Let’s explore why the conventional statements about coffee acidity and the methods for determining it are not as straightforward as they seem.
Acidity is one of the primary criteria for evaluating the quality of coffee beans and their flavor profile. It is particularly prominent in light-roasted coffee, which is gaining popularity alongside the growing interest in specialty coffee culture. To guide coffee enthusiasts through the variety of available coffee types, experts create detailed descriptions. These descriptions often highlight the intensity of acidity and sometimes specify the types of acids that can be detected in the cup. Acids are considered a crucial factor influencing the organoleptic sensations derived from coffee.
However, in practice, there can be discrepancies between the chemical analysis of a beverage and the sensory experience. For instance, Kenyan coffee is perceived as significantly more acidic than Brazilian coffee, even though Kenyan varieties have a more alkaline environment and a lower concentration of citric acid.
Experts at Barista Hustle sought to unravel this contradiction. They analyzed the results of over 40 studies on this topic. In this article, we will share some of the most intriguing findings from their publication.
What creates acidity in coffee?
Acid is a chemical substance whose aqueous solution has a pH of less than seven. The lower the pH, the higher the acidity. Our sensory organs perceive it as a fresh, sour taste, often associated with fruits. In green coffee beans, the primary acids are chlorogenic, quinic, citric, and malic acids. The composition of these acids in a particular coffee variety depends on numerous factors:
- Type of coffee;
- Coffee bean variety;
- Geographic origin;
- Altitude of growth;
- Processing method.
Acids influence not only the perception of sourness but also all the essential characteristics of coffee. For example, citric acid is responsible for citrus descriptors in the cup. Acetic acid, in moderate concentrations, creates a sensation of pleasant sharpness and a lime-like flavor, but in high concentrations, it has a distinct fermentation aroma. When acetic acid combines with simple sugars, it produces a taste reminiscent of wine or champagne. Tartaric acid primarily affects mouthfeel—it triggers strong salivation and leaves an astringent aftertaste. It is one of the key ingredients in sour candies.
The phrase “citrus acidity” does not imply that the cup tastes like citrus. The correct interpretation is that the nature of the acidity in the cup resembles citrus. © Kamran Aydinov, Source: freepik.com
After roasting, the acidic composition of coffee beans undergoes significant changes:
- Chlorogenic, citric, and malic acids decompose under high temperatures;
- The concentration of quinic acid increases due to the breakdown of chlorogenic acid;
- The alteration of simple sugars, such as sucrose, glucose, and fructose, leads to the formation of acetic, formic, lactic, and glycolic acids.
In brewed coffee, the predominant acids are chlorogenic, quinic, citric, malic, acetic, formic, lactic, glycolic, and phosphoric acids. The character of the acidity imparted by these acids is established through the process of cupping—commonly used to describe a new lot when updating a product lineup.
International certification organizations train students to distinguish and identify specific acids. The Coffee Quality Institute (CQI) focuses on citric, malic, acetic, and phosphoric acids, while the Specialty Coffee Association (SCA) emphasizes citric, malic, lactic, and tartaric acids. Both organizations have developed protocols to assess the ability to recognize sour flavors. Trainers add tenths of a gram of each acid to a cup and ask students to identify and describe the acidity.
In CQI, students undergo a “matching pairs” test: the trainer asks them to identify the two cups out of four that contain added acid, as well as to name the acids.
Researchers at the University of Southern Denmark questioned the effectiveness of these methods. Along with specialists from Coffee Mind, they decided to conduct similar tests using a more conventional brewing method. The Coffee Mind team brewed coffee in a French press to a strength of about 1.1% TDS (Total Dissolved Solids), believing this method to be the simplest to prepare, thus enabling a more systematic procedure. It is also the closest to industry standards for coffee evaluation.
French press coffee complicates the identification of acids: due to the absence of a filter, this method results in a high content of oils, melanoidins, and micro-particles that "mask" other flavor nuances.
The study involved a group of professionals from the specialty coffee industry who had previously been trained in recognizing organic acids. To ensure the experiment's accuracy, the concentration of acids in some samples was doubled, considering that some non-experimental varieties might have higher acid concentrations.
The findings led to the following conclusions:
- The actual concentration of acids in brewed coffee is lower than in the organoleptic tests of SCA and CQI, with the only exception being citric acid.
- Test participants were unable to identify samples with doubled acid content. The excessive acidity went unnoticed.
- At concentrations typical of brewed coffee, baristas were unable to accurately identify the acid composition.
This experiment introduced the term "detection threshold" — the amount of acid that needs to be added to coffee for the difference in flavor to become noticeable and the acid to be easily recognizable. In brewed coffee, the average concentration of malic, lactic, and acetic acids is below this threshold, while the average concentration of orthophosphoric acid is slightly above. Citric acid was the only substance whose concentration matched the detection threshold.
