Green Bean Composition
A raw, unroasted coffee bean is approximately 50 percent carbohydrate, 10 to 15 percent lipid, 11 percent protein and amino acids, 6 to 9 percent chlorogenic acids, 1 to 2.5 percent caffeine, 1 percent trigonelline, and 3 to 5 percent minerals, with the remainder being water (10 to 12 percent) and miscellaneous organic compounds. These proportions vary by species (arabica vs. robusta), variety, origin, altitude, processing method, and agricultural practices.
Carbohydrates
Carbohydrates are the dominant fraction. Polysaccharides, primarily cellulose, galactomannans, and arabinogalactans, make up the structural framework of the bean. These complex sugars do not dissolve into the brew directly but their partial thermal decomposition during roasting produces caramelization products, furanones, and pyranones that contribute sweetness and body.
Sucrose is the most important simple sugar, comprising 6 to 9 percent of arabica green beans (robusta contains roughly half as much). Sucrose is almost completely destroyed during roasting, but its thermal degradation is a primary driver of caramelization reactions and a precursor to many volatile flavor compounds. The higher sucrose content of arabica compared to robusta is one chemical explanation for arabica’s generally sweeter, more complex flavor profile.
Small amounts of glucose, fructose, and other reducing sugars participate directly in Maillard reactions during roasting.
Lipids
Coffee lipids, primarily triacylglycerols and diterpene esters (cafestol and kahweol), reside mainly in the bean’s endosperm oil. Arabica contains approximately 15 to 17 percent lipids; robusta contains 10 to 11 percent. These lipids are crucial for flavor: they act as solvents for volatile aromatic compounds during roasting, carry flavor in the cup as emulsified oils, and contribute mouthfeel and body.
The diterpenes cafestol and kahweol are unique to coffee and have received attention for their cholesterol-raising effects (primarily relevant to unfiltered brewing methods that allow lipids into the cup) as well as potential anti-inflammatory and hepatoprotective properties.
During roasting, some lipids migrate to the bean surface, creating the oily sheen visible on dark-roasted beans. This surface oil is vulnerable to oxidation, which is why oily dark roasts stale more quickly than dry light roasts.
Proteins and Amino Acids
Proteins account for roughly 11 percent of green bean dry weight. Free amino acids, while a small fraction by mass, play a critical role as reactants in Maillard browning during roasting. The amino acid profile influences which specific Maillard products form, thereby affecting flavor. Key amino acids include asparagine, glutamic acid, aspartic acid, and various sulfur-containing amino acids that generate potent volatile sulfur compounds during roasting.
Chlorogenic Acids (CGAs)
Chlorogenic acids are a family of phenolic compounds formed by esterification of quinic acid with hydroxycinnamic acids (caffeic, ferulic, and p-coumaric acids). They comprise 6 to 9 percent of arabica green beans and 7 to 12 percent of robusta. The dominant subclass is 5-caffeoylquinic acid (5-CQA), commonly called chlorogenic acid in singular.
CGAs are the primary source of perceived acidity in brewed coffee. They also serve as major antioxidants and contribute to astringency and bitterness at higher concentrations. During roasting, CGAs are progressively degraded: light roasts retain 50 to 80 percent of green bean CGA content, medium roasts retain 30 to 50 percent, and dark roasts retain as little as 5 to 15 percent. CGA degradation products, particularly chlorogenic acid lactones, contribute to the bitterness profile of darker roasts.
Robusta’s higher CGA content relative to arabica contributes to its typically harsher, more bitter, and more astringent cup profile.
Caffeine
Covered in detail in the dedicated caffeine entry, caffeine comprises approximately 1.2 percent of arabica and 2.2 percent of robusta by dry weight. It is thermally stable through roasting and highly water-soluble, extracting quickly during brewing. Caffeine contributes approximately 10 to 15 percent of perceived bitterness in brewed coffee.
Trigonelline
Trigonelline (N-methylnicotinic acid) is present at roughly 1 percent in arabica and 0.7 percent in robusta. It is thermally labile: during roasting, trigonelline decomposes to produce nicotinic acid (niacin, vitamin B3), pyridines, and other volatile compounds that contribute to coffee’s roasty aroma. A cup of coffee provides a meaningful dose of niacin, partly due to trigonelline decomposition.
Minerals
Coffee beans contain potassium (the dominant mineral, approximately 1 percent of dry weight), calcium, magnesium, phosphorus, sodium, iron, zinc, and manganese. Potassium is highly water-soluble and extracts readily, contributing to the alkaline buffering capacity of brewed coffee. The mineral content of brewed coffee, while modest in nutritional terms, influences pH, ionic strength, and can affect perceived flavor balance.
Roasting: The Chemical Transformation
Roasting transforms green coffee from a dense, grassy, hay-like seed into the aromatic, flavorful bean we recognize. This transformation occurs through three overlapping reaction systems.
Maillard Reaction
The Maillard reaction, a cascade of reactions between reducing sugars and amino acids, is the primary driver of flavor and color development during roasting. It begins around 150 degrees Celsius and accelerates as temperature rises. The initial condensation of a sugar with an amino acid produces an Amadori compound, which then undergoes a series of rearrangements, dehydrations, fragmentations, and polymerizations that generate hundreds of distinct flavor compounds.
Key Maillard products in coffee include pyrazines (nutty, roasty, earthy notes), furans and furanones (caramel, sweet, burnt sugar), thiophenes and thiols (roasty, meaty, sulfurous), and pyrroles (sweet, grain-like). The specific mix depends on the precursor amino acids and sugars present, which vary by origin and variety, and on roasting time-temperature profile.
Caramelization
Caramelization, the pyrolysis of sugars in the absence of amino acids, occurs at higher temperatures (above 170 degrees Celsius) and contributes additional sweet, butterscotch, and bitter-sweet flavor compounds. Sucrose caramelization is particularly important in coffee due to arabica’s high sucrose content.
Strecker Degradation
Strecker degradation, a subset of the Maillard cascade, involves the reaction of alpha-amino acids with dicarbonyl compounds to produce aldehydes, aminoketones, and CO2. The aldehydes formed through Strecker degradation are potent aroma compounds. Methylpropanal (malty), 2-methylbutanal and 3-methylbutanal (chocolate, dark fruit), phenylacetaldehyde (honey, floral), and methional (potato, cooked vegetable) are all Strecker aldehydes that contribute to coffee aroma.
Melanoidins
Melanoidins are high-molecular-weight, brown-colored polymers formed in the late stages of the Maillard reaction. They comprise up to 25 percent of roasted coffee dry weight and are the primary source of coffee’s brown color. Melanoidins also contribute body and mouthfeel, exhibit antioxidant activity, chelate metal ions, and may have prebiotic effects by serving as fermentation substrates for gut microbiota.
Volatile Aromatics
Roasted coffee contains over 800 identified volatile compounds, making it one of the most chemically complex foods. Major aroma impact compounds include 2-furfurylthiol (roasty, the single most potent coffee aroma compound by odor activity value), 4-vinylguaiacol (spicy, clove-like), several alkyl pyrazines (nutty, earthy), furaneol (caramel, strawberry), sotolon (maple, fenugreek), damascenone (fruity, jammy), and a variety of sulfur compounds, aldehydes, and ketones.
The volatile profile is inherently unstable. Aromatic compounds begin escaping from roasted coffee immediately, and oxidation reactions transform the aroma profile over time. This is why freshness is paramount: the aromatic complexity that makes specialty coffee distinctive is the most perishable aspect of the bean.
How Brewing Extracts Chemistry
Not all of the roasted bean’s chemical content ends up in the cup. Brewing extracts water-soluble compounds while leaving behind insoluble cellulose, lipids (except in unfiltered methods), and high-molecular-weight polymers. The extraction sequence matters: acids and caffeine dissolve first, followed by sugars and melanoidins, with dry, astringent, and bitter compounds extracting last.
TDS (Total Dissolved Solids) typically ranges from 1.15 to 1.35 percent for filter coffee and 8 to 12 percent for espresso. This means brewed coffee is 98 to 99 percent water and 1 to 2 percent dissolved coffee solids. Within that small dissolved fraction lies the full complexity of coffee flavor, a testament to the concentration and potency of the chemical compounds involved.