Roasting Chemistry

Coffee roasting occurs in three major phases: the drying phase (150 to 160 degrees C), the Maillard phase (160 to 190 degrees C), and the development phase (190 to 230 degrees C). During the drying phase, water evaporates from the bean, and the bean structure changes from dense green to yellow. At approximately 160 degrees C, the Maillard reaction begins: amino acids react with reducing sugars to form melanoidins and hundreds of aromatic compounds. At approximately 170 degrees C, caramelization begins: sucrose and other sugars decompose into caramel compounds. At approximately 196 degrees C, the first crack occurs: steam pressure builds inside the bean, causing an audible cracking sound as the bean structure fractures. During the development phase (after first crack), pyrolysis occurs: complex molecules break down into smaller aromatic compounds, and the bean darkens. The roasting process is exothermic after first crack, meaning the bean releases heat.

Major chemical reactions during roasting: 1. Maillard Reaction (160 to 190 degrees C): amino acids + reducing sugars -> melanoidins, furans, pyrazines, pyrroles, thiols. Produces browning, body, and savory/nutty aromas. 2. Caramelization (170 to 200 degrees C): sucrose -> caramelan, caramelen, caramelin, volatile aldehydes, ketones. Produces sweet, caramel aromas and darkening. 3. Strecker Degradation (throughout): amino acids + dicarbonyl compounds -> aldehydes (Strecker aldehydes). Produces key aroma compounds like 2-methylbutanal (malty), phenylacetaldehyde (floral), 3-methylbutanal (cocoa). 4. Pyrolysis (190 to 230 degrees C): thermal decomposition of complex molecules. Produces volatile phenols, guaiacols, and other smoky, spicy compounds. 5. Degradation of Chlorogenic Acid (throughout): chlorogenic acid -> caffeic acid + quinic acid. Reduces acidity and astringency. 6. Lipid degradation: <a href="/coffee-science/lipids-and-coffee-oil">triglycerides</a> -> free fatty acids + volatile aldehydes. Contributes to rancidity if over-roasted. 7. Caffeine is relatively stable during roasting (minimal degradation). The net result: green coffee has ~250 identified compounds; roasted coffee has over 1,000.

Roasting coffee involves complex thermodynamics governed by conduction, convection, and radiation. In a standard drum roaster, convection provides 70% to 80% of the heat transfer via heated airflow, while conduction accounts for 20% to 30% through contact with the drum surface. The bean undergoes a glass transition phase at temperatures exceeding 100°C, moving from a rigid, glassy state to a flexible, rubbery state. This transition increases the bean's specific heat capacity and allows for significant structural expansion. Internal pressure within the bean cells reaches 20 to 25 atmospheres (2.0 to 2.5 MPa) due to the vaporization of internal moisture and the generation of carbon dioxide. This pressure eventually exceeds the tensile strength of the cellulose walls, resulting in the 'first crack' between 190°C and 200°C. The bean volume expands by 50% to 100% while its mass decreases by 12% to 20% through moisture loss and organic matter decomposition. Final bean density typically drops from 0.65 g/cm³ to approximately 0.35 g/cm³, creating the porous structure necessary for subsequent water penetration.

The chemical and physical modifications during roasting directly determine the solubility and diffusion rates during brewing. Thermal degradation of the bean's cellulose matrix increases porosity, facilitating the entry of water and the exit of soluble solids. Darker roasts exhibit greater micro-porosity and lower structural integrity, leading to faster extraction and higher Total Dissolved Solids (TDS) compared to lighter roasts under identical brewing parameters. Roasting degrades chlorogenic acids (CGA) from roughly 8% in green Arabica to less than 1% in dark roasts, which shifts the flavor profile from bright acidity to increased bitterness. The formation of CO2 during the Maillard reaction and pyrolysis creates an internal gas reservoir; this gas acts as a barrier to water contact, requiring a 'bloom' phase during extraction to degas the grounds. Lipids, which comprise 15% to 17% of the bean mass, migrate toward the surface as the roast progresses, influencing the emulsification process and the stability of crema in espresso. Research by the Specialty Coffee Association (SCA) and the Coffee Renaissance project confirms that the Roasting Degree (measured via Agtron scales) is the primary predictor of extraction efficiency and chemical yield.

Roast phases: drying (ambient to 160C), Maillard (160-190C), development (first crack to end). Key temperatures: first crack ~196C, second crack ~224C. Exothermic phase begins at first crack. Rate of Rise (RoR) should decrease monotonically for even development. Total roast time: 8-14 minutes for drum roasters. Development Time Ratio (DTR): 20-25% of total time after first crack. Sugar browning: sucrose degrades from ~8% (green) to ~0.5% (dark roast). Chlorogenic acid: degrades from ~7% (green) to ~2% (medium) to ~1% (dark). Total acid content decreases. Acidity (pH) increases from ~5.5 (light) to ~6.0 (dark). Aroma compounds peak at medium roast and degrade at dark roast. The ideal roast profile depends on bean origin, processing method, and intended brewing method.

Choose roast level based on brewing method and preference. Light roasts work well for pour over and AeroPress, where clarity and acidity are desired. Medium roasts are versatile for most methods. Dark roasts work well for espresso, where body and bitterness complement milk drinks. If your coffee tastes too acidic, try a slightly darker roast. If it tastes too bitter or burnt, try a lighter roast. Freshly roasted coffee should rest for 3 to 7 days before brewing to allow degassing.

• •Dark roasts have more caffeine. In reality, caffeine is relatively stable during roasting. Dark roasts may have slightly more caffeine by volume (less dense beans) but slightly less by weight.
• •All coffee should be roasted the same way. In reality, different origins, processing methods, and intended brewing methods require different roast profiles.
• •Roasting longer always means more flavor. In reality, over-roasting destroys volatile aromatic compounds, producing a flat, bitter, ashy cup.

• •Over 1,000 volatile compounds have been identified in roasted coffee, compared to ~250 in green coffee.
• •The Maillard reaction in coffee produces melanoidins (brown polymers) that contribute to body, antioxidant activity, and flavor.
• •Research has shown that sucrose content in green coffee correlates with roast development: higher sucrose produces more caramel and sweet notes.
• •Chlorogenic acid degradation during roasting reduces acidity and astringency but also reduces antioxidant content.

• •Chapter 4: Roasting
• •Chapter 5: Roasting Chemistry

• •Clarke, R.J. & Macrae, R. (1987). 'Coffee: Volume 2: Technology.' Elsevier Applied Science.
• •Illy, A. & Viani, R. (2005). 'Espresso Coffee: The Science of Quality.' Academic Press.
• •Baggenstoss, J. et al. (2008). 'Roasting and Degassing Kinetics.' Journal of Agricultural and Food Chemistry.
• •Dutta, R. et al. (2006). 'Coffee Roasting Monitoring.' Journal of Food Engineering.

• •Illy Coffee Quality Book
• •SCA Roasting Standards
• •Coffee Science Foundation
• •Roasters Guild