When the green beans are heated, the water vapor in the green beans will evaporate, and the green beans will turn from green to white at about 135 degrees.
As the heating progresses, the green beans turn from green to light yellow. When the temperature reaches about 160 degrees, the aroma of roasted grains will be emitted. With continuous heating, the beans will turn light brown.
3. First crack
After the dehydration at about 190 degrees is completed, the cell wall ruptures due to internal heat expansion to form a “first explosion”. At this time, a series of thermal decomposition reactions will occur inside the beans, and the caramelization reaction will bring the sweetness, dark brown, and alcohol of coffee beans The first crack will last for about one and a half minutes.
4. Second crack
As the heating progresses, it enters the “second explosion”, at which time a more violent reaction will occur inside the beans. And it emits a lot of heat. With the end of the second explosion, the green beans have basically turned black at this time, the body of the beans has expanded to 1.5 times the original size, the surface is oily, and the weight has decreased to about 12% – 20%.
Generally, the roasting ends when the temperature reaches 230 degrees at the latest about one minute after the end of the second crack. At this time, it will be a darker French or Italian roast. When the temperature exceeds 230 degrees and continues to heat, the coffee beans may spontaneously ignite.