Which organic acids in coffee shape the final cup?

Green coffee beans hold many organic acids, but roasting breaks down or drives off most of them. What survives in roasted beans is mainly chlorogenic acid, quinic acid, citric acid, malic acid, phosphoric acid, acetic acid, and lactic acid. Of these, chlorogenic acid is not water-soluble, so it adds texture rather than perceived sourness.

Perceptible acidity comes largely from citric, malic, phosphoric, acetic, and lactic acids, while quinic acid tastes bitter and non-volatile. The SCA cupping form splits acidity into sweet and dry types, which helps separate quality fruit acidity from a defect. The fruity notes tasters describe often come from associations between sourness and other flavors.

Is the acidity in coffee good or bad? Some people believe that sourness indicates stale coffee, while others regard it as a hallmark of high-quality coffee. In fact, green coffee beans themselves contain a rich variety of organic acids, most of which break down or disappear after roasting, leaving only a limited range of organic acids in the final product. Recent research shows that the taste buds can clearly identify only a few acids, and many of the fruity notes we perceive actually come from associations between sourness and other flavors. The latest SCA cupping form classifies acidity into two categories: "sweet type" and "dry type," helping us more accurately judge the quality of coffee acidity. This article, from the professional perspective of Front Street Coffee, provides an in-depth analysis of the sources and evaluation criteria of coffee acidity.

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Some coffee enthusiasts feel that sourness is a negative flavor, meaning the beans are of poor quality or no longer fresh.

Others regard acidity as an important characteristic of specialty coffee, because through it one can capture floral notes, fruity notes, and the bright, uplifting flavor layers brought by various sugars.

The acidity in coffee can be either a plus or a minus. So in everyday tasting, how do we distinguish whether the acidity in a cup of coffee is high-quality or a defect?

Where does coffee acidity come from?

Green coffee beans contain a wide variety of organic acids, but after high-temperature roasting, many acidic compounds gradually decompose or volatilize. Therefore, in the roasted beans after roasting is complete, the remaining organic acids are mainly chlorogenic acid, quinic acid, citric acid, malic acid, phosphoric acid, acetic acid, and lactic acid.

Among these, chlorogenic acid is insoluble in water, so the taste buds cannot perceive its sourness; it only brings a rough texture to the tongue. Quinic acid is a degradation product of chlorogenic acid during roasting, is non-volatile, and tastes bitter.

"So by that logic, the acidity you can taste in coffee is determined by citric acid, malic acid, phosphoric acid, acetic acid, and lactic acid together?"

In the past, Front Street's answer might have been yes. But a recent research report points out that the taste buds can actually identify only citric acid in coffee liquid, and possibly phosphoric acid.

The acidity that can be tasted in coffee liquid does not entirely come from the dissolution of organic acids.

Not long ago, CoffeeMind and researchers from the University of Southern Denmark jointly conducted a study on the sensory thresholds of organic acids in coffee. The conclusion showed that when people taste a certain organic acid in coffee, there may not actually be neurologically effective evidence in the coffee to trigger stimulation from that substance.

The research team selected coffee beans from three origins—Kenya, Bolivia, and Brazil—and processed them at three roast levels: light, medium, and dark. They then recorded the concentration of each organic acid in the coffee liquid (green) through cupping and compared it with the sensory threshold required for the human body to identify each organic acid (orange).

The results showed that, aside from citric acid (Citric) and phosphoric acid (Phosphoric), the actual concentrations of acetic acid (Acetic), lactic acid (Lectic), and malic acid (Malic) in coffee liquid cannot be perceived by the taste buds at all.

What does this mean? For example, the malic acid you perceive does not actually come from the malic acid in coffee; the Yakult flavor you taste is not imparted by the easily soluble lactic acid in coffee either.

Then why can we perceive acidity in coffee similar to that of apples or Yakult?

According to the latest SCA cupping form's definition of coffee acidity, the reason we can taste acidity beyond citric acid is mostly that the acidity of the brewed coffee stimulates the sensory organs of taste, thereby triggering a combined response of related surrounding perceptions.

In other words, the perception of acidity is not determined by acid alone, but is an association the senses make about food after receiving varying degrees of acidity, sweetness, or other perceptions. The reason we can perceive apple-like acidity may be that the fresh sweetness is more prominent than the sourness, leading us to associate it with the fresh, sweet-and-sour taste of apple juice.

So how should we judge whether coffee acidity is good or bad?

In the old SCA cupping form, acidity existed as an independent scoring item. But judging acidity actually needs to be combined with other senses, and acidity alone cannot accurately evaluate the quality of the acidity in the cup.

Therefore, in the latest cupping form, the SCA incorporated "acidity" into the scope of other senses, constructing the experience of acidity through taste together with other perceptions, and provided two ways of describing and distinguishing it: "dry-type acidity" and "sweet-type acidity."

Bad "dry-type acidity/astringency" refers to: herbaceous acidity (usually caused by poor quality of the coffee beans themselves or incomplete fruit maturation); grassy acidity (usually caused by incomplete roasting, underdevelopment, or problems with the beans themselves); wine-like acidity (usually caused by too light a roast or incomplete, underdeveloped roasting).

Good "sweet-type acidity/sweet-and-sour" refers to: acidity with a juicy, fruity, and bright quality. Coffee can present this kind of acidity usually because the coffee trees are grown at high altitudes (above 1,200 meters) and the fruit is harvested only after ripening (when sugar content is sufficient).

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