Coffee roasting: What happens when coffee is roasted?

Roasting coffee brings out the aroma and flavor trapped in green coffee. The conversion of green coffee into roasted coffee beans is therefore one of the most important processes in coffee, alongside the type of coffee, the growing region and the processing method. Green coffee beans have none of the properties of roasted coffee beans. They are softer, contain around 11-12% moisture and smell grassy . Roasted beans, on the other hand, have a strong aroma, are brittle and weigh less because the moisture has evaporated during roasting.

When coffee beans are roasted, chemical and physical changes occur because the coffee beans are brought to a high temperature. But what happens when coffee is roasted? Let's look at some of the chemical and physical changes, including the development of flavor and aroma, when roasting coffee

Roasting coffee

What happens when coffee is roasted?

Green coffee is dense, green seeds composed of carbohydrates in various forms, water, proteins, lipids, acids and alkaloids (caffeine and trigonelline). Green coffee beans hardly have any of the properties we would typically associate with roasted coffee.

After coffee roasting, the raw coffee bean undergoes a significant chemical and physical transformation, producing 800 to 1000 chemical compounds that are responsible for the taste of the coffee . The number of aromatic compounds produced after roasting depends on the quality of the green coffee and the roasting process.

Roasting coffee usually involves three main phases: drying, browning (also called the Maillard reaction), and development. These phases refer to different stages of the chemical and physical transformation of the coffee beans during roasting. How you as a roaster react during these different phases can change the flavor profile of the coffee and, if not done correctly, can even ruin the coffee.

Coffee Roasting Phase 1: Drying

Drying the coffee is an important initial phase of coffee roasting. The green coffee has a moisture content of around 10-12%, which is essential to maintain its quality. As soon as we put the green beans into the roasting machine, the temperature of the beans will gradually start to rise. During drying, the water content begins to evaporate, resulting in the development of steam in the beans. As a result, the volume and internal pressure of the beans increase. At this stage, the coffee beans lose density and the color of the beans gradually changes from green to yellow.

Coffee roasting drying phase

As a rule of thumb, depending on the roasting machine and its capacity, the drying phase takes around 35% to 45% of the total roasting time and usually ends when the beans have reached a temperature of around 160 degrees Celsius . At this point the beans enter the browning phase, also known as the Maillard reaction. If the drying phase proceeds too quickly, there will be uneven heat distribution in the bean, which increases the risk of burning.

Coffee Roasting Phase 2: Maillard Reaction

The Maillard reaction is a series of chemical reactions that play an important role in the development of flavors and the brown color of roasted coffee. Yellow beans turn light brown when sugar reacts with amino acids. This reaction also creates a wealth of flavors and aromas in the coffee, which contribute to its special taste.

Small changes in the temperature and duration of the Maillard reaction can have a big impact on the final profile of a coffee. In general, it is important not to rush the roasting process with the Maillard reaction, otherwise many organic acids will not be developed sufficiently, resulting in a sour, unpleasant coffee . If the Maillard reaction is delayed for too long, the acids that produce fruity and sweet notes are destroyed, resulting in an earthy, bitter coffee . It is important to experiment with the roast profiles of each coffee, including varying the duration and intensity of the Maillard reaction and recording the effects on the coffee's profile.

The Maillard reaction produces gases such as carbon dioxide, water vapor and some volatile compounds . The internal pressure increases to the point where the cell walls of the beans break, producing a cracking sound . This is called the first crack . The First Crack marks the end of the Maillard reaction and the beginning of the development phase of coffee.

Roasting Coffee Phase 3: Development

The development phase, better known as the roasting phase, is the period after the "first crack" until the end of the roasting . After the first crack, the beans are technically coffee and can be removed from the roast at any time.

The duration of the development phase is usually referred to as development time . The length of the development time and the speed at which the coffee is roasted determine the roasting profile of the coffee. Much of a coffee's character, such as aroma, sweetness and acidity, develops during the development phase . Therefore, the development phase is the phase in which we determine the flavor notes that we want to highlight.

There is no strict rule that dictates the exact length of the development phase, but generally it is considered to be 15-25% of the total roasting time . However, the exact duration depends on the specific flavors we want to highlight or the degree of roasting we want to achieve.

For a light, fruity and acidic coffee ("light roast"), the roasting should be finished immediately after the first crack or shortly thereafter. A light roast is the roast level commonly used for cupping. If you want a coffee with a balanced acidity, lots of body and sweetness, a medium roast is the right choice. A medium-dark roast has lower acidity, a characteristic favored by some coffee lovers. With such a roast, the citrusy notes that are typical of a light roasted Ethiopian coffee , for example, would transform into a note reminiscent of dark chocolate.

Coffee roasting profiles

It is important to recognize the importance of each phase of the roasting process in order to achieve the desired result in the development phase, as the previous chemical reactions in the previous phases form the basis for the development phase. For this reason, roasters carefully monitor and record temperature adjustments during the roasting process using charts called coffee roast profiles . This technique allows them to repeat a successful roast for a particular bean at a later date .


When roasting coffee, the green coffee is transformed through various physical and chemical reactions that are triggered by heating the beans. Roasting begins with the drying (evaporation) of the water contained in the bean. During the roasting process, the color, shape, weight, volume and taste of the coffee beans change. More than 800 aromatic substances are released during the roasting process. In the Maillard reaction or browning phase, amino acids and sugar combine to form new flavoring substances. These compounds then serve as the basis for the subsequent chemical reactions that occur during the development phase of the coffee. A small adjustment in the temperature and duration of the roasting process can greatly influence the flavor profile of the coffee. Paying attention to the temperature and duration of the individual roasting phases is therefore essential for better and consistent results.