Where Does Coffee's Acidity Come From? Why Does Coffee Become More Acidic When It Cools?

A Brief Explanation by FrontStreet Coffee: Where Does the Acidity in Coffee Come From?

Professional coffee knowledge exchange, for more coffee bean information, please follow Coffee Workshop (WeChat public account: cafe_style)

Acidity Components in Raw Coffee Beans

Raw coffee beans contain acidic components such as citric acid, malic acid, quinic acid, and phosphoric acid, but these are not the acidity flavors we perceive when drinking coffee. The acidity we taste mainly comes from acids produced during the roasting process.

Chemical Changes During Roasting

When roasting raw coffee beans, certain components in the beans undergo chemical changes to form new acids. For example, chlorogenic acid decomposes upon heating to produce quinic acid, while sugars decompose to generate volatile formic acid and acetic acid.

The Role of Quinic Acid

Quinic acid is also a type of phenolic acid, accounting for only 0.3% to 0.5% of raw bean weight. However, during coffee roasting, chlorogenic acid continuously degrades into quinic acid, gradually increasing its concentration. Finally, quinic acid accounts for approximately 0.6% to 1.2% of roasted bean weight—meaning the concentration of quinic acid after roasting is more than double that before roasting.

The concentration of quinic acid reaches its maximum after entering the second crack. If roasting continues into the middle and later stages of the second crack, quinic acid rapidly degrades from its peak into aromatic components including phenol (also known as carbolic acid), catechol, hydroquinone, and pyrogallol, making the flavors of dark roast and light roast significantly different. It's evident that quinic acid is quite stable during the roasting process and won't easily decompose unless subjected to unusually heavy roasting.

Acidity Changes in Cooled Coffee

Additionally, when brewed coffee cools down, the acidity intensifies, which is also related to quinic acid. This is because quinic lactide, which has no acidity, hydrolyzes into quinic acid, increasing the sour taste.

The Nature of Quinic Acid: Dead or Vibrant?

Is quinic acid a "dead acid" or a vibrant aromatic acid? There are two perspectives on this. R.J. Clarke, author of "The Flavour of Coffee," believes that excessive quinic acid causes the bitter taste in dark roast beans, and quinic acid is also the culprit behind the astringent taste that appears when coffee is left to sit in a thermos for too long after brewing.

However, research reports from the Specialty Coffee Association of America (SCAA) take a positive view, suggesting that quinic acid is water-soluble and will completely dissolve into the cup. Besides increasing body, it also adds complexity and brightness to coffee.

Other Acidic Components

Additionally, caffeic acid, another degradation product of chlorogenic acid, also belongs to the phenolic acid family. It has a slightly astringent taste and is also an antioxidant.

Important Notice :

前街咖啡 FrontStreet Coffee has moved to new addredd:

FrontStreet Coffee Address: 315,Donghua East Road,GuangZhou

Tel:020 38364473