Roasting: What Heat Does to a Green Bean

Drying, Maillard reactions, caramelisation, first crack and development — and why roast level dictates how you brew.

Green coffee is dense, grassy, and almost unbrewable. Roasting drives a cascade of chemical reactions that create nearly every aroma you associate with coffee, and physically transforms the bean from a hard seed into a brittle, porous, soluble one¹.

The stages

1. Drying (ambient → ~160 °C). Green coffee is 10–12 % water². The first minutes of a roast evaporate most of it. Little flavour develops here, but how evenly the bean dries determines how evenly everything afterwards happens³.

2. Maillard reactions (~150–200 °C). Amino acids and reducing sugars react to form hundreds of new compounds: melanoidins (brown colour, body), pyrazines (nutty, roasty), furans (caramel), and many aromatic intermediates¹³. This is where "coffee" flavour is born. The bean turns from yellow to tan to light brown.

3. Caramelisation (~170 °C+). Sugars break down and reform into caramel-flavoured compounds. Sweetness peaks and then declines as sugars are consumed — a dark roast has far less residual sugar than a light one¹.

4. First crack (~196–205 °C). Water vapour and CO₂ pressure inside the bean finally rupture the cell structure with an audible pop. The bean expands, becomes porous, and from here on extraction becomes possible¹. Almost all specialty coffee is dropped between the start of first crack and shortly before second crack; that is trade practice⁴.

5. Development. The time after first crack. Longer development converts acids to sweetness and body and increases solubility; too long and the roast loses origin character and moves toward generic "roasty" flavour. The chemistry is described in the roasting literature³; the flavour judgement is the roaster's.

6. Second crack (~225 °C+). Oils migrate to the surface, the cellular structure begins to fracture¹. This is dark roast territory: carbon, smoke, and very low acidity.

What roast level changes for the brewer

	Light	Medium	Dark
Density	high	moderate	low (porous)
Solubility	low — hard to extract	moderate	high — easy to over-extract
Acidity	high	moderate	low
Sugars remaining	most	some	few
Bitterness	low	moderate	high
Grind for same extraction	finer	medium	coarser
Water temperature	94–100 °C	92–95 °C	88–92 °C
CO₂ after roasting	less gas, released slowly	moderate	more gas, released fast

Density and solubility trends are physical consequences of expansion and cell-wall degradation described in the roasting chemistry literature¹. The acidity and bitterness rows match what trained panels reported in a 2020 UC Davis study across light, medium and dark drip brews: acidity fell and bitterness and roast character rose with roast degree at matched strength and extraction⁵. The CO₂ row follows a 2014 study that measured gas content and release across roast levels: darker roasts contain more CO₂ yet lose it faster because their structure is more porous⁶. The grind and temperature rows are Cup Engineering practice, consistent with practitioner guidance⁷.

InteractiveWhat roast level does to the bean and the cup

Slide from a Nordic light roast to a dark roast. Physical properties on the left drive the brewing adjustments; the taste profile on the right is what panels consistently report.

Roast level — Medium-light35

In the bean

Solubility69%

Density88%

CO₂ retained51%

Aromatic compounds surviving74%

In the cup

Acidity70%

Sweetness96%

Body63%

Bitterness / roast43%

Water temp

~93 °C

higher for dense light roasts

Grind

your medium baseline

Rest after roast

7–14 days

before it peaks

How this is computed: The bars are qualitative tendencies (0–1), not measurements: acidity falls and bitterness rises monotonically with roast degree; sweetness peaks around medium as caramelisation outruns acid loss before burnt notes take over; body rises as cell walls degrade; solubility rises and density falls as the bean expands and its matrix becomes more porous; CO₂ retained after roasting rises with roast degree. Shapes follow the sensory trends UC Davis found across roast levels and the physical changes described in the roasting chemistry literature. Temperature and grind suggestions are Cup Engineering practice, consistent with the higher solubility of darker roasts. Sources: [5] [1] [3] [6]

Light roasts hide an enormous amount of flavour behind their density; they reward hot water, fine grinds, high ratios and agitation. Dark roasts give everything up immediately and punish exactly the same treatment.

Acidity and chlorogenic acids

Green coffee is rich in chlorogenic acids (CGAs), typically several percent by dry mass⁸. Roasting progressively degrades them, cleaving them into quinic and caffeic acid and, through dehydration, into chlorogenic acid lactones, which are among the compounds associated with the bitterness of roasted coffee; at darker roast levels the losses reach most of the original CGA content⁸. This is why bitterness is a roast property as much as an extraction one: a dark roast brewed at a perfect 20 % EY is still bitter⁵.

Degassing and freshness

Roasting generates CO₂ that stays trapped in the bean. In the first days after roasting the gas escapes rapidly and the rate then slows over weeks⁹; brew too early and it repels water, causes erratic blooming, and suppresses extraction — the cup tastes flat and sour. The common resting windows — roughly 5 to 21 days off roast for filter, and a bit later (10–30 days) for espresso, where CO₂ causes wild crema and channelling — are practitioner consensus rather than measured optima⁷. After a month or two, oxidation takes over and aromatics fade³. The freshness article goes into the timelines.

Reading a bag

Roast date, not "best before". Aim to use the coffee in the window above.
Roast level is often unstated; "filter roast" is generally lighter than "espresso roast" from the same roaster, but there is no standard⁴.
Agtron / colour numbers where given: higher number = lighter. The figures often quoted — specialty light roasts around 70–85, dark roasts around 40–50 — are trade conventions on the Agtron scale, not a standard⁴.