When the green beans are heated in a coffee roaster, 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. This is the stage where you can find some coffee roasters for sale online, as they are suitable for light roasts.
- First crack
After the dehydration at about 190 degrees is completed, the cell wall ruptures due to internal heat expansion to form a “first crack”. 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. This is the stage where you can use a best home coffee roaster to achieve a medium roast, as it gives you more control over the temperature and time.
- Second crack
As the heating progresses, it enters the “second crack”, 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 crack, 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%. This is the stage where you can use a probat coffee roaster or an electric coffee roaster to achieve a dark roast, as they have more power and efficiency.
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. This is the stage where you need a commercial coffee roasting machine to handle large batches of beans and prevent fire hazards.