The Three Stages of Coffee Roasting

Coffee roasting involves applying heat to green coffee beans, causing physical and chemical changes that produce the flavor compounds we want. When the roast reaches the desired balance of acidity, sweetness, and bitterness, we stop. Afterward, the beans are ground and brewed—using various methods—to dissolve these flavor compounds (along with acidity, sweetness, and bitterness) into water, creating the beverage we know as coffee.

Green coffee beans are actually the seeds of ripe coffee cherries. Farmers process and dry these cherries to obtain green coffee beans, which have a grassy or slightly sweet, fruity aroma before roasting. During roasting, beans develop 800 to 900 different aromatic compounds, which contribute to coffee’s complex aroma and flavor.

The Three Stages of Coffee Roasting: Drying Stage, Maillard Stage, and Development Stage

1. Drying Stage (脫水期)

Green coffee beans typically contain 8–12% moisture. As soon as the beans enter the roaster, controlling moisture evaporation becomes crucial. This stage usually lasts 4–8 minutes. Proper drying is essential for accumulating enough heat energy in the beans because, after the first crack, the beans enter an exothermic (heat-releasing) phase.

2. Maillard Stage (梅納期)

Beginning at around 160°C (320°F), the beans start to smell like toasted bread or sweet butter. These aromas come from the Maillard reaction, which starts around the time the beans begin to yellow. During this reaction, reducing sugars react with amino acids to form melanoidins (“browning compounds”), hundreds of different aromas, and various volatile organic compounds.

3. Development Stage (發展期)

After the Maillard stage, the temperature continues to rise, and you’ll hear popping sounds—known as the first crack—indicating that the beans are releasing heat. Throughout the drying and Maillard stages, the beans have been building up internal heat. As the temperature increases, steam pressure and carbon dioxide accumulate inside the beans, and that stored heat is suddenly released, causing the first crack.

From the start of the first crack to the end of the roast is known as the development time. If development is too short, the coffee will be overly acidic and lose sweetness. If it’s too long, the coffee can taste dull or burnt and bitter. Thus, controlling both development time and temperature is critical. In general, the development phase makes up about 18–22% of the total roasting time.

Roast Level (烘焙度)

Roast level is one of the most important indicators in coffee roasting. The Specialty Coffee Association (SCA) promotes using infrared caramelization analysis values, the Agtron 10-level Roast Color Chart, or an Agtron Roast Analyzer to determine roast degree.

• Light Roasts have the highest acidity and often showcase floral and fruity aromas.
• Dark Roasts offer the strongest “coffee flavor,” with caramelized or bitter notes more pronounced.

During light roasting, many volatile aromatic compounds—particularly those contributing floral and fruity notes—are produced, along with organic acids like formic acid and acetic acid. If roasting continues, high-temperature degradation reactions occur. This reduces astringency, darkens the beans, and develops the characteristic roast flavors and more intense coffee notes.

Roasting Time (烘焙時間)

While roast level has the greatest impact on flavor, total roasting time and each stage of the roast are all interrelated. A fast roast typically produces more aromatic compounds—such as stronger fruit, floral, or nutty notes—because these compounds develop more intensely during the early stages.

However, not every coffee bean is suited to fast roasting. For instance, a fast roast often preserves a bright acidity, which many people prefer to tone down in espresso. Slowing the roast allows more time for organic acids to break down, reducing the perceived acidity.

In recent years, roasting methods have become more flexible. For light-roast beans, a quicker roast can maximize aroma while maintaining lower body. By fine-tuning the approach, it’s possible to achieve a balanced acidity with plenty of sweetness. On the other hand, for espresso roasts, extending the development stage can help reduce acidity while enhancing body and overall balance.