Not all sour tastes the same: Where does the acidity in coffee come from?

Natural acidity is as much a part of coffee as roasted aromas and fragrance. “Acidity” in coffee is by no means a single substance: numerous different acids and other compounds shape the acidity profile of a cup. They develop and change during cultivation, processing, and finally during roasting. However, there’s a world of difference between the natural acidity of the coffee bean and a sour off-flavor: lively acidity is something entirely different from a sour cup of coffee.

This also shows why a blanket negative assessment of acidity in coffee falls short. In fruity coffees, acidity is part of the desired flavor experience and is therefore perceived as pleasant by many.

The situation is different when the acidity does not stem from the coffee’s natural flavor profile but is instead attributable to the brewing process. In that case, the acidity in the finished brew can quickly become off-putting—it is often perceived as sharp, sour, and out of place. As we’ve already described here, this can have various causes: The coffee may not have been freshly brewed, the grind size may have been set incorrectly, or the water may not have been hot enough.

Coffee contains not just one, but a variety of different acids. The pH value indicates how acidic or basic a liquid is: a value below 7 is considered acidic, 7 is neutral, and anything above that is basic.

Coffee typically has a pH between 4 and 5.5, making it mildly acidic. However, the measured pH value does not automatically indicate how acidic a coffee actually tastes. For example, a coffee can have a comparatively high pH value and still possess a distinctly perceptible, pleasant acidity. Conversely, a coffee with a lower pH value may taste less acidic.

This is because our perception of taste depends on far more than just the pH value. The type and composition of the acids, as well as other flavors, also play a role. For example, we often perceive a coffee with fruity notes as more acidic than a chocolatey coffee—even if the pH value suggests otherwise. The pH level alone, therefore, cannot explain why we do or do not enjoy a coffee’s natural acidity.

What's Inside a Coffee Bean?

A coffee bean contains various acids and acid-forming compounds, including chlorogenic acids and trigonelline. However, not all of these compounds are present in green coffee. Some are formed only during the roasting process. Which acids are present and in what quantities depends on numerous factors—including the degree of ripeness of the coffee cherry, temperature and light during cultivation, and later, storage conditions.

Many of these compounds play an important role for the coffee plant. They are classified as antioxidants and can, among other things, protect the plant from natural predators. Caffeine also serves as a natural defense against pests.

For us as a roastery, a coffee’s acidity is an important criterion right from the cupping stage. We regularly taste new coffees, specifically seeking out varieties that complement our portfolio well. For a coffee intended for a fully automatic machine, for example, we tend to choose low-acidity varieties. The reason is simple: the lower the coffee’s natural acidity, the smaller the risk that minor errors in preparation will result in an unbalanced, sharply acidic cup.

With a fruity summer coffee, it’s a whole different story. Here, a lively, pleasant fruit acidity can be a decisive plus—especially when it’s paired with a pronounced natural sweetness. This interplay can make a coffee particularly complex and exciting.

Trigonelline also changes during roasting and is broken down into niacin and N-methylpyridinium, among other compounds. These changes contribute to the fact that, as the roast level increases, not only do the color and aroma of the bean change, but so does its acidity profile.

Roasting is therefore a crucial moment for the subsequent perception of acidity: It helps determine which acids end up in the cup, in what concentration they are present, and how they interact with sweetness, roasted flavors, and the coffee’s other nuances.

Why Coffee Tastes Differently in Terms of Acidity: A Conclusion

Let’s briefly return to the aromas in coffee and the roast time mentioned at the beginning: Our Rungeto is very light-roasted, tastes fruity, and may initially seem unusually tangy to consumers who aren’t used to this type of coffee.

Aroma and acidity are closely linked in coffee, but they are not the same thing. Every coffee bean has its own natural flavor profile. The roasting process influences this profile—among other factors, through roast time and roast temperature. A short roast time at a high temperature can affect certain compounds and antioxidant compounds differently than a longer, darker roast. Light roasts generally preserve more of the coffee’s original, fruity characteristics, while dark roasts are more strongly characterized by roasted aromas and caramelized notes.

Rungeto doesn’t taste fruitier and more acidic than our Toleyo simply because it’s roasted for a shorter time or at a higher temperature. The growing region, variety, degree of ripeness, processing method, and many other factors also influence which flavors and acids develop in the bean and how they’re later perceived in the cup.

In short: Good acidity tastes like ripe fruit—tangy like citrus, crisp like a green apple, or juicy like red berries. If the coffee tastes purely sour, it usually just lacks the necessary extraction. With the right combination of grind size, water temperature, and brew time, you can easily balance acidity and sweetness.