Coffee is a complex matrix formed by the interaction of hundreds of distinct compounds. The green coffee bean holds molecules, including carbohydrates, proteins, lipids, and organic acids, which are largely flavorless precursors. Roasting subjects this matrix to intense heat, triggering chemical reactions that create the characteristic color, flavor, and aroma compounds. The final chemical profile is determined by the initial bean composition and the degree of change induced during roasting and brewing extraction.
Caffeine and Related Methylxanthines
Caffeine is the most recognized chemical in coffee, belonging to the class of methylxanthines. Its structure is similar to adenosine, a molecule that promotes relaxation and sleepiness. This similarity allows caffeine to act as an antagonist, binding to adenosine receptors in the brain without activating them, effectively blocking the signal for fatigue and increasing alertness.
This antagonism is the primary mechanism behind caffeine’s stimulating effects on the central nervous system. By inhibiting adenosine, caffeine indirectly enhances the release of natural stimulants like dopamine and glutamate, leading to an energy boost and reduced reaction times.
Coffee also contains less concentrated methylxanthines, including theobromine and theophylline. Caffeine concentration varies significantly by bean species. Coffea canephora (Robusta) beans contain a higher content (1.7% to 4.0% by dry weight) compared to Coffea arabica (Arabica) beans (0.8% to 1.4%).
Roasting influences the final caffeine level, though the difference is minor since caffeine is stable and does not degrade easily with heat. Lighter roasted beans generally contain a slightly higher concentration of caffeine by weight because they lose less mass. The final amount in the cup is heavily influenced by the brewing method, as extraction efficiency determines how much is dissolved into the water.
Organic Acids: Defining Taste and Antioxidant Power
Organic acids are responsible for the bright, fruity, or wine-like qualities in coffee and function as antioxidants. Chlorogenic Acids (CGAs) are the most abundant acid in the green bean, making up a significant portion of its dry weight. CGAs are studied for their antioxidant properties, which are largely preserved after roasting.
During roasting, intense heat causes CGAs to break down, dramatically affecting the final flavor. They decompose into simpler molecules, primarily quinic acid and caffeic acid. Quinic acid contributes to sourness and astringency, which can become bitter in dark roasts. Caffeic acid also contributes bitterness.
Other organic acids contribute distinct flavor notes. Citric acid is associated with bright, citrusy flavors; its concentration is highest in light roasts and decreases significantly as roasting progresses. Malic acid imparts a pleasant, apple-like tartness, and its concentration also decreases steadily during roasting. The balance of these acids creates the nuanced acidity valued in specialty coffee.
Carbohydrates and Lipids: Structure and Body
Carbohydrates make up about 50% of the green bean’s dry mass, serving as both structural elements and flavor precursors. Insoluble polysaccharides, such as cellulose and galactomannan, form the rigid skeletal structure of the cell walls. This structural matrix remains intact through roasting, contributing to the physical body and mouthfeel of the brewed coffee.
Simpler, soluble carbohydrates and amino acids are the foundational ingredients for the Maillard reaction during roasting. This non-enzymatic browning transforms these molecules into hundreds of compounds that contribute to color and flavor. The reaction forms melanoidins, large brown-pigmented polymers responsible for the dark color and the perceived viscosity of the brew.
Lipids, or fats and oils, constitute 10% to 18% of the bean’s dry weight, primarily triglycerides and diterpenes like cafestol and kahweol. Since lipids are largely preserved during roasting, they are crucial for the sensory experience. A small amount of these poorly water-soluble compounds is emulsified and extracted into the brew, where they trap volatile aromatic compounds. This emulsified fat provides the velvety mouthfeel and viscosity, and it is responsible for forming the reddish-brown foam, known as crema, on espresso.
The Volatile Compounds of Coffee Aroma
The complex scent of coffee is due to hundreds of Volatile Organic Compounds (VOCs) created almost entirely during roasting. Intense heat initiates the Maillard reaction and subsequent Strecker degradation. These reactions break down precursor molecules, such as amino acids and sugars, into small, odor-active chemicals that define the aroma.
Pyrazines are a significant chemical group formed by the Maillard reaction. These compounds contribute classic roasted, nutty, earthy, and cocoa-like notes, and their concentration increases as the roast darkens. Furans, formed from the thermal degradation of sugars, are also abundant, imparting sweet, caramel, and bready notes.
Sulfur-containing compounds, despite being present in trace amounts, have extremely low odor thresholds and are highly impactful on the final scent. For example, 2-furfurylthiol is considered the most important molecule for the characteristic “roasted coffee” aroma. Other sulfur compounds contribute savory or fruity notes. The combination of these volatile chemical families creates the complex retronasal flavor that distinguishes coffee.