What is necessary for the beans to swell during the roasting process?

During the roasting process, the swelling of coffee beans is primarily due to water evaporation and the glass transition phenomenon. As the beans heat up, the moisture inside them begins to evaporate. This evaporation creates steam, leading to an increase in pressure within the beans, which causes them to expand or swell.

The glass transition refers to the change in the physical state of the beans as they heat, where components within the beans, such as starches and sugars, transition from a solid state to a more pliable, plastic-like state. This transition allows the coffee beans to expand further as the heat and pressure rise, ultimately contributing to the desired development of flavors and aromas during the roasting process.

In contrast, consistent airflow can be important for even roasting and for removing smoke and chaff, but it is not the primary factor causing the beans to swell. High humidity can negatively affect the roasting process by introducing unwanted moisture at the wrong stages. Low temperatures would not facilitate the necessary conditions for swelling, as the reactions that lead to evaporation and glass transition require sufficient heat. Therefore, the correct answer highlights the essential roles of water evaporation and glass transition in the physical changes that occur in coffee beans during roasting.