Why this matters for professionals

Two roasters can take the same green coffee and produce cups that taste like different beans. The difference is the profile — the temperature curve and time spent in each chemical stage between charge and drop. Profile design is the heart of specialty roasting, and the difference between a competent operator and a master is the ability to read the curve in real time and steer it.

This article walks the five recognized phases of a typical drum roast, explains the chemistry of what happens in each, and gives you concrete numbers (rate of rise, development time ratio, charge temperature) that you can plot and adjust at the wheel.

The Five Phases of a Roast

1. Drying (Charge → ~150°C bean temp). Green coffee enters the drum at roughly 11% moisture content. The first job of the roast is to drive that moisture down to ~3% before pyrolysis begins in earnest. This takes typically 4-5 minutes. Apply too much heat too fast and the surface scorches while the interior is still wet ("baked" defect).

2. Yellowing (150 → 175°C). Chlorophyll breaks down. The beans turn from green to pale yellow to gold. Grassy and herbaceous compounds dissipate.

3. Maillard Reaction (175 → ~196°C). Amino acids and reducing sugars combine to form hundreds of aromatic compounds: nutty, malty, bready notes. This phase builds body and sweetness. The roast should slow down here so chemistry can complete — typically 3-5 minutes.

4. First Crack (~196-205°C). Internal CO2 pressure ruptures the bean cell walls audibly. Beans expand 50-100% in volume. Lipid migration begins. This is the start of "development" — the period where the coffee transitions from underdeveloped to drinkable.

5. Development to Drop. The window from first crack to drop is where you set roast level. Light roasts drop 60-90 seconds after first crack; medium 90-180 seconds; dark roasts go to second crack at ~225°C. The Development Time Ratio (DTR) — percentage of total roast time after first crack — typically sits at 18-22% for balanced filter coffees.

Key Metrics

Practical Profile Design

Start every new origin with a baseline: 12-minute total time, 4:30 drying, FC at 9:30, drop at 11:30 for medium-light. Cup it. Adjust the next batch one variable at a time. If the cup is grassy, extend drying. If the cup is baked or flat, drop sooner after FC. If acidity is too aggressive, lengthen development. If body is thin, raise charge temperature.

Profile changes you make at one stage cascade. Slowing the Maillard phase shifts FC later, which shifts drop, which extends DTR. Always log every variable: charge temp, gas levels by minute, airflow, and the audible cues. Without logs you cannot replicate success.

Common Mistakes

• Crashing rate of rise. A sudden drop in RoR at FC produces baked, lifeless coffee. Maintain a gentle declining RoR.
• Flicking RoR. An RoR spike during development can produce harsh, raw flavors.
• Ignoring batch size. Charge temp must adjust with batch mass. Roasting 50% of capacity needs different settings than full charge.
• Cupping too soon. Coffee needs 24-72 hours post-roast to outgas before cupping reveals true character.
• Chasing competition profiles. Profiles are roaster-specific. Heat transfer geometry differs between machines.
• Skipping moisture readings. Different lots of the same coffee can vary 1-2% in moisture, requiring profile adjustment.

Further Reading

Scott Rao's "The Coffee Roaster's Companion" remains the canonical practitioner reference. Rob Hoos's "Modulating the Flavor Profile of Coffee" goes deeper on chemistry. Cropster and Artisan are the two standard data-logging platforms; both let you overlay profiles and compare iteration to iteration. The SCA Roasting Foundation and Intermediate modules are recommended formal training paths.