The green bars indicate the concentration of acids in coffee brewed with a French press, while the yellow bars represent the amount that needs to be added for each acid to be detectable. © Graph by Barista Hustle
The results demonstrated that the concentration of acids in brewed coffee is so low that they are almost imperceptible to our senses. Let's now attempt to determine what actually creates the perception of acidity in the cup.
Acidity and Acids: What Lies Behind the Taste of Coffee
The sour taste we perceive when drinking coffee is not solely a sensory response to the concentration of acids in the cup and their chemical composition. Other factors also influence our perception, which must be considered during organoleptic evaluation.
Individual acids act as modulators of coffee flavor, even if they lack their own aroma. Although they cannot be directly sensed, they alter the perception of other elements.
Bitter compounds, such as caffeine, trigonelline, or quinin acid, reduce perceived acidity.
Chlorogenic acids, which are almost tasteless, eventually begin to decompose, introducing sour, bitter, and astringent notes into the coffee.
Astringency and tactile sensations in the mouth influence the perception of sourness more strongly than bitterness or acidity.
Aromas modify taste perception. Researchers suggest that citrus aromas make coffee seem more acidic, while caramel aromas reduce the perception of acidity and give the beverage a sweeter flavor.
In barista competitions, individual organic acids are often used to describe the taste of the cup.
The intensity of sour taste cannot be directly correlated with the acid composition or their concentration. It is more accurate to rely on the term "titrable acidity." This refers to the measure of alkali needed to neutralize all acids in a solution. Experiments have shown that titrable acidity depends on the amount of dissolved solids: the higher the TDS (Total Dissolved Solids), the more pronounced the acidity.
We will examine how the extraction process is related to coffee acidity and whether it can be influenced in the next section.
How to Influence Coffee Acidity
The flavor profile of coffee is established during the extraction process when aromatic compounds from the beans are transferred into the water. The amount of these compounds is measured by TDS: the higher the TDS, the stronger the coffee.
However, increasing this measure does not mean that the acid content in the beverage will rise or become easier to identify. On the contrary, strong coffee contains many different flavor compounds in high concentration, making it more challenging to isolate those responsible for sour taste. Therefore, the higher the TDS, the higher the detection threshold for acids in coffee.
If one tries to identify added acid in espresso or filter coffee, it is much more difficult to do so in espresso due to the beverage's density.
The level of extraction is not the only factor that influences the perception of acidity. A barista can affect the taste and aroma of the beverage by adjusting brewing time, grind size, roast degree, and brewing method. We will now explore the impact of each of these factors on coffee acidity.
Extraction time. Acids and organic salts are released at the beginning of the extraction process. Next, volatile aromatic compounds formed during roasting are extracted. Finally, organic substances responsible for bitterness are released. Therefore, under-extracted coffee tastes sour, while over-extracted coffee tastes bitter.
Grind size. The coarser the grind, the less surface area of the bean comes into contact with water. If the grind is too coarse, it leads to a dominance of acidity in the coffee and a lack of balance between sweetness and bitterness.
Water temperature. Different chemical compounds have their optimal extraction temperatures. If the water used for brewing is too cold, there will be insufficient extraction of sweet and bitter components, resulting in an unbalanced, acidic beverage.
Roast degree. The acidic composition of coffee changes during thermal processing. As beans transition from light to dark roast, the concentration of chlorogenic, citric, and malic acids decreases, while the levels of quinin, acetic, lactic, phosphoric, and glycolic acids increase.
Natural and honey processing methods in Ethiopia
Processing method. Natural and honey processing methods retain more sweetness in the beans. In contrast, the washed method enhances the perception of acidity. Research has shown that processing has the greatest impact on the chemical composition of green coffee. The natural method reveals the presence of glyceric and glycolic acids, while washed and honey methods leave traces of glutamic acid and galactinol. Geographic location and altitude are the next most significant factors influencing coffee's flavor profile.
After roasting the beans, it is crucial to consider the brewing method, pouring technique, and characteristics of the devices used. These factors greatly influence the taste of the brewed coffee. One experiment confirmed a direct correlation between acid concentration and the filters used. However, to track precise patterns, it is necessary to compare the results of this study with other similar experiments. The differences in brewing techniques make this challenging.
Key Points About Acids
The conducted studies have called into question long-standing beliefs about coffee acidity. The concentration of acids in a brewed cup is significantly below the detection threshold, meaning that even experienced baristas may not always be able to identify their type. This suggests that sensory training for coffee experts needs to be improved by incorporating chemical analysis and the established acid detection threshold.
When evaluating acidity, it is important to consider the presence of aromatic components, the chemical composition of the beans, and the beverage’s pH collectively. The processing method and brewing process also significantly influence coffee acidity. These factors have a much greater impact on perceived acidity than terroir. Therefore, describing the acidity of a bean based solely on its country of origin is inaccurate.
The experiments conducted indicate that further research into coffee acidity is necessary, as it is a vital component of coffee quality. Acidity helps experts assess beans and aids consumers in making their selections. It is now crucial to revisit the principle of acidity calibration to find a more meaningful way to evaluate it.