Coffee Roasting

Coffee roasting involves applying high heat to green coffee beans, inducing chemical and physical transformations that enhance their aromatic qualities and flavors, preparing them for brewing.

1. Stages of Coffee Roasting

(1) Dehydration Stage

When green coffee beans enter the roaster, the bean temperature initially drops. In this stage, moisture within the beans (known as free water) significantly reduces, preparing beans for subsequent caramelization and Maillard reactions.

(2) Heat Ramp Stage

After dehydration, the beans become slightly softer. At this point, increasing the heat rapidly is crucial. If you don't sufficiently raise the temperature now and try to compensate later, the beans can become overly carbonized and bitter.

(3) First Crack and Temperature Reduction

Around 180°C, sugars within the coffee bean cells begin caramelizing (caramelization reaction), while amino acids interact with carbohydrates, initiating the Maillard reaction. These reactions create significant amounts of carbon dioxide and steam, causing the beans to audibly "crack." Bean volume increases, density decreases sharply, and the roasting process transitions from endothermic to exothermic (heat-releasing).

(4) Second Crack and Fine-tuning Stage

About 1 to 2 minutes after the first crack ends, at approximately 210°C, the roast becomes endothermic again. The beans enter a second crack, signaling the transition to medium-dark roasts. Over 60% of organic acids degrade, decreasing acidity, and aromatic components begin to volatilize.

(5) Cooling Stage

The beans should be cooled rapidly to around 170°C within 2 to 3 minutes. Delays in cooling, especially beyond 5 minutes, can seriously degrade bean quality.

2. Changes in Coffee Beans After Roasting

• Weight Loss: Beans lose approximately 12–21% of their weight (moisture content dropping from around 12% to 1%).
• Volume Expansion: Bean size increases by roughly 60%.
• Formation of Pores: Internal structure becomes porous.
• Generation of Carbon Dioxide (CO₂): Beans produce CO₂ as a by-product.
• Chemical Changes: Significant alterations in sugars, aromatic compounds, and organic acids.

3. Levels of Coffee Roasting

• Very Light Roast: Toward the end of the first crack, intensely acidic.
• Light Roast: Completion of first crack, strong acidity.
• Medium-Light Roast: About 30–40 seconds after the first crack, moderate acidity.
• Medium Roast: Between the end of the first crack and before second crack, bean surface is dry, acidity decreases.
• Medium-Dark Roast: 20–40 seconds into the second crack, roasted aroma emerges, slight oil droplets appear.
• Dark Roast: 40–100 seconds into the second crack, frequent cracks, noticeable oil on beans.
• Southern Italian Roast: About 100 seconds into the second crack, extensive oil on the surface.
• French Roast: After the second crack finishes, smoke changes from white to bluish tones.

4. Types of Heat Transfer in Roasting Machines

• Direct Fire: The roasting drum has small holes, allowing flames to directly touch bean surfaces, which can easily cause charring.
• Semi-Direct Fire: Drum surface has no holes; instead, small holes inside direct hot airflow, assisting in heat transfer without direct flame contact.
• Hot-Air Roasting: Utilizes high-velocity heated airflow within the drum, providing efficient and uniform heat transfer.