The Myth of Coffee Acidity | New Research Challenges Our Understanding of Coffee Acidity

"This coffee has a distinct malic character, and as the temperature drops it develops tartaric acid..."

If you’ve ever competed in a barista competition, you’ve likely heard similar descriptions. You may even be familiar with the flavors of the common organic acids in coffee—citric, malic, lactic, acetic—and feel confident that you can tell them apart in a blind test. However, newly published research has come up with a surprising idea: the levels of many organic acids in brewed coffee are so low that they are almost undetectable.

A new discovery that shook up the coffee world

Morten Münchow and Ida Steen of CoffeeMind worked with researchers from the University of Southern Denmark to investigate the actual levels of those acids we often refer to in brewed coffee. They also tested whether coffee experts could tell the difference between citric and tartaric acid in coffee. The findings challenge many of our assumptions about how different acids affect coffee flavor.

The study found that at typical acid levels in brewed coffee, coffee professionals were unable to correctly identify any organic acids in taste tests. In fact, for acetic, lactic, and malic acids, the concentrations in their brewed coffee were so low that they were below the detection threshold, meaning the acids were completely undetectable.

The researchers note that the findings call into question the sensory skills training and testing methods widely used in the specialty coffee industry. Even with lightly roasted specialty coffee beans, levels of most organic acids in brewed coffee are well below the levels used in the Specialty Coffee Association (SCA) and Coffee Quality Institute (CQI) sensory tests.

Even more surprising, the researchers found the lowest pH and highest concentration of citric acid in the Brazilian coffees they tested. Kenyan coffee, on the other hand, has less citric acid and a higher pH, although most baristas would consider Kenyan coffee to have a very high perceived acidity.

Standard Procedure for Acidity Testing

The standard procedure used by SCA and CQI to test the sourness discrimination in coffee is to “add” organic acids to brewed coffee. The SCA uses citric, malic, lactic and tartaric acids in the sensory skills professional test, while the CQI tests citric, malic, acetic and phosphoric acids in the Q Appraiser exam.

In each case, the trainers added 0.4 grams of a specific acid (0.5 grams in the case of lactic acid) to one liter of brewed coffee. For the CQI course, the trainer adds acid to two of the four cups and asks the students to find which two cups had the acid added (a “pairing test”). The SCA qualification goes a step further and tests students' ability to identify which acid has been added.

The purpose of these tests is to ensure that the amount of acid added is detectable - in fact, the SCA instructions specifically ask trainers to "adjust to taste" to ensure students can taste the difference between cups.

However, the study found that, with the exception of citric acid, the concentrations of these acids in brewed coffee were well below the levels used in the SCA and CQI tests. The researchers tested five different specialty coffees: two Brazilian semi-washed coffees, two Kenyan washed coffees, and one Bolivian washed coffee. For each coffee, they tested three different roast levels within the light roast range commonly seen in specialty coffees.

Lightly roasted specialty coffees should be fairly sour—but of the acids tested by the CQI and SCA protocols, only citric acid reaches the levels used in the tests (0.4 grams per liter). Two other important acids—chlorogenic and quinic acids—are present in higher concentrations, but their contribution to flavor is still under discussion and is not included in common sensory tests.

The challenge of detection threshold

The sensory acidity test is based on the assumption that common organic acids have a significant impact on coffee flavor and help distinguish different coffees. But studies have found that many of these acids are undetectable at low concentrations in brewed coffee.

The researchers added varying amounts of each acid to brewed coffee and tested it on a group of coffee professionals—baristas from the renowned Copenhagen roastery Coffee Collective. The baristas had all been trained to identify organic acids and received an additional 30 minutes of intensive training before the test.

It was found that at the concentrations found in coffee, baristas were unable to distinguish which acid was which in tests. In fact, for most acids, it is not even possible to detect whether any acid has been added. When tasting acids in pure water, the tasters performed slightly better—only acetic acid was correctly identified.

By adding varying amounts of each acid to coffee, the researchers calculated how much needed to be added to the coffee for the difference between cups to become detectable — the “detection threshold.” According to their results, the average concentrations of malic, lactic, and acetic acids in brewed coffee were below this threshold, while the average concentration of phosphoric acid just exceeded it. At the concentrations found in coffee, the only clearly detectable acid is citric acid.

