How Should Green Coffee Beans Be Roasted? What Happens During the Coffee Bean Roasting Process?

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Coffee Roasting

Coffee beans that have undergone various post-processing treatments are not actually the black beans we commonly see, but rather yellowish or greenish. To transform these beans into the familiar black coffee beans, they must go through a very important step—roasting.

Simply put, roasting beans is like cooking the raw beans as an ingredient, but behind it lies many more complex details and delicate techniques than one might imagine. The roasting process has a considerable influence on the flavor of coffee.

Among the factors that affect the taste of a cup of coffee, raw beans account for 60%, roasting accounts for 30%, and extraction accounts for 10%. Good roasting can maximize the personality of raw beans while minimizing the appearance of defective flavors. The most crucial key is to what degree the beans are roasted. Over-roasted coffee will only highlight bitterness; while under-roasted coffee will become too acidic. Depending on the characteristics of the beans, mastering the best timing to stop roasting is quite important.

How to Roast Coffee Raw Beans?

For the same bean, different roasting rhythms will greatly affect the taste, mouthfeel, and flavor profile of the coffee.

Generally, the coffee roasting process is divided into three stages: the dehydration stage before the beans turn yellow, the browning reaction stage from when the beans turn yellow and main chemical reactions (mainly Maillard reactions) begin until the first crack, and the development stage from after the first crack until the end of roasting.

Adjusting the roasting rhythm means regulating the time and temperature rise of these three stages, as well as the temperature of the turning point.

There are too many factors affecting roasting. Uncontrollable factors such as raw beans and weather aside, other factors that require manual adjustment include: flame adjustment, whether the air damper has been cleaned thoroughly, gas usage conditions, etc. Whether roasting a bean consistently requires the roaster to record in detail every batch: bean-in temperature, bean-in weight, bean turning point, drum temperature per minute, dehydration point, first crack point, first crack time, discharge time, dehydration rate, temperature rise rate, etc. (the more detailed the better). Then correct the curve through cupping and make corrections in the next roasting session.

Example: FrontStreet Coffee's Costa Rica Mozart Roasting

FrontStreet Coffee uses Costa Rica Mozart as an example, roasted with a Yangjia 800N semi-direct flame roaster (roasting amount 500g)

Preheat the drum to 200°C, adjust flame to 160, air damper open to 3, turning point at 1'36", maintain flame; at 140°C, adjust flame to 160, air damper open to 4. At 5'20", beans turn yellow, grassy smell disappears, entering dehydration stage. At 166°C, flame adjusted down to 130, air damper remains at 4. At 176°C, adjust flame to 100, air damper unchanged.

At 8'05", dehydration completed, bean surface shows wrinkles and black spots, toasted bread smell turns to coffee aroma, prelude to first crack, at this time pay attention to listen for the sound of first crack. At 8'55", first crack begins, air damper fully open to 5, flame unchanged. Development time after first crack 1'30", discharged at 193.2°C. (Light roast)

The Coffee Roasting Process

1. Drying

When raw beans are heated, the water vapor inside the beans begins to evaporate. At about 135 degrees, the raw beans start to turn from green to white.

2. Dehydration

As heating continues, raw beans turn from green to light yellow. When the temperature reaches around 160 degrees, they emit the aroma of roasted grains. With continued heating, the beans become light brown.

3. First Crack

After dehydration completes at about 190 degrees, due to internal thermal expansion causing cell walls to rupture, the "first crack" occurs. At this time, a series of thermal decomposition reactions occur inside the beans. The caramelization reaction brings sweetness, dark brown color, and body to the coffee beans. The first crack lasts for about one and a half minutes.

4. Second Crack

As heating continues, the "second crack" occurs, during which more intense reactions happen inside the beans, releasing large amounts of heat. With the end of the second crack, the raw beans have basically turned black, the bean body expands to 1.5 times its original size, the surface becomes oily, and weight reduces to about 12-20%.

5. Stop

Generally, at the latest about one minute after the second crack ends when the temperature reaches 200 degrees, roasting is concluded. This would be a darker French or Italian roast. If heating continues when the temperature exceeds 230 degrees, the coffee beans might spontaneously combust.

The timing when roasting ends is called the "roasting point," which varies depending on the type of raw beans. Raw beans from Ethiopia and Papua New Guinea have lower density, so they absorb heat faster. If roasting to the same medium roast level, the roasting time will be shorter than other types of raw beans. Conversely, higher-density raw beans from Peru, Colombia, and Indonesia require longer roasting times, and deepening the roast degree as much as possible can bring out the best flavor of the coffee beans.

Important Notice :

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