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?

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 the acids produced during the roasting process.

Chemical Changes During Roasting

When raw coffee beans are roasted, certain components in the beans undergo chemical changes that form new acids. For example, chlorogenic acid decomposes upon heating to produce quinic acid, while sugars decompose to generate the 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. During roasting, however, chlorogenic acid continuously degrades into quinic acid, gradually increasing its concentration, until quinic acid finally accounts for approximately 0.6% to 1.2% of roasted bean weight - meaning its concentration after roasting is more than double what it was before roasting.

The concentration of quinic acid reaches its maximum after the beans enter second crack. If roasting continues into the middle and later stages of second crack, quinic acid rapidly degrades from its peak into aromatic components including phenol (also known as carbolic acid), catechol, hydroquinone, and pyrogallol, which is why dark roasts and light roasts taste so dramatically different. Clearly, quinic acid is quite stable during roasting and will not easily decompose unless subjected to an unusually heavy roast.

Acidity Changes in Cooled Coffee

Additionally, when brewed coffee cools down, its acidity intensifies, and this is also related to quinic acid. Quinic lactide, which has no sourness, hydrolyzes into quinic acid as the coffee cools, 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 roasted beans, and that quinic acid is also the culprit behind the astringent taste that appears when coffee is left sitting in a thermos for too long after brewing.

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

Other Acidic Components

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

FrontStreet Coffee Recommendation

During roasting, three elements decide the final flavor and must be kept in balance: sourness, sweetness, and bitterness. The darker the roast, the more bitter and full-bodied the coffee; the lighter the roast, the more acidic and lighter the body. If you are chasing a well-rounded cup, medium roast coffee beans are the best place to start.

If you dislike both sourness and bitterness, FrontStreet Coffee's Brazil Fazenda Rainha Estate is an excellent choice. Grown in the world's largest coffee-producing country, this Yellow Bourbon offers a quiet, sweet cup, and a medium roast brings out notes of creamed peanut, chocolate, toast, and sugarcane. It works for pour-over, moka pot, and espresso alike. For a sweeter medium roast, try the Strawberry Candy blend, which combines Costa Rican honey-processed coffee with Panamanian washed Geisha; its medium roast reveals orange-honey aromas and a rounded sweetness that suits cold brew, pour-over, and espresso.

Yunnan coffee lovers should consider FrontStreet Coffee's 2013 Natural Typica, a representative of Chinese specialty coffee with the classic Yunnan arabica profile. A medium roast preserves its balanced, gentle character with notes of nuts, brown sugar, black tea, and dried apricot - great value for pour-over, cold brew, or drip machines. And for the ultimate in balance, FrontStreet Coffee's Jamaica Blue Mountain No. 1, a Typica variety known as the "Emperor of Coffee," is roasted medium so that its acidity, bitterness, sweetness, and body merge into one elegant, harmonious cup of chocolate, roasted nuts, cream, and caramel.