How Coffee Beans Are Roasted: Tools and Process Explained

Ever wondered why green coffee beans taste grassy and dull—but the same beans, after roasting, deliver complex aromas and flavors? The secret isn’t in the bean itself, but in how it’s roasted. Roasting is what unlocks the flavors you love.

Coffee roasting transforms raw beans through heat, creating over 12% weight loss, 60% volume expansion, and hundreds of new aromatic compounds. The entire process hinges on three key phases: drying, pyrolysis (first and second crack), and rapid cooling—all within minutes.

The Tools of Coffee Roasting: 3 Main Types

Roasting machines fall into three categories: direct-fire , semi-hot-air direct-fire , and hot-air . Today, the latter two dominate commercial and specialty roasting.

Direct-fire : The oldest method, using flames to heat a metal drum directly. Drawbacks include slow heat transfer, risk of burning beans against the drum, chaff buildup that muddies flavor, and inconsistent results. Not favored today for fine coffee.

Semi-hot-air direct-fire : Developed in Germany between 1870–1920 and refined by Van Gupen. In 1907, the German Perfect roaster introduced a gas-heated drum with a fan directing some heat inside the drum and some around it. This design improved efficiency. By 1987, Idaho-based Diedrich introduced gas-powered infrared heating for finer temperature control. Probat, a German brand, remains globally respected for this style. These roasters blow heated air through the drum while applying direct flame, improving speed and cleanliness. The result: balanced, clean-tasting coffee.

Hot-air : A 20th-century innovation. The 1934 Jabez Burns Thermalo was an early large-scale hot-air roaster. Hot air circulates around beans to roast them quickly.

Fluid-bed : Pioneered in 1976 by American Michael Sivetz and also developed by Australian Ian Bersten. Here, beans are suspended and tumbled by a stream of hot air—like popcorn popping. This allows even roasting but lacks the “maillardized” depth some associate with drum roasting.

The Science of Roasting: 3 Key Phases

Roasting is a high-temperature chemical process called pyrolysis. It breaks down and reforms bean compounds into new ones that create aroma, acidity, sweetness, and bitterness. This only happens under intense heat.

Roasting is both a science and an art. It unfolds in three main stages:

1. Drying Phase

At the start, beans absorb heat. Internal moisture—typically around 13%—evaporates. Color shifts from green to yellow or light brown. Silver skin begins to flake off, and a grassy aroma develops. This phase takes up roughly half the total roast time. Although removing moisture is the goal, skilled roasters control the pace to use water’s heat-conducting properties effectively without losing too much too fast.

2. Pyrolysis (First and Second Crack)

Around 160°C (320°F), internal moisture turns to steam and escapes. The beans switch from absorbing heat (endothermic) to releasing it (exothermic). This triggers the first crack—a loud, distinct popping sound. Pressure inside can reach 25 atmospheres. Beans begin developing complex flavors as high heat and pressure break down old structures and form new compounds.

As the roast continues toward 190°C (374°F), the energy dynamic shifts again: from exothermic back to endothermic. The beans darken from light brown to deep brown. This leads into the second crack, a softer, more rapid sound indicating further structural breakdown and movement into darker roast levels.

3. Cooling (Quenching)

Roasting stops, but the chemical reactions don’t. To lock in flavors, beans must cool immediately. Two methods exist:

• Air cooling: Slower but clean, preserving delicate aromatics. Preferred for specialty coffee.
• Water cooling: Sprays beans with mist for rapid cooling. Alters bean weight and requires precision. Common in commercial settings but less ideal for high-end profiles.

What Changes During Roasting?

Roasting physically and chemically transforms the bean:

• Weight loss: From ~13% moisture to ~1%, total loss of 12–21% depending on roast level.
• Volume increase: Up to 60% or more.
• Cell structure: Cells become porous and fragile, increasing oxidation risk.
• Gas formation: Maillard reactions and pyrolysis generate CO₂ trapped inside the bean.
• Chemical shifts: Carbohydrates drop from 58.9% to 38.3%; acids drop from 8.0% to 4.9%. These reorganize into sugars (like caramelized compounds, making up ~25% of the bean and creating sweetness), fats rise slightly from 16.2% to 17% (contributing body), while caffeine stays nearly unchanged. Bitterness in dark roasts doesn’t come from more caffeine.

Understanding Roast Levels

Different regions classify roast levels uniquely:

• Light Roast: Grassiness remains; little aroma or body.
• Cinnamon Roast: Light tan color.
• Medium Roast: Bright acidity.
• City Roast: Balance starts forming between acid, sweet, and bitter.
• City+ Roast: Reached just before second crack.
• Full City Roast: Roasted into the beginning of second crack; a favorite of single-origin specialists.
• French Roast: Heavy bitterness.
• Italian Roast: Used for espresso.

In the U.S., the SCAA uses Agtron color readings (0 = black, 100 = white), scaled into 8 grades to standardize roast color and caramelization.

Specialty vs Commercial Roasting

Specialty roasting differs significantly:

• Roast profiles are tailored to bean attributes like moisture, hardness, crop year, and pre-roast cupping.
• Beans are roasted in small batches.
• Roasters monitor every stage—adjusting time, temperature, listening for cracks, watching color and sheen.
• Coffee is shipped the same day it’s roasted to preserve peak freshness.

Frequently Asked Questions

What are the three main types of coffee roasting machines?

The three main types are direct-fire , semi-hot-air direct-fire , and hot-air . Semi-hot-air and hot-air are now the most common in both commercial and specialty settings due to better heat control and cleaner roasting.

What happens during the first and second crack in coffee roasting?

The first crack occurs around 160°C (320°F) when moisture turns to steam and beans expand, creating a loud, popping noise. It signals light to medium roast levels. The second crack, softer and faster, happens around 190°C+ (374°F+) as beans darken further, marking entry into medium-dark and dark roasts.

How much do coffee beans change during roasting?

Roasted beans lose 12–21% of their weight mainly due to moisture loss, and their volume increases up to 60%. Internal structures break down, sugars caramelize, and gases like CO₂ form inside the bean.

What’s the difference between city roast and full city roast?

City roast is roasted to the verge of balance between acidity, sweetness, and bitterness. Full city roast goes deeper, into the beginning of the second crack, and is often preferred for highlighting origin characteristics in specialty coffee.

Why is cooling important after roasting?

Cooling stops pyrolysis immediately after roasting. Air cooling is slow but preserves aroma, favored in specialty coffee. Water cooling is faster but can affect bean weight and is mainly used commercially.

What is the most widely used professional roasting method today?

Semi-hot-air direct-fire and hot-air roasting are the most widely used today. Brands like Probat (semi-hot-air drum) and Diedrich (with infrared gas heating) are industry standards for controlled, repeatable roasts.

Recommended FrontStreet Beans for Roast Exploration

Try FrontStreet Coffee’s Ethiopia Humbera for bright, fruity notes in a light to City roast; their Yirgacheffe offers floral and citrus tones ideal for exploring first crack profiles; and the PWN Golden Mandheling delivers body and sweetness perfect for Full City to French styles. All showcase how roast levels shape flavor. Freshly roasted within 5 days · Orders placed before 17:00 ship the same day · Next-day delivery across most of Guangdong Province.