Coffee Roasting Process: A Perspective on Rubbery and Glassy States

Coffee Roasting Process: A Perspective on Rubbery and Glassy States

When analyzing the roasting process through the lens of material states—specifically the transition between rubbery and glassy states—we begin with the fundamental nature of heat absorption in green beans based on two core principles:

Core Principles

1. Phase of Maximum Efficiency: The most efficient heat absorption occurs before 150°C. Because the bean temperature is low, the temperature gradient ($\Delta T$) between the roaster's heat source and the beans is at its peak, maximizing heat transfer efficiency.

2. The Rubbery State Transition: As beans turn white/yellow, they enter the rubbery state. Due to the physical characteristics of this state, heat mobility within the bean slows down, leading to a natural decline in the Rate of Rise (RoR).

Application of Principle 1: Optimizing Initial Heat Intake

To monitor heat absorption, we typically use visual cues such as the expansion of the center cut and surface color. A common challenge is a mismatch between bean temperature and physical transformation (e.g., at 130°C, the center cut remains closed regardless of whether the RoR is high or low).

Recommended Strategy:

Utilize a low Turning Point (TP) combined with high heat to maximize energy intake during this window.

• Method: Charge at a high temperature with the gas off, then apply maximum heat at the Turning Point.

• Target: Reach 150°C within 4:30–5:30. At this point, the bean surface should be yellow, the center cut expanded, and a distinct toasted grain aroma should be present.

Application of Principle 2: Managing State Transitions

Heat absorption efficiency varies between the rubbery and glassy states. Beans transition into the rubbery state after 130°C and gradually move toward the glassy state after 150°C. This progression can be divided into three stages: Entering Rubbery State, Full Rubbery State, and Transition to Glassy State.

Observational Key Points:

You can physically verify these states by removing a bean and breaking it open; the hardness changes significantly with temperature.

• 150°C: Full rubbery state (softest texture).

• 150°C – 180°C: Transition phase.

• 180°C: Full glassy state. The bean becomes shriveled and the hardness increases significantly (losing the "softness" of the rubbery state).

Extension 2.1: Heat absorption efficiency slows down upon entering the rubbery state as the internal temperature gradient narrows.

Extension 2.2: The timing of the glassy state transition varies depending on the processing method (moisture distribution).

Heat Management and Equipment Specifics

After the full rubbery state (150°C), efficiency begins to rise again. However, since the bean temperature is already high, the roaster only needs to maintain a temperature slightly above the bean temperature to provide sufficient energy for the glassy transition.

• Semi-Hot Air Roasters: The drum stores significant thermal energy as the roast progresses. To avoid excessive heat before First Crack (1C), it is often necessary to reduce gas early or even turn it off.

• V-Series Roasters: These utilize high-velocity hot air that far exceeds the bean's 1C temperature, with minimal interference from drum heat storage.

The "High-to-Low" Technique:

1. Pre-150°C: Use high heat to reach the full rubbery state (yellowing) quickly.

2. Transition to Glassy State: Reduce heat significantly, bringing the RoR below 8 or even 6 before entering First Crack.

3. Airflow Note: Increase airflow after yellowing. Significant gas discharge occurs just before yellowing; maintaining higher airflow here is effective for increasing flavor cleanliness.

Conclusion

This approach effectively enhances flavor clarity and prevents excessive surface caramelization (scorching). Furthermore, by managing the transition this way, you can preserve desirable acidity even with a higher Development Time Ratio (DTR) after First Crack.