Specialty coffee has a longstanding reputation for being acidic ­– that much is true.

Acidity brings vibrancy and life to our coffees, and is greatly sought after when we brew coffees. In the right amounts, the effect of a good tinge of tartness in a cup of coffee is staggering: it provides a refreshing twist in flavour that whets the senses, and accentuates the drink’s sweetness.

However, specialty coffee may also be sour. Although the two might sound interchangeable in nature, coffee professionals perceive both very differently.

One should normally expect acidity in a coffee. When describing coffee, acidity is often used positively, and is seen as a valuable contribution to the final taste profile, such as in a bright citrusy acidity, or a creamy mouthfeel. On the other hand, sourness in coffee is used to represent an unpleasant sourness that can indicate under extraction. The line distinguishing the two can often be very fine, and in some cases, it is a matter of balance that makes an acidity good or bad.

So, what then makes up the prized and delicious acidity in our coffee?

Citric acid

Easily identifiable in a cup of coffee as the acidity present in lemons and oranges, citric acid is often described as bright and crisp. It tends to bring an element of juiciness to the coffee.

Malic acid

Malic acid is tart and leaves a lingering taste on the tongue. Characterised by the taste of pears and green apples, malic acid often provides a fruity, acid-forward note in a cup of coffee.

Tartaric acid

Generally associated with its primary contributor, grapes, tartaric acid provides a slightly astringent mouthfeel that alludes to the winey taste of grapes.

In the right amounts, it provides a pleasant mouthfeel and a desired finish that leaves us wanting more. Too much, and it gives an overwhelmingly sour and dry aftertaste.

Acetic acid

While vinegar and acetic acid are the same, this does not mean that our coffee should taste outwardly like vinegar!

A result of the fermentation and roasting processes, small amounts of acetic acid give rise to a pleasant, clean-tasting, lime-like notes in the finish. In larger doses, predominant fermented flavours take the place of the cleaner ones, covering any potential benefits of acetic acid in the cup profile.

Lactic acid

Produced by bacteria undergoing anaerobic fermentation, the presence of lactic acid greatly enhances the mouthfeel of a cup of coffee. Commonly found in milk, it gives a somewhat creamy and milky texture to the coffee, supplementing the body of what might otherwise be a light cup of filter coffee.

Phosphoric acid

The only inorganic acid in this list, phosphoric acid found in coffee beans comes purely from the hydrolysis of phytic acid in the soil. Generally odourless, it adds a sparkling attribute of acidity to coffee, and works in tandem with the organic acids found in coffee by enhancing the perceived flavours from those acids.

Quinic acid

Chlorogenic acid inherently present in green coffee beans decomposes to produce quinic acid during roasting. This acid lends to an astringent and sour taste, and an unpleasant bitterness in a cup of brewed coffee.When roasting coffee, one aims to minimize the amount of quinic acid produced and balance it with the other acids and sugar in the cup to present a well-rounded and complete taste profile.

A balancing act

Acidity shares a relationship with the level and extent of roast of a coffee bean. The amount of acids in the coffee beans varies across origins and varietals. Some acids are developed through the roasting process, while others are gradually lost over the course of it. Balance in this process is the key to good coffee, for both the coffee roaster and the coffee brewer.

When we drink our coffees at home, it takes an acute sense of taste to be able to discern different tasting notes in the cup. Yet, we often lack an explanation for understanding the difference in nuances between the tasting notes, especially with acidity.

With the knowledge of the various acids at work, we can understand the tasting notes we perceive when we drink a cup of coffee, and truly appreciate the wonders that a good acidity is able to bring.

Deciphering Acidity

Journal article by Homeground Coffee Roasters.