Re: What are all the ingredients in coffee?

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.

Addendum

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.