Why Does Your Coffee Taste Sour? What Makes Coffee Sour and What Causes It?

In the article "Why Most Coffee Today is Acidic," FrontStreet Coffee explained why most coffee has acidity and the reasons behind it. This led many friends to say: "I understand why coffee should be acidic, but another question arises—why is coffee acidic in the first place?"

The answer is simple because coffee itself contains very rich acidic compounds. The reason most people find it surprising that coffee has acidity is largely because before the millennium, mainstream coffee roasting levels were predominantly dark.

Dark roasting causes the acidic compounds within coffee to continuously break down and convert into other substances. Although not all acids are eliminated, the burnt aroma brought by dark roasting also masks the remaining acidity. Therefore, in the past, people believed coffee was bitter and lacked acidity.

With the arrival of the specialty coffee era, mainstream roasting levels shifted from dark to light. Without prolonged roasting and the masking effect of bitterness, people were finally able to perceive acidity in coffee. However, coffee isn't necessarily better the more acidic it is.

Why Isn't Coffee Better the More Acidic It Is?

Because the acidity we want to experience in coffee is "acidity quality" rather than "acidity intensity." Acidity intensity refers to the strength of sourness in a cup of coffee.

As we all know, the main flavors we can perceive from coffee are four: sour, sweet, bitter, and salty. These four flavors exist in mutual balance! When one flavor significantly exceeds the others, its expression in the coffee becomes prominent and sharp, making it difficult to accept.

It's similar to oranges and lemons, both citrus fruits—one tastes sweet and sour, while the other has a stimulating sourness. This difference is formed by varying levels of acidity. FrontStreet Coffee believes few friends would enjoy eating lemons directly! Therefore, coffee isn't necessarily better the more acidic it is.

Acidity quality refers to the acidic sensation we experience in coffee, combining not only our taste buds but also other senses. Most flavors are associated after we perceive the acidity quality. Therefore, acidity quality is the type of acidity we want to experience in coffee.

But acidity quality can also be good or bad! Good acidity quality comes with noticeable sweetness, allowing us to associate rich fruit aromas—a sweeter type of acidity. For example, lighter expressions include citrus, starfruit, pineapple... while deeper ones include dark plum, purple grapes, ripe fruits... in short, acidity quality that allows us to associate positive flavors.

Bad acidity quality, on the other hand, makes us perceive unpleasant flavors, such as acidity that reminds us of the sourness when tasting herbal plants, or the sourness of certain vinegars, and even more intensely, an astringent sourness with strong puckering sensations.

It's important to know that roasting level is not entirely the determining factor for the quality of coffee acidity. Although roasting can control the flavor direction of coffee—the lighter the roast, the more acidic the coffee—just as FrontStreet Coffee mentioned earlier, what we pursue is acidity quality rather than acidity intensity. Therefore, the degree to which coffee should be roasted mainly depends on at which level the coffee beans perform best.

The acidity quality we perceive in coffee is mainly formed by the decomposition and conversion of various organic acids during roasting. This means the content of organic acids in coffee beans determines the expression level of acidity quality. It's a bit convoluted, but that's the principle~

The organic acid content in a coffee bean mainly depends on its growing altitude and soil fertility, as these factors positively contribute to the formation of organic compounds in coffee beans. The higher the altitude and more fertile the soil, the more organic acids can accumulate in the coffee beans, and vice versa.

FrontStreet Coffee offers another intuitive example to help everyone understand: when coffee beans grown at an altitude of 1600 meters are lightly roasted, they exhibit very solid and full fruity acidity quality; however, if these coffee beans were grown at 1000 meters altitude and then lightly roasted, the acidity they can express will be quite hollow, with no obvious fruit aroma, low sweetness, and only strong nutty flavors. This is the gap caused by growing environment.

Of course, it's not to say that the quality of acidity in a cup of coffee is entirely determined by the growing environment. Coffee processing, roasting, and even brewing all affect the acidity quality of a cup of coffee. When mistakes occur in these "cooking" stages of coffee, the acidity quality will develop in a negative direction, causing us to experience undesirable flavors. Therefore, regardless of which stage it is, we should focus completely when proceeding to avoid any mishaps~

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