Aillio Bullet R1 / R2 咖啡烘焙中的乾燥階段

The drying phase in coffee roasting is often treated simply as the period when moisture leaves the beans before yellowing.

On the Aillio Bullet R1 / R2, however, this stage also establishes how heat begins moving through the coffee’s different layers. In this article, I explore the relationship between heat transfer, moisture release, yellowing, and RoR, and explain why drying time alone cannot tell us whether the deeper layers of the bean have received the heat they need.

The period before yellowing is commonly referred to as the drying phase and is usually understood as the stage in which coffee beans release moisture. This description is not wrong, but if we focus only on moisture loss, it becomes easy to overlook what is actually important during this period: thermal energy is beginning to enter the different layers of the coffee bean, while moisture movement is only one part of that process.

After the coffee is charged into the roaster, the outer layers are the first to come into contact with the hotter environment and absorb thermal energy. As the temperature of the outer layers rises, moisture begins to move and gradually leave the bean. At the same time, thermal energy must continue passing through the outer layers and moving toward deeper regions. These two processes are not separate from each other. Moisture has a major influence on heat transfer inside the coffee bean, so the drying phase cannot be understood only in terms of how much water has been removed.

From my perspective, the drying phase is better understood as the stage in which the conditions for subsequent heat transfer are gradually established. The thermal energy applied during this period affects not only how quickly the bean surface turns yellow, but also whether heat can continue moving into the middle layers and the core.

If insufficient heat is applied early in the roast, the surface may still turn yellow over time, while the deeper layers may not accumulate enough thermal energy. If too much heat is applied, thermal energy may become overly concentrated in the outer layers: the bean surface may become excessively hot before the interior has been sufficiently affected by heat.

Comparing drying time alone cannot tell us whether this part of the roast was handled well. The same yellowing time can result from completely different heat-transfer histories. One batch may use sufficient and continuous available heat to move thermal energy progressively into the deeper layers, while another may simply reach a similar color more quickly through a higher bean-surface temperature. The same amount of time does not mean that the inside of the coffee bean received the same amount of thermal energy.

What I truly pay attention to during the drying phase is whether Gain and Loss remain in balance. Once they become unbalanced, the RoR can easily crash. If you have ever roasted coffee outdoors, you may have noticed that the smoke becomes progressively heavier as the roast continues.

In my experience, larger amounts of water vapor tend to be released when the IBTS reaches approximately 130–140°C and again around 170–180°C. These are also points at which a crash is more likely to occur. If a crash happens, the roast plan needs to be reconsidered.

My IBTS is the first-generation Aillio model, and its readings are usually around 6°C higher than those of the latest IBTS.

In “Pay Attention to RoR While Thinking About Gain and Loss,” I discuss five typical RoR fluctuations. Each represents a different Gain-and-Loss condition and may lead to a different flavor result. I recommend reading that section.

Although Rob Hoos notes that there are “no real significant chemical reactions happening” during the drying phase, this stage lays the foundation for everything that follows. How much water is removed and how deeply heat reaches into the bean determine how much water is available for the Maillard reaction and how many layers of the coffee can actively participate in it.