Why is Coffee Sour and Bitter - The Origins of Coffee's Acidity and Bitterness

For many people, the impression of coffee is stuck between acidity and bitterness. In fact, acidity and bitterness are basically the fundamental components of coffee. For today's specialty pour-over coffee, without acidity and bitterness, it's impossible to bring out so many flavors of coffee, such as FrontStreet Coffee's mellow Blue Mountain coffee and the outstanding citrus-floral Geisha coffee. They may be acidic or bitter, but they won't overpower your taste buds; instead, they balance everything on your palate.

The taste of coffee is primarily composed of bitterness and acidity. However, where do these flavors come from? Additionally, why does the balance between bitterness and acidity change with the degree of roasting? FrontStreet Coffee believes that if you understand the reasons behind this, coffee will become much more interesting.

Why Deeper Roasting Results in Stronger Bitterness

When talking about coffee's flavor, many people probably first think of that unique bitterness! Most people assume that coffee's bitterness comes from caffeine, but when looking at the proportion of all components in coffee beans, the caffeine content is actually very small. The bitterness of coffee is mainly due to components other than caffeine. The components of dried raw beans are primarily polysaccharides (such as fibers that form the plant's structure), along with proteins, lipids, sugars (sweet sucrose, etc.), chlorogenic acid, malic acid, citric acid, quinic acid and other acids, as well as caffeine.

After roasting, coffee beans turn brown mainly because sugars, chlorogenic acid, and amino acids produce brown pigments, which are also one of the elements of bitterness.

Brown pigments can generally be distinguished by size—the larger the molecule, the stronger the bitterness. Light-roasted coffee beans contain many small-molecule pigments, while large-molecule pigments tend to increase as the roasting degree deepens. In other words, the reason why deeper roasting results in stronger bitterness is due to the influence of pigment changes.

Quinic Acid is the Main Component of Coffee's Acidity

The acidity you feel when drinking coffee doesn't come from acidic components like malic acid or citric acid in the raw beans. Instead, it's mainly due to new acids generated through chemical reactions caused by roasting heat. It primarily comes from quinic acid produced after chlorogenic acid decomposition, which has a more significant effect on enhancing acidity. Additionally, there are also large amounts of acetic acid and phosphoric acid.

Even before raw beans reach the light roast level, even at a very light stage, the total amount of acids (especially quinic acid) will continue to increase. When the temperature begins to rise further, thermal decomposition begins, and as the roasting degree deepens, the amount of acid gradually decreases. The reason why acidity weakens with deeper roasting is also based on this principle.

The content of components like sugars and chlorogenic acid in raw beans varies depending on differences between original varieties such as Arabica and Canephora, as well as factors like growing environment. These differences also affect the personality of the coffee beans.

Important Notice :

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