Roasted coffee contains three essential groups of acids: aliphatic acid, chlorogenic acid and phenolic and alicyclic acids. Coffee also contains the inorganic acid, phosphoric acid. The specific types and concentrations of these various acids vary in coffee depending on the type of bean and factors of brewing.
Tannic acid, also known as gallotannic acid, is an astringent plant product present in coffee. Its biological purpose is still unknown, according to "A Dictionary of Biology", though biologists and ecologists theorize it may impart an unpalatable taste to discourage animals from grazing on the plants containing it, and it may shield the plants from attack by pathogens. The generic term "tannins" is used to describe any number of compounds containing hydroxy acids, phenolic acids and glucosides.
Approximately 7 percent of coffee's dry weight can be attributed to chlorogenic acids, as can much of some coffee's acidic taste, or perceived acidity. Chlorogenic acids have an astringent taste and may contribute to a heightened "body" in the coffee. Over 17 different chlorogenic acids have been isolated in coffee beans, most of which degrade during the roasting process. A 2007 study in the "Journal of International Medical Research" suggests that the chlorogenic acids in coffee may help the body absorb and utilize dietary glucose, thereby encouraging weight loss.
Aliphatic acids exist in coffee mostly in the form of proteins, as amino acids. Coffee contains at least two dozen aliphatic acids including acetic, citric, lactic, malic and pyruvic acid. In its green form, coffee beans contain only a negligible amount of these acids. But when roasted, the concentration of these acids increases exponentially. Next to the chlorogenic acids, aliphatic exist in the highest proportions in roasted coffee.
Phenolic & Alicyclic Acids
Coffee is rich in phenolic acids, such as caffeic, ferulic and quinic acids. Phenolic acids are types of polyphenols, which possess antioxidant properties. A 2009 study in "Carcinogenesis" suggested that the caffeic acid in coffee may possess anti-carcinogenic properties as well.
Among the lipids in coffee oil are fatty acids in similar concentration as is found in edible vegetable oil. A 2000 study in "Talanta" found at least 10 fatty acids in coffee oil extracts, including palmitic, stearic, oleic, linoleic and linolenic acids. A 2006 study in the "Journal of the Science of Food and Agriculture" determined that the oil in coffee could be used as a valid source for essential fatty acids.
Factors Influencing Acidity
Several factors seem to influence the acidity in a cup of coffee, including how long and at what temperatures the beans have been roasted, what kind of roaster was used and the method of brewing. The lighter the roast, the higher the concentration of acids tends to be; the darker the roast, the lower the acid concentration. Coffee grown at higher altitudes and in volcanic, mineral-rich soil typically has a higher acid content. Washed coffee has a higher perceived acidity than dry roasted coffee.