leathery" state. At this point, the beans undergo significant physical and chemical changes, transitioning from a solid-like state to a more fluid state. The glass transition is a critical phase during which the starches and other carbohydrates within the beans start to soften. This softening is essential for subsequent development of flavors and aromas as roasting progresses.

In the "leathery" state, you begin to notice the beans transforming in texture, which indicates that the moisture content is changing and the structure of the beans is becoming more pliable. This state is crucial before reaching the first crack, as it prepares the beans for the expansion and transformations that occur with thermal application.

Understanding this transition highlights the importance of monitoring temperature and time during roasting, as it directly influences the quality and characteristics of the final coffee product.