Where does coffee acidity come from during roasting?

Coffee acidity comes from organic acids naturally present in the bean, including chlorogenic, malic, citric, quinic, and tartaric acid. Yes, coffee does have citric acid, along with the others, and roasting transforms them. Light roasts keep more of the bright, fruity acids, while darker roasts break them down and push the cup toward bitterness and darker, heavier notes.

An arabica coffee bean typically holds more sugars and fewer harsh compounds than robusta, which carries more chlorogenic acid and tastes rougher. Altitude and origin also shift the balance: higher-grown beans ripen slowly, build more sucrose, and often show clearer fruit and floral acidity after roasting, which is what tasters read as layered brightness.

People who have tasted specialty coffee are often deeply impressed by its bright acidity—not a harsh, mouth-puckering sourness, but a pleasant tartness with fruity and floral notes. Where exactly does this acidity come from? Starting from the chemical composition of coffee beans, this article analyzes the changes in organic acids such as chlorogenic acid, malic acid, citric acid, quinic acid, and tartaric acid during the roasting process, as well as the relationship between sucrose content and origin and altitude, helping readers understand the sources of coffee's acidity, sweetness, bitterness, and aroma. Mastering this knowledge not only enhances your tasting ability but also provides practical reference for selecting coffee beans, roasting green beans, and brewing coffee.

After tasting specialty coffee, what do you think is the most significant difference between it and regular coffee? This editor believes it's the acidity. Of course, this is not to say that specialty coffee must be acidic, nor that other coffees have no acidity at all, but rather that specialty coffee typically presents a more prominent acidity—an interesting and pleasant tartness, with each cup having its own character and rich aromatic layers, possibly featuring flavors like grape, mango, or even wine.

So do you know why specialty coffee is so acidic?

In summary, coffee beans are rich in organic acids, aromatic hydrocarbons, proteins, lipids, alkaloids, and sugars. After chemical changes during roasting, they give rise to various interesting aromas, acidity, sweetness, and bitterness. Today let's explore the acidity, sweetness, bitterness, and aroma in coffee beans together—this knowledge is helpful for selecting coffee beans, roasting green beans, and brewing coffee.

The healthy but unpleasant harsh acidity in coffee: chlorogenic acid

Coffee itself is an acidic beverage, and one of the important aspects of appreciating specialty coffee is enjoying its layered acidity. Most specialty coffees use Arabica coffee beans, while common commercial beans are mostly Robusta. Robusta coffee beans contain more chlorogenic acid. Chlorogenic acid, also known as coffee tannic acid, has an unpleasant taste and is disliked by insects, which is why Robusta coffee trees generally have strong pest resistance.

Not only do insects reject chlorogenic acid, but humans also dislike its somewhat bitter and astringent taste, even though it is actually a good antioxidant that benefits health. The good news is that chlorogenic acid decomposes into quinic acid and caffeic acid during roasting, which is one of the reasons why the darker the roast, the less acidic the coffee.

Specialty coffee has less chlorogenic acid, so even a light roast won't be too harshly sour, while Robusta coffee beans are mostly roasted at medium or above so they won't be too sour to drink. But this is only a general guideline, not an absolute rule.

Wait, isn't specialty coffee supposed to be more acidic? So why is specialty coffee more acidic when it has less chlorogenic acid?

Because the layered, bright, and interesting acidity of specialty coffee is not produced by chlorogenic acid alone—chlorogenic acid is actually a bitter and astringent acid. The pleasant acidity in specialty coffee mostly comes from citric acid, malic acid, quinic acid, acetic acid, tartaric acid, and more, but actually far more than these, because coffee beans contain thousands of chemical substances that affect flavor. Today we'll mainly introduce these key flavor contributors.

Malic acid and citric acid: not what their names suggest

Malic acid and citric acid sound youthful and colorful, but did you know? Malic acid and citric acid actually don't have fruity aromas, because they are non-volatile, so you can't smell them. They are called malic acid and citric acid simply because apples and lemons are rich in these two acids.

But! If malic acid and citric acid combine with alcohols, they can burst forth with many charming aromas such as floral, fruity, and fresh scents—this is precisely the main source of aroma in green beans and roasted beans.

And there's also quinic acid and tartaric acid

Quinic acid is formed from the degradation of chlorogenic acid. It is currently uncertain whether it brings astringency to coffee or adds bright flavors. Both claims have supporters, but this editor believes the two are not contradictory. Did you know? Actually, a tiny bit of unpleasant odor is added to high-end perfumes to enhance the layering of the fragrance…

As for tartaric acid, it is commonly found in fruits such as grapes and bananas, and is also one of the main organic acids in wine. With these acids, combined with various chemical reactions with the many sugars and lipids in coffee beans, the ever-changing acidity, aroma, and sweetness can be formed.

The relationship between specialty coffee's acidity and sucrose

More interestingly, the bright, lively acidity of coffee is proportional to the amount of sucrose contained in the coffee beans, and the amount of sucrose in coffee beans is also greatly related to origin and altitude.

For example, coffee beans from Kenya have a strong, distinctive acidity, with a sucrose weight ratio reaching 8.45%. Second place is Colombia, also at 8.3%. This is why Colombian coffee beans are often used in espresso blends to adjust acidity.

By continent, African coffee beans generally have higher sucrose content, as well as pronounced fruity acidity and fruity aromas. Next is South America, with a milder, gentler, smooth acidity and aroma.

Additionally, research has found that for every 100 meters increase in altitude, the temperature drops by 0.6 degrees Celsius, and for every 300 meters increase, the sucrose content in coffee beans increases by 10%. This is because at higher altitudes, temperatures are lower and the temperature difference between day and night is greater, which can slow down coffee growth and store more nutrients. Sucrose derives more aromatic and acidic compounds during roasting—this is why the acidity and aroma of high-altitude hard beans far surpass those of lower-altitude soft beans.

Roasting removes coffee's acidity

Anyone who has roasted beans themselves knows that the darker the roast, the more the acidity in the coffee is destroyed, replaced by more caramel flavors, smoky notes, and bitterness. That's why specialty coffee rarely comes in roasts darker than medium. Of course, rare doesn't mean impossible—dark roasting of specialty coffee beans is an interesting craft. Those who roast their own beans can try experimenting with how the same bean tastes at different roast levels.

The factors affecting coffee's acidity, besides the coffee beans themselves, include the water quality used for brewing, the timing of tasting, and so on. However, these topics are quite broad, so let's save them for next time~

The above content is compiled by CoffeeHunters, a coffee news website.