What Happens During the Roasting Process?

The roasting process runs on recorded curve data: drum temperature before charging, heat and airflow adjustments, the turning point, yellowing, Maillard and caramelization reactions, first crack, and the rate of rise. Logging these values lets a roaster repeat a batch instead of guessing, because bean temperature, environmental temperature and rate-of-rise curves show how the bean actually responded to the heat applied.

Green beans smell grassy and taste of little before roasting; heating triggers chain chemical reactions that release the flavors sealed inside. Drum temperature is read by a probe set in the roasting chamber, usually opposite the bean probe and not touching the beans, and some roasters lack that probe entirely. A typical example designs the roast rhythm around the bean's properties so acidity, body and aroma stay balanced.

Every coffee bean's roasting process requires detailed recording of curve data, including the turning point, heat adjustments, temperature changes, and first crack time. These data are not only the foundation for replicating flavor but also an important basis for roasters to understand the characteristics of the beans. This article will break down the key nodes in the roasting curve one by one—from the preheating drum temperature setting, the coordination of heat and airflow, to the turning point, yellowing point, Maillard reaction, caramelization reaction, and then to the determination of first crack and the calculation of the rate of rise—helping you build a clear logic for reading roasting curves. At the end, using [Front Street Jamaica Blue Mountain] as an example, it shows how to design the roasting rhythm according to the bean's properties, so that acidity, body, and aroma achieve a delicate balance.

Roasting is essentially a process of heating green beans. Unroasted green beans, when smelled closely, have only a faint grassy and fishy odor and a hint of fruitiness, and when put in the mouth, almost no flavor can be tasted. After heating, however, a series of chain chemical reactions is triggered inside the bean, and the various flavors originally sealed in the green bean are released.

When we roast each bean, we fully record the roasting curve, including information such as the turning point, heat level, temperature values, and the time when first crack appears. The reason for doing this is to use the previous batch's data as a reference, replicate the same flavor performance as much as possible, and keep the output stable. The most core data include the coffee bean temperature curve, the air and environmental temperature curve, and the rate-of-rise curve. By observing changes in bean color, airflow size, and gas pressure, one can further deepen understanding of the bean's state.

So, which nodes need special attention during the roasting process?

[Preheating drum temperature]

Before charging the beans, the inside of the drum needs to maintain a stable temperature value. The drum temperature is measured by a temperature probe inside the roasting chamber. In the case of a drum roaster, the probe is usually installed opposite the bean temperature probe and does not directly touch the beans. Some roasters do not have a separate drum temperature probe, so the temperature of the exhaust port probe of the roasting drum is used as the drum temperature.

[Heat level]

The heat level directly determines the final roast degree of the coffee beans and also affects the direction of the flavor and the roasting duration we expect. The fiercer the heat, the more flavor substances are released, but not all flavors are positive. Sufficient heat can allow the beans to release more flavor, while also amplifying the defective flavors of the beans themselves. Therefore, throughout the roasting process, it needs to be used in coordination with the airflow to adjust the flavor to its best state while ensuring the cleanliness of the coffee.

[Adjusting airflow]

The airflow plays a role to a certain extent in assisting heat and storing heat. The degree of airflow opening affects the heating effect of the hot air inside the drum on the green beans. Under the premise of sufficient heat, appropriately increasing the airflow can enhance the penetration of hot air into the green beans; but if the heat itself is insufficient or the airflow is opened too much, it will cause the drum to lose temperature. Opening the airflow wider when necessary can more quickly draw away the silver skin produced during roasting and the particles that can give the coffee off-flavors. In addition, the airflow size also affects the pressure inside the drum, which in turn affects the development of the coffee beans.

[Turning point]

After the beans enter the drum, they immediately absorb heat, and the drum temperature drops sharply. Then, at a certain point in time, the temperature begins to rise again. The equilibrium point where the temperature changes from falling to rising is the turning point.

[Yellowing point]

After the green beans enter the endothermic stage, heat must be transferred to the inside of the bean, relying on the conductivity of water. If the bean moisture content is low, the time needed to reach the yellowing point is shorter; conversely, beans with high moisture content take longer to reach the yellowing point. Water evaporates into steam, the internal vapor pressure and temperature rise simultaneously, and at this point the beans begin to expand, chlorophyll is destroyed, and the yellowing point is reached.

[Maillard reaction and caramelization reaction]

Maillard reaction process

In coffee roasting, the Maillard reaction roughly starts at the stage after dehydration is completed, when the bean color begins to turn yellow and then brown, and continues until roasting ends and the beans are completely cooled. The unique aroma of roasted coffee beans combines the green-blue smoky burnt smell of burning plant fibers, the creamy yellow sweetness, the brown bread aroma of toast, and many other odors. Most of these rich flavors come from the Maillard reaction during roasting.

After a large amount of thermal energy is absorbed, pyrolysis begins, the first crack sound appears, some sugars are converted into carbon dioxide, moisture continues to evaporate, and new aromatic components are gradually generated, forming the so-called coffee oils, and combining with hundreds of aromatic substances such as nicotinic acid, citric acid, quinic acid, malic acid, acetic acid, and caffeine.

