What are the Basics of Coffee Roasting? How to Judge the Process and Color Changes of Coffee Bean Roasting

The Five Stages of Coffee Roasting

Coffee roasting can be initially divided into five stages:

Warm-up Phase

Preheating the coffee roaster's drum to the starting temperature for roasting, known as the "charging temperature."

Coffee Bean Roasting Process and Color Changes

FrontStreet Coffee explains the coffee bean roasting process and color changes.

When determining the degree of coffee bean roasting, one can judge the roasting progress through sound, color, and aroma.

The color ranges from light gray to dark brown, and the aroma gradually intensifies from subtle to rich; there are two distinct cracking sounds. Taking heat-resistant raw beans as an example, the first crack begins around the 7-minute mark and lasts about 2 minutes. The sound is deep, indicating the beginning of thermal decomposition, starch converting to caramel, moisture evaporation, and degassing. Around the 12-minute mark, the second crack occurs, with fierce and sharp sounds, indicating cell fiber rupture and deeper roasting.

The renowned Specialty Coffee Association of America (SCAA) promotes using caramelization analysis values measured by infrared (Agtron number) to determine roasting degree. The values range from 0 to 100, where higher numbers indicate lower caramelization, grayish-white color, and lighter roasting. Conversely, lower numbers represent higher caramelization, dark glossy color, and deeper roasting.

Dehydration Phase

Raw coffee beans typically contain 8-12% moisture. Water serves as a heat conduction medium. As temperature increases, free water in the beans gradually evaporates, and gas is produced internally. The so-called "dehydration" doesn't mean completely removing water. During roasting, water also acts as a heat conduction medium. As temperature rises, free water in the beans gradually evaporates, producing gas internally that applies pressure to the bean's structure, promoting gradual expansion. Meanwhile, beans become elastic with changes in temperature and humidity until water activity decreases, returning to a hard, brittle glassy state. At this point, a large portion of cellular content has transformed into gas, and the outer layer has thinned due to thermal expansion. In cases of overly rapid roasting, different evaporation rates between the bean surface and core can lead to uneven moisture distribution, causing the surface to vitrify while the core remains under-roasted, resulting in coffee with more astringency.

Maillard Reaction Phase

When most free water has been removed and water activity drops below 0.7, the temperature and pressure inside and outside the beans reach similar levels. At this point, beans expand rapidly. Raw beans change from green to white, then begin turning yellowish-brown, emitting an aroma similar to toasted bread. This is followed by the so-called Maillard reaction and caramelization stages. The Maillard reaction refers to a series of reactions between reducing sugars and proteins/amino acids in food when heated, producing brown-black macromolecular substances also known as flavor compounds.

Development Phase

Turning to a darker brown color, with temperature climbing to nearly 185°C (values vary based on measurement points), one should pay attention to the approaching cracking sounds. The first crack is a reaction where carbon dioxide and moisture in raw beans expand due to heating, causing the beans to split open. This is also a critical point affecting flavor. After beans begin their first crack, they enter the development stage. The Maillard reaction is nearing its end while caramelization accelerates. During the first crack, water vapor and carbon dioxide inside cells are rapidly expelled in large quantities, causing a slight decrease in bean temperature and rapid increase in humidity. After the first crack ends and free water is completely released, humidity decreases again. Many reactions occur in this short period, so the airflow and heat control during the late Maillard reaction significantly impact the final flavor expression of the beans.

Bean Discharge and Cooling

When roasting reaches your chosen roast level or temperature, you can discharge the beans for cooling. Cooling is the final crucial step in coffee roasting. Reducing the coffee's temperature as quickly as possible is essential to lock in the coffee's flavor at your determined temperature and discharge condition.

Important Notice :

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