Coffee Roasting Explained by James Hoffmann
The Basics of Coffee Transformation
Raw green coffee beans have virtually no aroma or flavour; all the familiar characteristics are created during roasting. The roasting process affects three main flavour characteristics:
- Bitterness: Increases the longer the coffee is roasted (the darker the colour), similar to caramelising sugar.
- Acidity: Initially increases, but then begins to decrease as the roast gets darker.
- Origin Characteristics: These are the unique flavours of the specific bean (based on soil, climate, processing, etc.). These flavours are created early in the roast, but the longer the roast progresses, the more these unique characteristics are replaced by a generic "roast taste" . This is why specialty coffee, which is interested in unique flavours, tends to favour lighter roasts.
The Roasting Process in Steps
The roasting process can be broken down into stages based on what's happening inside the bean:
- Drying Phase (Initial Minutes): The roaster must first dry out the green coffee, which has a lot of moisture, before the browning reactions can properly take place. The beans change from green to pale, then yellow.
- First Crack: As heat is applied, built-up steam and gases like CO2 create pressure inside the bean. "First crack" is the point when this pressure forces the bean to crack open, sounding like popcorn. This is a key moment for roasters, as the browning reactions become very rapid and complex afterward.
- Progression to Darker Roasts: As the roast continues, the beans get darker, less wrinkly, and smoother.
- Second Crack: If roasting continues, a second, less audible release of gases occurs, called "second crack" . At this point, oils may begin to appear on the surface of the bean, as internal pressures force them out.
Roast Levels and What They Mean
The terms light, medium, and dark can differ between specialty and commercial coffee:
| Roast Level | Specialty Coffee Definition | Commercial Coffee Definition |
|---|---|---|
| Light | From the earliest drinkable stage up until the beginning of the medium stage. | Starts around the same point but runs a little further in. |
| Medium | Begins after light, running up until any sign of oil appears on the surface. | Starts slightly later than specialty medium, running a little further in. |
| Dark | Begins the minute oil appears on the surface, extending to very dark roasts. | Starts slightly later than specialty dark, and often roasts even darker. |
How Roasters Control Quality
Roasters aim for consistency in flavour. Since modern expectations demand coffee be shipped the same day it's roasted, roasters rely on technical measurements instead of waiting to taste every batch.
- Roast Profile: Using multiple temperature probes and software, roasters track the bean temperature and the rate at which it's heating up, making small adjustments to the gas burner and air flow to achieve a specific curve.
- Roast Loss: Roasters track the percentage of weight lost during the process (mostly water). An unexpected loss percentage signals that the roast did not go as planned.
- Colour Testing: A machine measures the brownness (lightness/darkness) of the ground coffee, often using a standard like Agtron. This ensures the color falls within an acceptable window of quality.
Roasting Equipment
The type of machine influences how heat is transferred to the coffee:
- Drum Roaster: The most common in specialty coffee. Heat is transferred three ways:
- Convection: Hot air flows through the coffee (the primary method).
- Conduction: The beans touch the hot, spinning metal drum.
- Radiant Heat: Heat radiates from the hot metal drum.
- Hot Air Roaster: Uses a separate heat source and vast amounts of hot air to roast and agitate the coffee (no spinning drum). Heat transfer is primarily convection.
- Hybrid Air Roaster: Uses a rotating drum to tumble the coffee, but the heat source is separate and only heats the air, not the drum itself.