External changes of coffee beans during roasting

In the first few minutes of putting the coffee beans into the roasting chamber, we don't see much change in appearance. At this time, the coffee beans are still gray-green and there is no sound. After a while, the beans will gradually turn yellow and give off a bag-like smell, followed by a steam smell like toast or shells.

Between 2 and 15 minutes of roasting (depending on the amount of beans roasted and the intensity of the roasting heat), this neat smell becomes heavier and more coffee-like, and later you hear the "First Crack/Crack/Popping" sound of the coffee beans!

This phenomenon, which we call the "first explosion", represents the true beginning of the transformation of the components inside the coffee beans, that is, the beginning of "pyrolysis", when the sugar inside the coffee beans begins to "caramelize"(Caramelize), and the moisture originally locked in the coffee beans will also begin to volatilize with the carbon dioxide gas. At this stage, the static water in the coffee beans changes to a gradually greasy roasting smoke that continues to grow thicker.

The deeper the roasting of coffee beans, the deeper the degree of coloring, resulting in this phenomenon is the main cause of caramelization, as well as certain acids caused by thermal transformation. Roasting can be stopped at any point after pyrolysis begins: initially, the beans appear very light brown; towards the end of roasting, the beans appear very dark brown, dark to near black. For those with a more sensitive sense of smell, the smell of baking smoke also changes slightly as the baking pattern deepens.

As the beans approach a medium brown in appearance, the relatively faint first pop fades away until there is no pop at all. After that, the beans are heated to a deeper roasting mode, and a second violent cracking period begins, called the Second Crack. The reason for the second explosion is presumed to be that more volatile substances rush out quickly, resulting in the breakage of the wood structure of coffee beans; when the second explosion becomes more and more dense, the roasting smoke becomes more and more concentrated, and the appearance of coffee beans at this time is darker, the smell is more pungent, and the beans are larger!