What happens to the color gradient of the beans at the beginning of the roast?

During the initial phase of roasting, as the beans are exposed to high temperatures, the color gradient decreases due to heat diffusion. Initially, the heat penetrates the outer layers of the beans first, causing them to begin the roasting process. However, the interior of the beans heats more slowly, which can create a disparity in color between the outer and inner layers.

As the roast progresses, the outer layers may start to take on a darker color due to the Maillard reaction and caramelization, while the interior remains relatively lighter and less roasted for a short period. This phenomenon creates a less pronounced gradient, as the color difference between the interiors and exteriors of the beans decreases with continued exposure to heat. Understanding this concept helps roasters manage time and temperature effectively to achieve the desired roast profile and flavor outcomes.