Why do coffee beans make crackling sounds when they are roasted?

Roasters often hear crackling sounds when roasting beans. Where does the crackling sound come from and why does it occur?

Coffee roasting refers to placing green coffee beans in a heat-resistant container and then heating them over fire, which causes a series of physical and chemical reactions inside and outside the coffee beans. In this process, various flavors of coffee, such as sourness, bitterness, and sweetness, are generated, the body and color are formed, and the green beans are transformed into dark brown original beans.

The crackling sound of coffee roasting is the result of the "first pop" and "second pop" of the roasted coffee beans. The pyrolysis process occurs when the coffee beans are roasted. Taking high-temperature resistant green beans as an example, the first pop starts at about the 7th minute and lasts for about 2 minutes. Its sound is low, indicating that pyrolysis begins, starch turns into caramel, water evaporates, and exhaust is carried out. The second pop is emitted at about the 12th minute, and the sound is fierce and sharp, indicating that cell fibers are broken, and the darker the roasting.

When the internal temperature of the coffee beans reaches 200°C, the internal steam is released and the first burst occurs. After about two minutes, the first burst of all coffee beans stops.

After the first explosion, some of the components produced begin to decompose while generating heat. When the temperature inside the coffee beans reaches about 230°C, the internal gas will be released again. During this process, gas will be generated, causing the beans to continue to expand. As the coffee beans expand, the cells are destroyed again, followed by a crackling sound. This is the so-called "secondary explosion" or "tertiary explosion".

First crack is the sound that the roaster is looking for during the roasting process. When the first crack occurs, it is particularly important for the roaster to control the firepower and time. Any adjustment will definitely affect the flavor of a bean. First crack is both a reminder and an indispensable element in the development of flavor in coffee bean roasting. Listening to the sound of the beans rolling during roasting and the density and sound intensity when they crack will help with coffee bean roasting. In addition to the physical reaction, the first crack also represents the chemical transformation of the coffee beans. The sound of the first crack of the coffee beans sounds similar to the sound of popcorn popping, and the first crack is louder than the second crack, which sounds more like the sound of Rice Krispie dissolving in milk.

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