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 taste sensation. When coffee beans are heated the acids they contain will 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. For example, a typical medium roast loses 50 percent of the citric acid concentration originally found in the unroasted beans. (Weers et al., 1995). On the other hand, as we discuss in the next lesson, other organic acids including acetic, formic, lactic, and glycolic acid are formed during roasting as soluble carbohydrates like sucrose, glucose and fructose are degraded as they are heated (Ginz et al., 2000).
Taking its lead from the wine industry, over the last twenty years, the coffee industry has placed a lot of importance on the presence of fruit acids in coffee. However, recent evidence has shown that coffee professionals cannot distinguish between most organic acids (Birke Rune et al., 2023) with the exception of acetic and formic acids which are aromatic even at room temperature. Birke Rune and colleagues also demonstrated that most acids present in brewed coffee (including malic and tartaric acid) sit below the sensory detection threshold. Citric acid is a notable exception to this rule.
There is much left to be discovered about the role of organic acids in the sensory perception of coffee beverages. It is clear there is a non-linear relationship between the quantity of acids present in brewed coffee and its perceived acidity or sourness (Birke Rune et al., 2023). Serving temperature for example can significantly alter the perceived acidity of a brewed coffee (Steen et al, 2017).
There are 38 organic acids in coffee (H.H.