What characterizes the "leathery" state in coffee roasting?

The "leathery" state in coffee roasting refers to a specific phase where the structure of the coffee bean undergoes significant changes due to heat. This is linked to the concept of the glass transition, which is a state that allows the internal structure of the beans to be more malleable. During this phase, the hard and brittle characteristics of the beans soften, and they start to change from a solid state to a more pliable one, similar to leather.

This transformation is particularly important as it allows the materials inside the coffee beans, such as carbohydrates and oils, to begin to shift and meld, leading to the development of flavors and aroma that are integral to the final cup of coffee. The glass transition is crucial for flavor development during roasting, as it promotes the release of volatile compounds and aids in the complex reactions that produce the rich tastes and aromas expected in roasted coffee.

In contrast, the other options primarily focus on physical characteristics or states that do not define the leathery phase accurately, such as brittleness, moisture loss, or surface appearance like glossiness. The essence of the leathery state rests in the structural remodeling facilitated by the glass transition, which allows the progression toward the desired roasting levels.