Where Do the Crackling Sounds in Coffee Roasting Come From?

 Roasters frequently hear crackling sounds when roasting beans. Where do these crackling sounds originate, and why do they occur?

Coffee roasting is the process of placing green coffee beans in a heat-resistant vessel and applying heat, which causes a series of physical and chemical reactions both inside and outside the bean. During this process, various flavors, such as acidity, bitterness, and sweetness, are generated, and the coffee's body (mouthfeel) and color are developed, transforming the green beans into dark brown roasted beans.

The crackling sounds in coffee roasting are the result of the pyrolysis process occurring during the coffee bean's "First Crack" and "Second Crack." Taking a heat-resistant green bean as an example, the First Crack begins around the 7-minute mark and lasts for approximately two minutes. This sound is low and dull, signifying the onset of pyrolysis, the caramelization of starches, moisture evaporation, and the release of internal gases. The Second Crack typically occurs around the 12-minute mark, producing a violent, sharp sound, which indicates the fracturing of the cellular structure as the roast progresses deeper.

The First Crack occurs when the internal temperature of the coffee bean reaches 200℃, causing the release of internal steam. The First Crack for all the beans typically ceases after about two minutes.

Following the First Crack, certain newly formed components begin to decompose while simultaneously generating heat. When the internal temperature of the bean reaches approximately 230℃, internal gases are released again. This gas generation causes the bean to further expand. Along with this expansion, the cell structure is fractured once more, immediately followed by another cracking sound. This is known as the "Second Crack" (and rarely, a "Third Crack").

The First Crack is the sound roasters intently look for during the roasting process. Once the First Crack occurs, the roaster's control over heat and time becomes exceptionally important, as any adjustment can definitively affect the flavor profile of the batch. The First Crack serves as both a reminder and an indispensable element for flavor development, and it involves not only physical reactions but also chemical transformations within the coffee bean. Listening to the sound of the beans rolling and the density and intensity of the cracking sound helps guide the roast. The First Crack sounds similar to popcorn popping, and it is generally louder than the Second Crack, which is often described as sounding more like Rice Krispies cereal dissolving in milk.


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