The Ultimate Guide to Coffee Roasting: Stages and Flavor Impact

When it comes to understanding coffee flavor, it all starts with roasting. This is the process that transforms raw green beans into the aromatic, flavorful coffee we know and love—triggering complex chemical reactions that define the final taste in your cup. So what exactly happens during roasting, and how does each stage shape the coffee’s profile? In this guide, we break down the process, the science, and how roast levels influence flavor.

Understanding Coffee Roasting: What It Is and Why It’s Essential

Coffee beans don’t start out as the roasted, aromatic seeds we know—they begin as the inner seeds of coffee cherries, the fruit of the coffee tree. Once harvested, the cherries go through a processing stage to remove the outer skin and pulp, leaving behind the raw, green coffee beans. These green beans are then roasted using high heat to trigger complex chemical reactions that bring out aroma, flavor, and color.

Roasting times and temperatures aren’t fixed—they’re adjusted based on the desired roast level and how the beans respond to heat. Typically, coffee is roasted at temperatures between 180–230°C (356–446°F) for about 8 to 15 minutes.

Several key chemical reactions take place during roasting, most notably the Maillard reaction, caramelization, and carbonization. These reactions drive the transformation of raw green beans into flavorful roasted coffee, each contributing to different aspects of the final taste profile—a topic we’ll explore in detail in the sections below.

Maillard Reaction

The Maillard reaction is a fundamental chemical process in food science, where sugars and amino acids interact under either ambient or heated conditions. It leads to browning and the formation of aromatic compounds, giving many cooked or processed foods their distinctive color and flavor.

In coffee roasting, the Maillard reaction is especially important during the early stages of high-temperature exposure. It rapidly generates a wide spectrum of aromatic compounds that give roasted coffee its rich, layered fragrance.

Caramelization

Caramelization is the process in which sugars break down and recombine under heat to form new aromatic compounds and brown pigments. It typically occurs at temperatures above 160°C (320°F).

Unlike the Maillard reaction, caramelization involves only sugars and does not require amino acids to proceed. Caramelization plays a relatively smaller role in coffee roasting because green beans contain limited amounts of simple sugars. However, it still contributes to flavor development during the later stages of roasting by adding sweet, toasted notes and enhancing color.

Carbonization

Carbonization typically occurs in the final stages of roasting, but not all coffee undergoes this process. It’s mainly observed in dark or very dark roasts, when the beans are exposed to prolonged, intense heat.

At this point, the surface of the beans may begin to char, introducing more pronounced bitterness and smoky aromas. These characteristics often overpower the coffee’s original flavors, resulting in a cup with a markedly different taste profile.

As high heat continues to act on the beans, both their flavor and appearance are transformed. Depending on roast duration and visual cues, coffee roasts are generally categorized into light, medium, and dark levels.

In the next section, we'll take a closer look at the three main roasting stages and how different roast levels influence the final flavor.

The 3 Stages of Coffee Roasting

As coffee beans are exposed to heat, they undergo a series of physical and chemical transformations. The roasting process is generally divided into three main stages: the drying phase, the yellowing phase, and the development phase.

Roasters monitor changes in aroma, color, sound, and temperature to determine which stage the beans are in and when to transition to the next.

Once roasting is complete, the beans must be rapidly cooled to room temperature. This step is essential to halt residual heat from continuing to alter the flavor.

Drying Phase

The drying phase is the first stage of roasting, where moisture is removed from the outer and inner layers of the green coffee bean. According to the International Coffee Organization (ICO), green coffee typically contains between 8% and 12.5% moisture by weight. During the drying phase, heat energy drives evaporation, preparing the bean for the complex chemical reactions that follow.

Though the drying phase may seem straightforward, it plays a critical role in setting the stage for later transformations. The thermal energy absorbed by the beans here will directly influence how effectively subsequent reactions unfold. Visually, the beans shift in color from a pale grayish-green to a light yellow—one of the first clear signs of physical change.

Yellowing Phase

This is the stage when both the Maillard reaction and caramelization begin to take place. Inside the coffee bean, flavor compounds—such as acidity and aromatic precursors—start to form. Visually, the color shifts from pale yellow to light brown, signaling the start of deeper chemical transformation.

Development Phase

In the development phase, the flavor compounds formed earlier continue to evolve. The beans darken in color and release more steam and gases as internal pressure builds. This stage marks the most active period of chemical transformation and is typically divided into two key events: the first crack and the second crack.

• First Crack
  First crack occurs at the beginning of the development phase, when pressure, moisture, and carbon dioxide inside the bean reach a critical point. The beans emit an audible cracking sound, indicating a major physical shift. This event typically happens around 196–204°C (385–399°F).
• Second Crack
  If roasting continues beyond the first crack, the beans may enter a second crack stage, typically occurring around 224–230°C (435–446°F). At this point, internal gases and pressure build up again, causing a second round of audible cracking. The bean's cell structure breaks down further, oils begin to migrate to the surface, acidity diminishes, and bitterness increases. If the roast continues beyond this point, the beans may enter carbonization.

** The actual temperature range of both cracks can vary depending on the roasting equipment and probe placement. It's best to evaluate roast progress by combining sensory cues—sound, aroma, and bean appearance—rather than relying solely on temperature.

Understanding Light, Medium, and Dark Roast Coffee

Coffee is typically classified into three roast levels—light, medium, and dark—each offering distinct flavor characteristics depending on roast time and development.

Light Roast

Light roast coffee is typically roasted to a point shortly after the first crack. Because the beans are exposed to heat for a shorter period, their cellular structure remains more intact, preserving more of the bean’s original characteristics—especially bright acidity and origin-specific flavors.

Because light roast beans are denser, they contain slightly more caffeine by volume. This has led to the common myth that light roasts have significantly higher caffeine content. However, when measured by weight, roast level has minimal impact on caffeine content. In fact, the difference in caffeine between light, medium, and dark roasts is negligible.

Visually, light roast beans appear lighter in color with an oil-free surface.

Medium Roast

Medium roast coffee is roasted between the first and second cracks. The flavor profile strikes a balance between acidity and bitterness, resulting in a smooth, well-rounded cup. The beans take on a medium brown or chestnut color.

Dark Roast

Dark roast coffee is typically finished after the second crack. At this point, more flavor compounds are produced, but origin characteristics become harder to distinguish. Acidity decreases, bitterness intensifies, and the overall profile becomes bold, heavy, and robust.

Roast Levels and the Agtron Scale

The Specialty Coffee Association (SCA) uses a color measurement standard developed by Agtron, a food analysis instrument company, to define coffee roast levels. The Agtron analyzer measures the color of whole beans or ground coffee: higher Agtron values indicate lighter roasts, while lower values correspond to darker roasts.

The Agtron values require specialized equipment to measure, and results may vary slightly depending on the machine or calibration method. Most consumers rely on general roast categories—light, medium, or dark—while some specialty roasters include the Agtron number for greater precision.

Agtron Scale Reference for Roast Levels