Caramelization reaction

The sugars in coffee beans undergo caramelization roughly between 170 and 200°C, which happens to fall within the melting point of sucrose (185°C) and the temperature range of the first crack stage in coffee roasting. The products of the caramelization reaction are divided into two parts:

1. Dehydration products of sugar, namely caramel or dark brown color

2. Cracking products, mainly some volatile aldehydes and ketones.

Overall, caramelization brings a fire-roasted aroma, caramel flavor, and color, and also produces other aromatic substances such as maltol, cyclotene, furan compounds, and so on. These compounds can likewise be found in foods such as red wine, fruit juice, and cream.

However, if caramelization is excessive, it will instead cause carbonization, making the coffee dry-bitter and throat-clenching. Insufficient caramelization makes the aroma monotonous and lacking in layers.

Green beans contain large amounts of sugars, proteins, and amino acids. After sugars and amino groups trigger a series of changes, they generate brown-black macromolecular substances—melanoidins (also called melanins). In addition to melanoidins, the reaction process also produces hundreds or thousands of intermediate molecules with different odors, including reductones, aldehydes, and heterocyclic compounds. These substances provide food with pleasant flavor and attractive color.

[First crack point]

First crack refers to the obvious and strong sound made by coffee beans inside the machine during roasting, somewhat like the sound of popcorn or burning firewood. When the pressure produced by the evaporation of moisture inside the bean after the coffee bean has absorbed heat exceeds the strength of the bean body itself, the cell walls cannot withstand the pressure of the bean center expanding and burst, and at this time a first crack sound is emitted, simultaneously releasing thermal energy and sound energy. This is the source of "first crack." Along with first crack, different flavor compounds (that is, aromatic substances such as floral and fruity notes) are released, while browning also occurs (which is why coffee beans are brown). This stage is also a period when coffee beans undergo major changes in development. Releasing the beans one second earlier or later may make a big difference in flavor.

[Rate of rise]

The temperature difference between two time points, for example, there is a temperature difference between the 2nd minute and the 3rd minute. Subtracting the former (108.8°C) from the latter (122.4°C) gives a value (13.6°C).

In the process of designing a roasting curve, the goal is to roast a bean "thoroughly" from the inside out. On the basis of roasting it through, use better roasting techniques, judge at which roasting stage the flavor can be expressed, how to create more possibilities for this stage, how to weigh the balance between controlling flavor and defective flavors, how to ensure the balance of acidity, sweetness, and bitterness in this coffee bean, and after first crack, with how much heat and for how long to develop.

Taking the roasting of [Front Street Jamaica Blue Mountain] as an example, it is necessary to first understand the properties of this bean. [Front Street Blue Mountain Coffee] is grown on a hillside at 1,310 meters, with fertile volcanic soil, fresh and pollution-free air, a humid climate, and fog and rain all year round. The average rainfall is 1,980 millimeters, and the temperature is around 27 degrees. The washed process allows it to ferment for 12 to 18 hours, and then the fermented beans are placed in a pool and moved back and forth, washed until smooth and clean, enhancing the steady and delicate acidity of Front Street Blue Mountain coffee beans. Front Street's Blue Mountain is NO.1 beans of 17 screen size or above, with a defect rate below 3%, a moisture content of about 13%, and medium bean density, suitable for roasting with a steadily increasing approach.

First, raise the drum temperature to 170°C and charge the beans, set the airflow to 3, and after 1 minute adjust the heat to 140°C, with the airflow unchanged. Roast until 5 minutes 10 seconds, at 153°C, when the bean surface turns yellow, the grassy smell completely disappears, and dehydration is complete. Adjust the heat to 115°C, and change the airflow to 4;

At 8 minutes 00 seconds, ugly wrinkles and dark streaks appear on the bean surface, and the toast aroma clearly turns into coffee aroma, which can be defined as the prelude to first crack. At this time, it is necessary to listen clearly for the sound of the first crack point. First crack begins at 8 minutes 12 seconds. Reduce the heat to 90°C, open the airflow fully (adjusting the heat must be done very carefully, and it must not be so low that there is no crack sound), and release the beans at 201.3°C.

Roasted this way, acidity, body, and aroma achieve a perfect and delicate balance. The acidity is bright and delicate, the texture is as smooth as velvet, and the aroma has a slight penetrating quality, accompanied by floral, spicy, and caramel tones. After entering the mouth, there is a hint of cocoa in the aftertaste.

In terms of taste, it is very clean, complex, very mild, chocolate-sweet, with a very strong body. The flavor is rich and mellow, and the coffee's sweetness, acidity, and bitterness are perfectly balanced, with no bitterness at all, only a moderate and perfect acidity, and a long-lasting fruity finish.

After roasting is completed, cupping evaluation is indispensable:

Dry aroma: roasted peanut, hazelnut, melon, chocolate

Wet aroma: oolong tea, caramel, honey, chocolate, almond skin, silky mouthfeel, brightness

So when looking at the roasting curve, has it become much clearer? Understanding the roasting curve actually allows you to see the roaster's understanding of this bean. We choose to release the beans 4 minutes after first crack because we want to retain more of [Front Street Blue Mountain Coffee]'s creamy chocolate and rich brown sugar flavors, as well as its heavy texture. Today's sharing is intended to help everyone understand the curve.

The above content is compiled by CoffeeHunters, a coffee news website.