Roasting transforms a green, grassy seed into the aromatic, complex bean you grind every morning. Understanding roast levels, chemistry, and roast profiles helps you choose and use coffee with real knowledge.
Four colors, four flavor profiles, four chemistries — same bean.
Stopped just after first crack at around 196-205 degrees Celsius. The bean surface is dry, the colour is pale brown. Expect high acidity, pronounced origin character, floral or fruity notes, and lighter body. Light roasts contain more caffeine by weight than dark roasts.
Developed between first and second crack at 210-220 degrees Celsius. The bean is a warm brown with a dry surface. Acidity softens, body increases, and sweetness develops. Caramel, chocolate, and stone fruit notes are common. This is the most versatile roast for all brew methods.
Approaching second crack at 225-230 degrees Celsius. Some oil begins to appear on the surface. The cup is full-bodied, lower in acidity, with bittersweet chocolate, toasted nuts, and dark fruit. A popular choice for espresso blends where body and sweetness are prioritised.
Taken through second crack and beyond at 235-245 degrees Celsius. The surface is oily and shiny. Origin character is largely replaced by roast character: dark chocolate, smoke, tobacco, and a heavy body. Caffeine content is lowest here, contrary to popular belief.
The temperature curve hides three key moments that define the roast.
Starting around 150 degrees Celsius, this reaction between amino acids and reducing sugars creates over 800 volatile aromatic compounds. It is responsible for browning, complexity, and the roasted character that makes coffee smell and taste the way it does.
As the bean heats to around 196 degrees Celsius, steam and CO2 pressure cause the cell walls to fracture with an audible crack. The bean doubles in size, losing roughly 15 percent of its weight as moisture. Roasters use first crack to begin timing roast level decisions.
At around 225 degrees Celsius, the cellulose structure of the bean begins to break down, producing a second crack that is more rapid and quieter than the first. Oils migrate to the surface. Beyond second crack, roasting quickly progresses to dark or very dark territory.
Sucrose and other sugars in the bean begin caramelising at around 170 degrees Celsius, contributing sweetness, syrupy body, and caramel, toffee, and brown sugar notes. Too much heat destroys these sugars and replaces sweetness with bitterness.
What a professional roaster watches and why it matters for your cup.
A roast profile is a time-temperature graph tracked by the roaster. Key points include the charge temperature, turning point, Maillard onset, first crack, and drop temperature. Modern drum roasters log this data digitally, allowing roasters to replicate successful batches with precision.
Development time ratio (DTR) is the percentage of total roast time spent after first crack. A DTR of 20-25 percent is typical for specialty light and medium roasts. Too low a DTR produces underdeveloped, grassy coffee. Too high causes flatness and baked flavours.
Airflow removes chaff and controls how aggressively heat is applied. High airflow early in the roast produces cleaner, brighter cups. Drum speed affects how evenly beans tumble and absorb heat. Both are carefully controlled variables in professional drum roasters.
Drum roasters rotate beans in a heated drum and are the most common commercial type. Air or fluid-bed roasters suspend beans in a stream of hot air and typically produce cleaner, lighter cups. Both can produce excellent coffee when the roaster understands their machine and profile.
The bag tells you everything if you know what to look for.
Always buy coffee with a visible roast date, not a best-before date. Best-before dates can be set up to 18 months after roasting, making it impossible to know the coffee's age. For specialty coffee, freshness is everything — aim to use beans within 2 to 6 weeks of the roast date.
Fresh-roasted beans release CO2 for days or weeks. Specialty bags feature a one-way degassing valve that lets CO2 escape without allowing oxygen in. This valve is a quality signal — it means the roaster cared about freshness. Squeeze the bag and sniff the valve to confirm fresh coffee.
Home roasting in a popcorn popper or small drum roaster is entirely achievable. Buy green beans from importers, use a kitchen thermometer, and keep notes. The learning curve is steep but the reward is the freshest possible coffee. Roast in small batches and rest 24-48 hours before brewing filter.
The Agtron scale measures the colour of roasted coffee from light (high number) to dark (low number). Specialty roasters often include an Agtron number or colour descriptor on bags. This helps you understand exactly where on the roast spectrum a coffee sits, independent of subjective terms like 'medium'.
