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Acid compounds

The perceived acidity of coffee brews has always been recognized as an important attribute of coffee quality. Acidity is typically a highly valued quality especially in Central American and some East African coffees. Sourness, however, is an extreme of acidity and can be considered as defect. Acidity has been correlated with coffees grown at very high altitudes and in mineral rich volcanic soils. The perceived acidity of washed coffees is also significantly higher than the acidity found in naturally (dry) processed coffees. This is likely due to an increase in the body of naturally processed coffees relative to wet processed coffees since body masks the coffee's acidity. The acid content in a brew is also greatly dependent upon the degree of roast, type of roaster, and brewing method. The pH of a coffee has been found to correlate with the perceived acidity of a coffee. A pH of 4,9 to 5,2 is the preferred range for a ‘good cup of coffee'.

Over 100 different acids have been isolated in green and roasted coffee. The major acids in green coffee are chlorogenic and quinic acid, with malic and citric acid as minor compounds. During roasting the complex chlorogenic acids are mainly degraded into smaller components. Other acids, such as acetic and formic acids, are formed during roasting, but, due to the high volatility of both compounds, also disappear at the later stages of the roasting process.

Chlorogenic acids

Chlorogenic acids are actually a family of esters formed between quinic acid and phenolic compounds known as cinnamic acids. The most abundant chlorogenic acid in coffee is 5-O-caffeoylquinic acid, an ester formed between quinic acid and caffeic acid, see image below. Over 17 different chlorogenic acids have been found in green robusta beans, but it is likely that more chlorogenic acids will be present in beans from different regions, plant species and varieties.

Coffee represents one of the richest dietary sources of chlorogenic acid. The chlorogenic acid content of a 200 ml cup of coffee has been reported to range from 70-350 mg, which would provide about 35-175 mg of caffeic acid. It has been claimed that the cholorgenic acids in coffee have a health promoting effect. Although chlorogenic acid and caffeic acid have antioxidant activity in vitro, it is unclear how much antioxidant activity they contribute in vivo because they are extensively metabolized in the body, and the metabolites often have lower antioxidant activity than the parent compounds.

Caffeic acid is an acid which is not only present in coffee, but in a large range of edible and non-edible plants. It is known as an antioxidant. On the other hand recent studies have revealed that caffeic acid may also be a carcinogen. The overall health effect of caffeic acid thus is unknown.

Sources :

• Olthof MR, Hollman PC, Buijsman MN, van Amelsvoort JM, Katan MB . Chlorogenic acid, quercetin-3-rutinoside and black tea phenols are extensively metabolized in humans. J Nutr. 2003;133(6):1806-1814.
• http://www.inchem.org/documents/iarc/vol56/03-caff.html
• http://www.coffeeresearch.org/science/sourmain.htm