Coffee Roasting Explained:
From Green Bean to Cup

Coffee roasting uses heat to drive a chain of physical and chemical changes in green beans: drying, the Maillard reaction, caramelization and development after first crack.

Green coffee has almost no coffee aroma. During roasting, sugars and amino acids react to form hundreds of aroma compounds and brown melanoidins, ending with around 800 aroma compounds. A roast usually takes about 8–15 minutes and ends at a bean temperature of about 200–230°C (390–445°F).

The difference is where the roast ends: light roasts are dropped around first crack, medium roasts after first crack ends and dark roasts into second crack.

Light roasts have bright acidity with floral, citrus and berry notes; medium roasts balance acidity and sweetness with prominent caramel and stone fruit; dark roasts have low acidity and heavy body, led by cocoa, nuts and smoky bittersweetness, with less origin character.

First crack is the popping sound of steam and CO₂ bursting the bean’s cell walls, usually at a bean temperature of about 196–205°C (385–400°F); second crack is the brittle cell structure fracturing further, at about 224–230°C (435–445°F).

After first crack the bean swells by about 50–100% and development begins. After second crack, oils are pushed to the surface and flavor turns smoky and bittersweet. Exact temperatures vary with moisture, origin and probe placement.

The development time ratio (DTR) is the time from first crack to drop divided by total roast time; specialty roasters commonly aim for 20–25%.

For example, an 11-minute roast with first crack at 8:30 has about 2:30 of development, a DTR of about 23%. Too low risks underdevelopment, with sharp acidity and grassy notes; too high lowers acidity and can make flavor heavy or flat.

Usually not by much: caffeine is heat-stable, so each bean’s caffeine barely changes during roasting.

By weight, dark beans have lost more mass, so the same grams hold more beans and possibly slightly more caffeine; by scoop, denser light beans can hold slightly more. Variety and brewing method matter more.

Per this issue’s guide: light roasts about 5–12 days for filter and 10–21 days for espresso; medium roasts about 3–10 days for filter and 7–14 for espresso; dark roasts about 1–5 days for filter and 3–10 for espresso.

Freshly roasted beans hold lots of CO₂, which causes uneven extraction and a sharp cup. Count from the roast date; beans and personal taste both shift the ideal window.

Not necessarily: in dark roasts, second crack pushes oils to the surface, so oiliness is normal.

Light roasts usually aren’t oily when fresh but may slowly turn oily with age. The roast date is the most reliable freshness check, along with whether the aroma has faded or smells rancid.

No: good light-roast acidity is bright, citrus- or berry-like and comes with sweetness.

Underdeveloped acidity is sharp and stinging, often with grassy, raw-peanut notes and little sweetness. If the cup is simply sour and thin, under-extraction from a coarse grind or cool water is more likely.

An Agtron number is a near-infrared reading of the color of roasted beans or grounds; higher numbers mean lighter roasts.

In Coffee Review’s scale, Agtron above 80 is light, 60–70 is medium and below 50 is dark. Roasters don’t agree on names like “City” or “French,” so Agtron readings make comparisons more objective.

Pour-over commonly uses light to medium roasts to highlight florals, fruit and bright acidity; espresso commonly uses medium to Full City roasts for more sweetness and body, and pairs better with milk.

These are only common pairings: light roasts can work as espresso with a finer grind and hotter water, and dark roasts can work as pour-over with water at about 85–90°C (185–195°F) to reduce bitterness.