Coffee Roasting Methods and Degree Guide

If you've ever wondered why the same coffee bean can taste bright and fruity one day and bitter and smoky the next, the answer lies in how it’s roasted—not just where it’s grown. Roasting transforms bland green beans into complex, aromatic coffee through precise application of heat. But how does equipment choice and roast degree shape the final cup? This guide breaks down every major factor.

In short: There are three main types of coffee roasters—direct-fire, semi-hot-air direct-fire (like Probat and Diedrich machines), and hot-air (including fluid-bed styles). Roast levels range from Light to Italian, with key benchmarks like City, Full City, and French roast defining flavor balance and intensity. Roasting also causes beans to lose 12–21% of their weight, expand over 60% in volume, and develop gases like CO₂ that influence freshness and crema.

How Coffee Roasting Works: The Basics

Coffee’s appealing aromas and flavors don’t come from raw beans—they’re created during roasting. Green coffee beans are nearly flavorless until heat triggers complex internal changes. This high-temperature process, known as pyrolysis, breaks down and reforms bean compounds to produce the oils, acids, and aromatic molecules that define your brew. Without roasting, there’d be no crema, no body, and none of the tasting notes you love.

The Three Main Types of Coffee Roasters

Today’s commercial and home roasting setups fall into three categories, each affecting flavor and efficiency differently:

• Direct-Fire : The oldest method, where flames touch the drum. Heat transfer is slow, roasting takes longer, and beans may scorch against the metal, causing burnt flavors. Chaff can also stick to beans, muddying taste. Most specialty roasters avoid this style today.
• Semi-Hot-Air Direct-Fire : Developed in Germany between 1870–1920 and refined by Van Gupen, this design mixes direct flame with forced hot air. The 1907 Perfect roaster introduced the concept, and modern examples include the famous German Probat machines and the 1987 American Diedrich roasters, which added gas-infrared heating for precision. These roasters heat the drum while blowing chaff away, resulting in cleaner, more balanced roasts.
• Hot-Air / Fluid-Bed : Introduced in the 20th century, hot-air roasting uses forced heat to roast beans more efficiently. The 1934 Jabez Burns Thermalo was an early large-scale example. In the 1976 fluid-bed design by Michael Sivetz, beans are suspended and tumbled by hot air currents, ensuring even exposure. While some argue this method lacks the depth from metal-drum contact, it’s valued for consistency and cleanliness. Australian Ian Bersten also contributed notable fluid-bed designs.

The Three Phases of Coffee Roasting

Every roast passes through three critical stages that determine flavor, body, and aroma:

1. Drying Phase

At the start, beans absorb heat and begin losing moisture—from around 13% down. The color shifts from green to yellow or light brown, silver skin starts flaking off, and a grassy scent emerges. This phase takes up roughly half the roast time. Though the goal is moisture removal, roasters control temperature carefully to use water’s heat conductivity to advantage without drying too fast.

2. Pyrolysis / First & Second Crack

At around 160°C, internal moisture turns to steam and escapes, shifting heat absorption from endothermic to exothermic. This triggers the first crack—a loud, crisp series of pops. Pressure inside beans can reach 25 atmospheres. Around 190°C, the process switches back to endothermic as heat continues breaking down structures, creating new flavor compounds. The beans darken from brown to deep brown, progressing toward second crack—a softer, more frequent sound marking the entry to dark roasts. This is where City to French roast levels emerge.

3. Cooling

Roasting stops, but pyrolysis continues unless cooled immediately. Two methods exist: air cooling (slow but clean, preferred for specialty coffee) and water cooling (fast but adds weight and is mainly used commercially). Quick cooling locks in aromatics and prevents over-development.

What Causes the Cracking Sounds?

The first and second cracks are key indicators roasters use to judge roast degree. The first is louder and more scattered; the second quieter and tighter. These sounds correlate tightly with internal bean temperatures and help determine whether you're aiming for a City or French profile.

