What are the Basics of Coffee Roasting? How to Judge the Process and Color Changes of Coffee Bean Roasting

The Five Stages of Coffee Roasting

Coffee roasting can be broadly divided into five stages:

Warm-up Phase

Preheating the roaster's drum to the starting temperature for roasting — known as the "charging temperature."

Coffee Bean Roasting Process and Color Changes

FrontStreet Coffee explains the roasting process and the color changes it drives.

When judging the degree of roast, you can track progress through sound, color, and aroma. The color moves from light gray to dark brown, the aroma builds from subtle to rich, and there are two distinct cracking sounds. Taking heat-tolerant green beans as an example, the first crack begins around the 7-minute mark and lasts about 2 minutes. The sound is deep and hollow — the beginning of thermal decomposition, with starch converting to caramel, moisture evaporating, and gas escaping. Around the 12-minute mark comes the second crack, with fierce, sharp sounds that signal cell-fiber rupture and a deeper roast.

The renowned Specialty Coffee Association of America (SCAA) promotes the use of caramelization values measured by infrared (the Agtron number) to define roast degree. The values run from 0 to 100: higher numbers mean lower caramelization, a grayish-white color, and a lighter roast; lower numbers mean higher caramelization, a dark glossy color, and a deeper roast.

Dehydration Phase

Green coffee typically contains 8–12% moisture, and water acts as a heat-conduction medium. As temperature rises, the free water in the beans gradually evaporates while internal gas pressure builds, pushing the bean's structure to expand. "Dehydration" doesn't mean removing all the water. As the beans respond to changes in temperature and humidity they turn elastic, until water activity decreases and they return to a hard, brittle glassy state. By then a large portion of the cellular contents has converted to gas, and the outer layer has thinned through thermal expansion. In an overly fast roast, the different evaporation rates between surface and core can leave moisture unevenly distributed — the surface vitrifies while the core stays under-roasted, yielding a cup with more astringency.

Maillard Reaction Phase

Once most free water has left and water activity drops below 0.7, temperature and pressure inside and outside the beans reach similar levels — and the beans expand rapidly. The color shifts from green to white, then begins turning yellowish-brown, emitting an aroma similar to toasted bread. What follows are the Maillard reaction and caramelization stages. The Maillard reaction is the series of reactions between reducing sugars and proteins/amino acids when heated, producing brown-black macromolecules — the very compounds that carry roast flavor.

Development Phase

As the color deepens toward dark brown and bean temperature climbs to nearly 185°C (values vary with the measurement point), listen for the approaching crack. The first crack is the moment when expanding carbon dioxide and moisture split the beans open — and it is a critical flavor turning point. After first crack begins, the beans enter the development stage: the Maillard reaction winds down while caramelization accelerates. During first crack, water vapor and carbon dioxide rush out of the cells in quantity, slightly lowering bean temperature and rapidly raising humidity; once the crack ends and the free water is gone, humidity falls again. Many reactions compete within this short window, which is why airflow and heat control during the late Maillard phase shape the beans' final flavor so strongly.

Bean Discharge and Cooling

When the roast reaches your target level or temperature, discharge the beans and cool them. Cooling is the final, crucial step: lowering the coffee's temperature as quickly as possible is essential to lock the flavor in at the exact point you determined at discharge.

FrontStreet Coffee Recommendation

How do you translate roast theory into a shopping decision? The rule of thumb: the darker the roast, the more bitter and full-bodied the cup; the lighter, the more acidic and delicate. For balance in between, medium roast is the sweet spot — and FrontStreet Coffee's medium-roast selection puts the theory into practice across three continents.

From Brazil, the Queen Estate Yellow Bourbon offers a quiet, gentle sweetness with cream-and-peanut, chocolate, toast, and sugarcane aromatics after medium roasting — equally good in a pour-over, moka pot, or espresso machine. From China, the 2013 Yunnan natural Typica shows the authentic Yunnan small-bean profile in a balanced, soft register: nuts, brown sugar, black tea, and dried apricot, suitable for pour-over, cold brew, or drip machines at excellent value. And from Jamaica, the Blue Mountain No.1 — Typica grown under the region's uniquely favorable terrain — is roasted medium precisely so its acidity, bitterness, sweetness, and body fuse into one elegant whole: chocolate, roasted nuts, cream, and caramel from the first sip.

Whoever roasts your beans, remember the final stage of this article applies to you too: after roasting, let the beans rest sealed in their one-way-valve bag for several days so the carbon dioxide vents and the flavors stabilize — and grind fresh just before brewing to catch everything the roaster worked to develop.