What is the primary factor in heating the internal structure of coffee beans?

The correct choice focuses on internal conductivity as the primary factor in heating the internal structure of coffee beans. During the roasting process, heat needs to penetrate the beans effectively to ensure an even roast throughout. Internal conductivity plays a crucial role here, as it refers to the ability of the coffee bean to transfer heat from the exterior to its interior. This is vital for achieving the desired roast profile and flavor development.

High internal conductivity means that heat can move quickly and efficiently through the bean, allowing the internal temperature to rise appropriately. This is particularly important because the chemical reactions that develop the flavors, aromas, and colors of the roasted coffee occur within the bean's interior, making internal heat transfer a critical component of the roasting process.

While factors such as external temperature control, moisture content, and bean density can influence roasting outcomes, they do not directly relate to how effectively heat transfers through the bean itself during roasting. Thus, internal conductivity stands out as the primary factor in heating the internal structure of coffee beans.