How Much is Sour?
An aim when coffee quality is high is fruitiness in the form of acidity, separate from the sensation of sourness. A good way to differentiate this is to think of acidity as a sparkling tingle on the tongue, and sourness as a mouth-curdling negative flavour. By adding heat to acids, they evaporate and/or break down so, in this way, the roaster controls acidity over sourness: the longer and hotter they roast, the more fruit acid will evaporate and escape up the roasting chimney.
Joseph A. Rivera has reported, “A typical medium roast will lose about 50% of its initial citric acid concentration and with progressive destruction in later stages of roasting progresses.”
There are 38 organic acids in coffee (H.H. Balzer, 2001); the most abundant in this group behind chlorogenic acid is citric acid. The farmer plays a big role in the impact of citric acid in the cup. During ripening citric acid converts into sugars, with unripe coffee beans having a higher citric acid content. Great cups of coffee come from lots that have been carefully harvested and screened to ensure all the beans come from perfectly ripe coffee cherries. From green to roasted, citric acid has been measured from 13.11g/kg, reducing down to 6.34g/kg. (Weers et al., 1995)
Malic acid is a prized fruity taste in coffee. It’s the main fruit acid found in apples. Malic acid is also significantly degraded in the roasting process, reducing from 2.5g/kg in green coffee to 1.5g/kg. (Weers et al., 1995)
Tartaric acid is an acid found in wine grapes. It has been measured at levels ten times higher in green coffee than roasted coffee, with only 0.04g/kg found in a commercial roast. (Coffee Recent Developments pg. 21) It could be that this level of concentration at 0.0004% may be so low as to render this acid almost undetectable,