Coffee beans undergo different changes at each roasting stage. After absorbing heat, raw coffee beans start to turn from green to yellow, and their moisture content begins to decrease. The heat converts the internal moisture of the coffee beans into steam → carbon dioxide is produced inside the beans → internal pressure reaches 25 bar (a unit of pressure) → first crack: the coffee beans produce a highly characteristic cracking sound during roasting.

Flavor development: the longer the roasting time, the more the acidity gradually decreases and bitterness increases.

After dehydration, the coffee beans begin to release heat and turn brown, shedding a layer of silver skin. Roasted coffee beans increase in volume by 1.5 to 2 times, and their weight decreases by at least 11% at this point.

　 Continuing to roast the coffee beans will keep producing carbon dioxide → second crack.

The coffee beans darken in color and enter the medium-dark roasting stage. By this point, the weight loss reaches 22% of the original. After the second crack, the coffee beans surface oil due to pyrolysis reactions and begin to carbonize. Without careful attention, they can easily burn.

During the drying stage, three to four types of aromas begin to develop. The aroma and flavor of coffee are released through two chemical reactions:

· Maillard reaction: when the moisture content of the coffee beans is less than 5%, the chemical change caused by the degradation of proteins from sugars and amino acids.

· Caramelization reaction: the chemical reaction occurring between water, sugar, and sucrose.

As the roasting time increases, acidity gradually decreases, bitterness increases, and many aromatic molecules begin to volatilize.

By the end of roasting, nearly eight hundred kinds of aromatic compounds are developed, and the flavor, acidity, sweetness, and body of the coffee beans reach maturity. However, roasting can sometimes also bring out flavors that are not well-received. In the final stage, aromatic compounds are destroyed, replaced by bitterness, with acidity completely gone and body reduced.

What are the taste characteristics of light roast, medium roast, and dark roast coffee beans?

Generally, light-roasted beans highlight lighter and crisper flavors, with very rich flavor expressions such as floral notes, fruity acidity, and certain tea-like characteristics. Medium roasts offer a more balanced profile, combining refreshing flavors with a heavier mouthfeel. Dark-roasted beans emphasize their texture more, often featuring richer flavors like smoky, chocolatey, or woody notes. They also come with a heavier mouthfeel.

The classification of these three roasting methods is just a very rough division. In reality, factors such as bean origin, variety, and the roaster’s style all influence the degree of roast. This answer is only a general explanation; baristas adjust the roasting curve based on the characteristics of each bean. A one-degree difference in dropping time or a two-second difference can result in vastly different flavors.