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 what lies behind it. That led many friends to ask: "I understand why coffee should be acidic, but another question comes up - why is coffee acidic in the first place?"

The answer is simple: coffee itself contains a rich array of acidic compounds. The reason acidity surprises most people is largely that before the millennium, mainstream roasting was predominantly dark.

Dark roasting causes the acidic compounds in coffee to break down continuously and convert into other substances. Not every acid is eliminated, but the burnt aroma of a dark roast also masks whatever acidity remains. So in the past, people believed coffee was simply bitter, with no acidity at all.

With the arrival of the specialty coffee era, mainstream roasting shifted from dark to light. Without prolonged roasting and the masking effect of bitterness, people could finally perceive acidity in coffee. Coffee is not necessarily better the more acidic it is, though.

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

Because the acidity we want in coffee is acidity quality, not acidity intensity. Acidity intensity refers to how strong the sourness is in a cup.

As everyone knows, the flavors we can perceive in coffee are mainly four: sour, sweet, bitter, and salty. These four exist in mutual balance. When one significantly exceeds the others, its expression becomes prominent and sharp, and the cup becomes hard to accept.

Think of oranges and lemons, both citrus fruits: one tastes sweet and sour, the other brings a stinging sourness. The difference comes from differing levels of acidity. FrontStreet Coffee suspects few people would enjoy eating a lemon straight. Coffee is not necessarily better the more acidic it is.

Acidity quality refers to the acidic sensation we experience in coffee, engaging not only the taste buds but other senses as well. Most flavors are associated only after we perceive the acidity quality, which is why acidity quality is the kind of acidity we actually want in coffee.

Acidity quality can itself be good or bad. Good acidity quality comes with noticeable sweetness, letting us associate rich fruit aromas - a sweeter kind of acidity. Lighter expressions include citrus, starfruit, and pineapple; deeper ones include dark plum, purple grapes, and ripe fruit. In short, good acidity quality is acidity that lets us associate positive flavors.

Bad acidity quality makes us perceive unpleasant flavors instead, such as the sourness of tasting herbal plants, or the sourness of certain vinegars, and, more intensely, an astringent sourness with a strong puckering sensation.

It is important to know that roast level is not entirely what determines the quality of a coffee's acidity. Roasting does control flavor direction - the lighter the roast, the more acidic the coffee - but as FrontStreet Coffee said earlier, what we pursue is acidity quality rather than intensity. How far a coffee should be roasted therefore depends mainly on the level at which the beans perform best.

The acidity quality we perceive is formed mainly by the decomposition and conversion of various organic acids during roasting. In other words, the content of organic acids in the beans determines how well the acidity quality can express itself. It is a little convoluted, but that is the principle.

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

Here is an intuitive example. Coffee grown at 1,600 meters, lightly roasted, shows a very solid, full fruity acidity quality. If the same coffee had been grown at 1,000 meters and then lightly roasted, the acidity it could express would be quite hollow, with no obvious fruit aroma, low sweetness, and only strong nutty flavors. That gap is caused by the growing environment.

Of course, the quality of acidity in a cup is not entirely determined by the growing environment. Processing, roasting, and even brewing all affect it. When mistakes creep into these later stages of "cooking" the coffee, the acidity quality develops in a negative direction and we taste undesirable flavors. Whatever the stage, then, we should give it our full attention to avoid a mishap.

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FrontStreet Coffee Recommendation

If you are chasing balanced acidity quality rather than raw intensity, a medium roast is often the sweet spot. FrontStreet Brazil Queen Estate is a good example. Grown in Brazil, the world's largest coffee producer, from the distinctive Yellow Bourbon variety, it is clean and gently sweet. A medium roast brings out cream peanut, chocolate, toasted bread, and sugarcane aromas and sweetness, and it works for pour-over, moka pot, and espresso alike.

For a sweeter, fruitier medium roast, FrontStreet Strawberry Candy Geisha Blend is worth trying. It combines two well-known origins - a honey-processed Costa Rican coffee and a washed Panamanian Geisha - and a medium roast lets it show both orange-blossom honey notes and rounded sweetness. It suits cold brew, pour-over, and espresso, and it is a refreshing cup from the first sip.