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Green coffee is dense, stable, and largely insoluble. Roasting converts it into something aromatic, soluble, and ready to brew. This transformation is chemistry, not craft alone.
Green coffee, in its raw state, is not what most people would recognise as coffee.
It is dense and hard. It smells grassy, vegetable-like, or faintly of raw grain. Brew it with hot water and you would extract something thin, sour, and largely unpleasant. The compounds that define roasted coffee's aroma, sweetness, and solubility do not yet exist in meaningful form.
Roasting creates them. Through the controlled application of heat over time, green coffee undergoes a sequence of physical and chemical transformations so extensive that it becomes, in fundamental ways, a different material.
Understanding what roasting does, not just the language around it, but the chemistry beneath it, gives you a framework for evaluating roast quality, diagnosing flavour problems, and understanding why different coffees respond differently to the same roast approach.
The Starting Material: What Green Coffee Contains
Before roasting can be understood, the starting composition of green coffee matters: carbohydrates (40–55% dry weight, primarily sucrose and complex polysaccharides, a primary precursor to Maillard and caramelisation reactions), proteins and free amino acids (10–13%, the other primary input for Maillard reactions), chlorogenic acids (6–10% in arabica, 9–12% in robusta, whose degradation contributes to colour, acidity, and bitterness), lipids (15–17%, concentrated in the bean's interior, responsible for body and aromatic retention), caffeine (1.2–1.5% in arabica, 2.2–2.7% in robusta, thermally stable through roasting), water (10–12%, the first thing driven off), and hundreds of aromatic precursor compounds that will transform under heat into volatile aromatic molecules.
Phase 1: Drying
The first phase of roasting is dominated by moisture evaporation. As the bean temperature rises from ambient (charge temperature, typically 160–220°C depending on roast profile) through the turning point (the temperature minimum as the beans absorb heat from the drum), water evaporates from the bean's surface and interior.
During this phase, the bean temperature rises, but relatively little chemical transformation occurs. The primary activity is physical: water evaporating, the bean warming, the cellular structure beginning to soften.
The duration and management of the drying phase significantly affect what follows. Insufficient drying, moving too quickly through this phase, can leave moisture trapped in the bean's interior, which then converts to steam under pressure and can disrupt even development later. Excessive heat during drying can begin scorching the bean surface before the interior has adequately warmed, the origin of a common roast defect.
Phase 2: The Maillard Reaction
As bean temperature passes approximately 140–150°C (internal), the Maillard reaction begins.
The Maillard reaction is not a single reaction but a cascade of hundreds of simultaneous and sequential chemical reactions between reducing sugars and amino acids. Named after French chemist Louis-Camille Maillard, who described it in 1912, it is responsible for the browning of bread crusts, the searing of meat, and critically, the development of roasted coffee's core flavour compounds.
During the Maillard phase in coffee roasting, sucrose and other sugars react with free amino acids to form N-glycosylamines, which rapidly rearrange and degrade through Amadori rearrangement into more reactive intermediate compounds. These intermediates react further to produce hundreds of heterocyclic compounds, pyrazines, furans, oxazoles, thiophenes, and many others, that collectively define roasted coffee's aroma profile. The bean colour changes from yellow-green through yellow to increasingly brown, and the grain-like, vegetative character of green coffee is replaced by progressively more complex roasted aromatics.
The Maillard phase is where most of roasted coffee's flavour complexity develops. Its duration and temperature profile are critical: too rapid, with excessive heat, and the reaction produces bitter, harsh compounds; too slow, with insufficient heat, and development is incomplete, producing cereal-like, astringent flavours.
Caramelisation
As temperatures rise further, approximately 170°C+, sucrose begins to caramelise independently of the Maillard reaction. Caramelisation degrades sucrose into simpler sugars which then undergo further thermal degradation, producing the diacetyl, furanones, and caramelans associated with caramel flavour notes.
Caramelisation contributes to the sweetness and caramel-toffee character in medium roasts. In very dark roasts, continued caramelisation produces increasingly bitter caramel degradation products.
First Crack: The Structural Transformation
First crack is the most audible event in roasting, a sequence of sharp cracking sounds produced by the explosive release of steam and CO₂ pressure as it exceeds the structural strength of the bean's cell walls.
At first crack, typically occurring at bean temperatures of approximately 195–205°C though this varies significantly by density, internal steam and CO₂ pressure breaks the bean's cell walls, the bean rapidly expands in volume by approximately 50–60% from green, density drops significantly as cell walls fracture and pores open, volatile aromatic compounds are released as the cellular structure becomes permeable, the silverskin separates as chaff, and solubility increases dramatically as the porous structure allows hot water to penetrate during brewing.
First crack marks the transition into what roasters call the 'development phase', the period during which the roast is finished and its final character determined.
Development After First Crack
What happens after first crack is the roaster's primary lever of flavour control.
Extending the roast beyond first crack, at reduced heat, allows further caramelisation, continued aromatic development, and progressive structural changes that shift the cup profile. Stopping shortly after first crack preserves a higher proportion of origin character and acidity. Extending further toward second crack, where cellulose in the bean walls begins to pyrolyse, produces darker, heavier roast character.
The period from first crack to the end of roasting, typically 1.5–4 minutes in most specialty profiles, is where the roaster's decisions have the most direct impact on what ends up in the cup.
Second Crack and Beyond
Second crack, less commonly used in specialty roasting, occurs at approximately 224–230°C bean temperature when the carbon framework of the bean's cellulose structure begins to break down. The sound is more subtle than first crack, a more rapid, papery crackling.
Beyond second crack, the coffee is significantly dark. Oils migrate to the surface, giving dark roasted beans their shiny appearance. Roast character increasingly dominates origin character. At extremes, French or Italian espresso roast, the coffee approaches carbonisation. Caffeine content is unchanged, but most other chemical compounds have been progressively degraded.
Roasting is amplification, not creation. It can only develop what the green coffee contains. A spectacular roast of poor green coffee produces a mediocre cup. A skilled roast of exceptional green coffee reveals what was already there.
What Roasting Cannot Do
This is as important as what roasting can do: roasting cannot remove physical defects from the green coffee, since black beans, sour beans, and foreign material enter the roaster and exit it; roasting cannot correct poor fermentation, since over-fermented character may be modulated by darker roasting but the underlying defect remains; roasting cannot improve underdense, underripe, or poorly harvested coffee, since its flavour potential is fixed before it reaches the drum; and roasting cannot add complexity that was not present as precursor compounds in the green coffee.
This is why the entire supply chain that precedes roasting, genetics, terroir, harvesting, processing, grading, matters so much. The roaster works with what they are given. A roaster with access to exceptional green coffee and the skill to develop it is operating in a different territory than one attempting to compensate for poor raw material with heat.