How Roasting Changes Coffee Beans

Though the science isn’t fully understood, roasting alters beans in measurable ways:

• Weight Loss: Moisture drops from ~13% to ~1%, with total weight loss between 12–21%. Darker roasts lose more.
• Volume Expansion: Beans swell over 60% in size.
• Cell Structure: Cells become more porous, making roasted beans more fragile and prone to losing aroma compounds.
• Gas Formation: Pyrolysis generates CO₂ and other gases trapped inside the bean, influencing freshness and crema formation.
• Chemical Shifts: Carbohydrates drop from 58.9% to 38.3%, acids from 8.0% to 4.9%. These transform into sugars (like caramelized compounds making up ~25% of the bean, creating sweetness), and fats rise slightly from 16.2% to 17%, enhancing body. Caffeine levels remain nearly unchanged—so heavy bitterness isn’t about more caffeine.

Coffee Roast Levels: Pacific vs SCAA Standards

Different systems classify roast levels based on color, cracking stage, and flavor balance:

• Light Roast: Grassiness remains; no developed aroma or body.
• Cinnamon Roast: Light brown, named for its color.
• Medium Roast: Bright acidity dominates.
• City Roast: Acidity, sweetness, and bitterness begin balancing.
• City+ Roast: Reached just before second crack begins.
• Full City Roast: Roasted into the start of second crack; a favorite of specialty roasters.
• French Roast: Heavy bitterness, very dark.
• Italian Roast: Used for traditional espresso.

The SCAA (Specialty Coffee Association of America) uses an Agtron color meter (measuring reflectance) to grade roasts from 0 (black) to 100 (light), divided into 8 levels for objective analysis.

Specialty vs Commercial Roasting

Specialty roasting differs significantly from mass-market practices:

• Roast profiles are tailored to bean attributes like moisture, hardness, age, and pre-roast cupping results.
• Batch sizes are small.
• Roasters monitor every stage—temperature, time, crack timing, color, and surface gloss.
• Coffee is delivered 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 (oldest, flame contacts drum, often leads to scorching), semi-hot-air direct-fire (combines flame and forced air, used in Probat and Diedrich machines), and hot-air or fluid-bed (uses circulating air, includes designs by Michael Sivetz and Ian Bersten). Semi-hot-air and hot-air methods are now dominant in specialty and commercial settings.

What causes the first and second crack during roasting?

The first crack occurs around 160°C when moisture turns to steam and pressure builds, causing a loud, crisp sound. The second crack, softer and more rapid, happens around 190°C+ as beans darken further, signaling structural breakdown and entry into darker roast levels like French or Italian.

How much do coffee beans change during roasting?

Beans typically lose 12–21% of their weight due to moisture loss, expand over 60% in volume, and develop internal gases like CO₂. Their cell structure becomes more porous, carbohydrates drop significantly, and fats increase slightly, contributing to body and sweetness. Caffeine content stays nearly the same.

What’s the difference between City and Full City roast?

City roast is roasted to the verge of the first crack’s end, balancing acidity, sweetness, and bitterness. Full City roast continues into the beginning of the second crack, favored by specialty roasters for deeper flavor development without heavy roast tones.

Why is cooling important after roasting?

Cooling stops pyrolysis immediately, locking in aromas and preventing over-roasting. Air cooling is slow but clean, preferred for specialty coffee. Water cooling is faster but adds weight and is mostly used in commercial settings.

What roast level is best for specialty coffee?

Specialty roasters often aim for City to Full City roasts, where the bean’s origin characteristics shine through. Darker roasts like French or Italian emphasize roast flavors over origin notes, and are less common in specialty offerings.

Recommended FrontStreet Beans for Roast Exploration

Try FrontStreet Coffee’s Ethiopia Humbera for bright citrus and floral notes in a light City+ roast, or their PWN Golden Mandheling for a balanced Full City profile with sweet caramel and almond. For darker profiles, their Brazil Queen Manor offers chocolatey richness at a City++ to Full City level. All are expertly roasted and arrive fresh within 5 days · Orders placed before 17:00 ship the same day · Next-day delivery across most of Guangdong Province.