Why Does Coffee Have Sour Taste? How to Judge Whether Acidity in Coffee Beans is Good or Bad?

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Some coffee enthusiasts believe that acidity in coffee is a negative flavor, indicating that the coffee is of poor quality or not fresh.

Others believe that acidic coffee is a sign of high quality, as it allows one to experience the uplifting flavors of flowers, fruits, and various sugars.

Acidity in coffee can be either a positive attribute or a drawback. In daily life, how do we determine whether the acidity we taste in coffee is good or bad?

Why Does Coffee Have Acidity?

Coffee beans naturally contain rich organic acids, but many of these acids gradually decompose or disappear during high-temperature roasting. Therefore, the organic acids remaining in coffee beans after roasting are limited to chlorogenic acid, quinic acid, citric acid, malic acid, phosphoric acid, acetic acid, and lactic acid.

Among these, chlorogenic acid is insoluble in water, so taste buds cannot detect its sourness, though it creates a rough sensation on the tongue's surface. Quinic acid, formed from the degradation of chlorogenic acid during roasting, is non-volatile and tastes bitter.

"So, the acidity we taste in coffee is determined by citric acid, malic acid, phosphoric acid, acetic acid, and lactic acid?"

In the past, FrontStreet Coffee's answer might have been "yes." However, a recent research report indicates that taste buds can only detect citric acid in coffee liquid and may potentially detect phosphoric acid.

The Science Behind Coffee Acidity Perception

The acidity we taste in coffee is not entirely from dissolved organic acids. A recent sensory threshold study on coffee organic acids, conducted by researchers from CoffeeMind in collaboration with the University of Southern Denmark, revealed that when people perceive certain organic acids in coffee, there may actually be no neurologically effective evidence in the coffee that could trigger such sensations.

Researchers took coffee beans from three regions—Kenya, Bolivia, and Brazil—and roasted them to light, medium, and dark levels. They then used cupping to record the concentrations of various organic acids in the coffee liquid (green) and compared these with the sensory thresholds required for human detection of these acids (orange).

As can be seen, except for citric acid and phosphoric acid, the actual concentrations of other acids like acetic acid, lactic acid, and malic acid in coffee liquid are below what our taste buds can detect.

What does this mean? For example, the malic acid we perceive doesn't actually come from malic acid in the coffee. When we taste something like Yakult, it's not due to the easily soluble lactic acid in the coffee.

How We Perceive Non-Citric Acid Flavors

Then why can we perceive acidity in coffee that resembles apples, Yakult, and other foods?

According to the latest SCA cupping form's definition of coffee acidity, we can taste these non-citric acid flavors because the acidity in brewed coffee stimulates our taste perception organs, triggering combinations of related peripheral perceptions.

In other words, acidity perception is not determined by acid alone but by the sensory experience of varying degrees of acidity combined with sweetness or other perceptions, leading to food associations. When we perceive apple-like acidity, it might be that the sweet and refreshing sensation is more prominent than the sourness, leading us to associate it with the sweet-tart profile of apple juice.

Evaluating Coffee Acidity: Good vs. Bad

So how should we determine whether coffee acidity is good or bad? In the old SCA cupping form, acidity was evaluated independently. In reality, acidity assessment needs to be combined with other sensory perceptions, making it difficult to properly judge whether the acidity in a cup of coffee is good or bad.

Therefore, in the latest SCA cupping form, "acidity" has been integrated with other sensory boundaries, using taste and other perceptions to construct the experience of acidity, providing two ways to describe and distinguish: "dry acidity" and "sweet acidity."

Undesirable "Dry Acidity/Astringency"

Refers to: Herbaceous plant-like sourness (typically caused by poor quality coffee beans that haven't fully matured); Grassy sourness when chewing (generally due to underdeveloped roasting or inherent quality issues); Wine-like acidity (usually caused by extremely light roasting or underdeveloped beans).

Desirable "Sweet Acidity/Brightness"

Refers to: Acidity with juice-like, fruit-like, and bright characteristics. Coffee exhibiting these acidity profiles is typically grown at high altitudes (1200m+) and harvested only after the cherries have fully ripened (high sugar content).

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