The roasting process of coffee after picking.

Roasting is the last and most important step in the processing of coffee beans after being picked. Raw coffee beans cannot be directly used as drinking coffee. All coffee beans must be roasted before grinding and brewing.

Unroasted coffee is often called "raw coffee". Compared with roasted coffee, raw coffee has a shelf life of up to one year. Raw coffee beans are small, dense and so hard that people who are not in the coffee industry may not recognize born coffee beans. Raw coffee has a grass-like taste, which is almost out of touch with the coffee beans we usually imagine in terms of appearance and flavor. It is the roasting process that produces the unique flavor and wonderful aroma of coffee.

Generally speaking, coffee roasting is a "time-temperature" interdependent process, in which physical changes and chemical reactions occur in raw coffee. In the initial stage of baking, the raw beans gradually turn yellow and the water is gradually discharged. When the temperature rises to a certain threshold, the coffee beans burst for the first time and the size of the coffee beans becomes larger. In the second stage of baking, with the rising temperature of beans, a series of complex chemical transformations begin to appear, and the size of coffee beans continues to increase and the color becomes darker. As the temperature continues to rise, aromatic oils form on the bean surface, resulting in a second burst. Many of the ingredients extracted from brewing coffee do not exist in raw coffee at all, but are produced during coffee roasting.

Finally, it is decided when to end the baking: the coffee beans go from the baking bin to the cooling chamber to cool, allowing the coffee beans to stop the chemical reaction.

The following pictures show the color changes of coffee beans during roasting: