|MadSci Network: Chemistry|
The ingredients in coffee are water and the roasted seeds of the coffee plant, Coffea arabica. The chemical constituents of coffee would be those compounds in the roasted coffee seeds that are soluble in boiling water. According to Dr. Duke's Phytochemical Database, unroasted, green coffee seeds contain the following soluble compounds:
2,3,5-TRIMETHYLPHENOL 2-ETHYLPHENOL 2-METHOXY-4-ETHYLPHENOL 2,4-METHYLENEPHENOL DICAFFEOYL-QUINIC ACID 4-ETHYLPHENOL 4-METHOXY-4-VINYLPHENOL ACETALDEHYDE CAFFEINE CAFFEOL CAFFEOYL-3-QUINIC ACID CAFFETANNIC ACID CHLOROGENIC ACID CITRIC ACID DATURIC ACID GUAIACOL HYPOXANTHINE ISOCHLOROGENIC ACID PUTRESCINE SCOPOLETIN SPERMIDINE SPERMINE SUGARS TANNIC ACID TANNIN THEOBROMINE THEOPHYLLINE THIAMIN TRIGONELLINE XANTHINE
Many of these compounds, like chlorogenic acids, will be destroyed by the roasting process, and many, will be oxidized to form new compounds not listed above. Most of these, like the phenolics at the top, are responsible for the aroma of the coffee, while others, like the xanthine derivatives give coffee its stimulant qualities.
Since posting this answer, I've come across several articles from the mid 1970's that describe the source of the bitterness associated with roasted seeds, such as in coffee, chocolate, and various malts. These chemists were able to isolate the compounds that give these foods their distinctive bitternesses, and found them to be cyclic dipeptides, also known as diketopiperazines, or DKP's for short. In chocolates, most of these have phenylalanine as one of the two amino acids, producing benzoyl-diketopiperazines, while the flavorant cyclic dipeptides in coffee all contain at least on proline, producing the heterodicyclic pyrrolidino-diketopiperzine. In coffee, the second amino acid is either another proline (bispyrrolidino-diketopiperzine), or a hydrophobic amino acid, as in cyclo(pro-ile), cyclo(pro-val), or cyclo(pro-phe): this last one is particularly interesting, since it contains phenylalanine, tying the bitterness of coffee with the bitterness of chocolate.
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