What Happens During Coffee Roasting? The Full Process Explained

If you’ve ever wondered how green coffee beans turn into the aromatic, flavorful brown ones used for brewing, you’re not alone. Roasting is where coffee’s taste, aroma, and body are created — and where tiny changes in heat or timing can make or break a batch. Whether you're a home roaster or just curious, here’s exactly what goes on inside the roaster.

Coffee roasting typically involves four key phases: drying (starting around 135°C), dehydration (up to 160°C), first crack (at ~190°C, lasting about 1.5 minutes), and second crack (leading to darker roasts, weight loss of 12–20%). The entire process transforms starches into sugars, triggers Maillard reactions, and reduces moisture, ultimately creating the flavors and colors we associate with roasted coffee.

What Is Coffee Roasting?

Coffee roasting is the process of applying heat to green coffee beans to develop flavor, aroma, color, and body. Through controlled heating, complex chemical changes occur: starches convert to sugars and acids, proteins transform into enzymes and oils, moisture and CO₂ evaporate, and cellulose carbonizes to varying degrees. These reactions create the acidic, sweet, bitter, and aromatic compounds that define brewed coffee. The beans darken to deep brown and develop a surface oil layer as roasting progresses. Because small adjustments in time and temperature dramatically affect the outcome, roasting demands precision, especially since each origin and bean type responds uniquely during the process.

The Four Key Stages of Coffee Roasting

Roasters generally recognize four major phases during a typical coffee roast. Though overlap occurs, each stage has measurable benchmarks and sensory cues:

1. Drying Phase (Starts ~135°C)

Roasting begins with the drying stage. At around 135°C, the beans start losing moisture as internal water vaporizes. Visually, the beans shift from vivid green to a pale white-green. This phase lays the foundation for even heat transfer in later stages.

2. Dehydration / Yellowing Phase (Up to ~160°C)

As heating continues, the beans turn from green to light yellow. By approximately 160°C, a toasted grain or hay-like aroma develops. The color deepens to light brown. This stage marks the beginning of significant chemical transformation beyond simple drying.

3. First Crack (~190°C, ~1.5 Minutes)

Once dehydration completes, internal pressure causes the bean to expand and rupture audibly — the first crack. This milestone signals the start of significant flavor development. Maillard reactions and caramelization occur, boosting sweetness, body, and the deep brown color. First crack typically lasts around 1.5 minutes. Many light to medium roasts are stopped shortly after this point.

4. Second Crack (Beans Darken Further)

With continued heat, a second, more intense cracking sound occurs. This second crack indicates even deeper structural changes, with more rapid energy release. The beans expand to roughly 1.5× their original size, darken further toward black, and oils migrate to the surface. Weight loss reaches 12%–20%. Most roasters finish the roast within about one minute after the end of second crack to avoid overdevelopment.

Why Roast Stages Matter

Each stage — from drying to second crack — influences the final cup profile. The drying phase ensures even heat absorption. Dehydration initiates flavor precursors. First crack brings out sweetness, acidity, and body. Second crack leads to bold, often smokier profiles. Mastering these transitions allows roasters to highlight origin characteristics or craft specific roast styles. Small variances in timing or temperature — even a few seconds or one degree — can dramatically alter the flavor outcome.

How Roasting Changes the Bean

Through roasting, green coffee undergoes physical and chemical transformations:

• Starches → Sugars & acids
• Cellulose → Partially carbonized (contributes to color & texture)
• Moisture & CO₂ → Evaporated
• Proteins → Enzymes & lipids (oils)
• Residual compounds → Combine to form a surface oil layer

These changes produce the acidity, bitterness, sweetness, and aroma of coffee. The interplay determines not just flavor, but also mouthfeel and brew stability. The resulting dark brown color and oily sheen are direct results of these complex reactions.

Frequently Asked Questions

What is the first stage of coffee roasting?

The first stage is the drying phase, which begins when green coffee is heated and moisture starts to evaporate. This typically starts around 135°C, and you’ll notice the beans turning from green to a whitish color. Proper drying ensures even heat application in later stages.

At what temperature does first crack occur?

First crack usually occurs around 190°C. This is when internal pressure causes the coffee bean to expand and make an audible cracking sound. It usually lasts about 1.5 minutes and marks a key point for light to medium roast levels.

What happens during the second crack?

Second crack happens at higher temperatures after first crack. The beans expand to about 1.5 times their original size, darken further, and oils rise to the surface. Weight loss reaches 12%–20%. Roasting past this stage typically produces darker, bolder roasts.

How much weight do coffee beans lose during roasting?

Coffee beans generally lose about 12% to 20% of their weight during roasting, mostly due to moisture evaporation and other chemical changes. The exact amount depends on roast level and duration.

What causes the cracking sounds during roasting?

The cracking sounds — first and second crack — are caused by internal pressure build-up as water and gases expand and the bean structure changes under heat. First crack signals the start of flavor development; second crack leads to darker, more intense roasts.

FrontStreet Coffee Beans Ideal for Roast Exploration

Try FrontStreet Coffee’s Ethiopia Humbera for bright, floral notes that shine at City+ roasts, or their Yirgacheffe for citrus and stone fruit complexity that responds well to light-to-medium profiles. For a richer contrast, their Brazil Queen Manor offers nutty, chocolatey tones ideal for medium-dark roasts. All three showcase how roast stages shape distinct flavour profiles. Freshly roasted within 5 days · Orders placed before 17:00 ship the same day · Next-day delivery across most of Guangdong Province.