These results contradict the notion that variations in the concentrations of individual organic acids determine coffee flavor. Adding extra acid to coffee in this way can double the acid concentration, but the difference won't even be noticeable.

The complexities of acidity perception

So why is Kenyan coffee considered to be high in acidity? Research shows that our perception of coffee acidity is influenced by a variety of factors:

1. Titratable acidity : A study found that the perceived acidity of coffee—how sour it tastes—seems to be more related to "titratable acidity" than pH or total acid concentration. Titratable acidity is a measure of how much base you need to add to neutralize all the acid in the coffee.

2. Acid Combinations : The specific combination of acids can also affect the flavor. For example, citric acid and malic acid work synergistically. Combined, they taste more sour and astringent than individually. On the other hand, bitter compounds like quinic acid may mask some of the coffee's acidity.

3. Influence of Aroma : An interesting study shows that our perception of coffee acidity is influenced by the aroma of the coffee. The researchers found that when they asked the tasting panel to wear nose clips to block out the coffee aroma, tasters rated the perceived acidity of Kenyan coffees as lower than that of other coffees—consistent with measurements of pH and titratable acidity. The researchers believe that Kenyan coffee’s aroma may be partially responsible for its higher perceived acidity—in other words, citrus aromas make coffee seem more sour than it actually is, while caramel aromas can reduce acidity perception and make coffee taste sweeter.

This may be similar to how we perceive sweetness in coffee: most coffees do not contain enough sugar to make the sweetness detectable; instead, the perception of sweetness is more likely to come from aromas in coffee that are associated with sweetness, such as caramel or vanilla.

Acid is more than just sour

The acid content in coffee affects more than just acidity. Some of the acids found in coffee have recognizable aromas, such as the vinegary taste of acetic acid or the caramel taste of pyruvic acid. Many acids can also add bitterness or astringency to coffee, or even act as flavor modifiers, changing the flavor of a coffee even if they have no substantial flavor of their own.

Chlorogenic acid may break down in brewed coffee, changing its impact on flavor over time. Chlorogenic acid in coffee is sometimes associated with a sour taste and sometimes with a bitter or metallic taste. It’s possible that chlorogenic acids themselves have little flavor, but rather the molecules formed by the breakdown of chlorogenic acids explain their effect on coffee flavor.

Rethinking our description of acid

This study clearly shows that the differences in flavor between coffees from different origins are not determined by the concentration of different acids. For example, the high perceived acidity of Kenyan coffee is not only determined by the acids it contains. At the same time, factors such as roasting degree or processing method seem to have a greater impact on the acid content of coffee than geographical origin.

Do these results mean that the concepts behind SCA and CQI testing are fundamentally flawed?

It is important to note that the tasters in this study were not Q assessors themselves. Perhaps the training involved in CQI (or SCA) certification would enable tasters to detect or even identify acids at lower concentrations. But the tasters in this study were still experienced coffee professionals, suggesting the test is not applicable to most of the industry.

On the other hand, the acidity of coffee is an important component of its overall quality and is closely related to customer enjoyment. The industry needs a way to calibrate acidity so that the perceived acidity of coffee can be rated in a meaningful way.

Clearly, the ideal test would include a more comprehensive approach to acidity, taking into account the different factors that influence perceived acidity. Until such a test is developed, fortifying coffee with organic acids may still help calibrate the way tasters describe acidity—even if it is by no means representative of the concentration found in brewed coffee.

In the meantime, maybe it’s time to reconsider descriptors like “malic acidity” that sound scientific but don’t reflect the reality of how we taste coffee. While acidity is measurable, how we perceive acidity is highly subjective. A descriptor like “green apple acidity” takes this into account—and is more meaningful to our customers.

Conclusion and Implications

This research provides valuable insights for the coffee industry, challenging our long-held understanding of coffee acidity. Key revelations include:

1. The specific acidity we perceive in coffee may not only come from the presence of a single acid
2. Aroma plays a big role in how we perceive acidity
3. The way coffee professionals are trained and tested to identify specific acids may need to be re-evaluated
4. Using more intuitive descriptors (e.g., “green apple acidity” rather than “malic acidity”) may be more accurate and more meaningful to consumers

Coffee acidity is more complex than we think, involving the interaction of multiple sensory and chemical processes. This understanding will not only help coffee professionals describe and evaluate coffee more accurately, but will also enhance consumers’ understanding and appreciation of coffee flavor.