Acids in Coffee

Where does coffee acidity come from? It sounds like this would be simple to answer, but it's actually quite complicated. This is due to the complex nature of the coffee brewed and the way our tongues work to perceive acidity.

It all comes down to how we perceive taste. Do we say a coffee tastes "citric" because coffee is rich in citric acid? Or could it be that citrus fruit notes that our taste comes from something completely unrelated to the real acids present?

Where does acidity come from?

The acids present in green coffee are the result of a number of factors: the nutrients in the soil, the health of the tree and even the biochemical reactions that take place during post-harvest processing . The Brewing method, the Brewing temperature and the additional variables involved in the preparation of the coffee are other areas with a significant impact on the acidity in the final cup .

Researchers are still investigating and learning more and more about exactly how acidity develops. The complex chemical process known as roasting also remains an exciting frontier of new research that sheds light on the development of acidity.

We have done our best to summarize some of the prevailing knowledge and explore exciting unanswered questions.

The Key Acids

Several acids are present in large amounts in green coffee. They represent about 11% (by weight) of the compounds in green coffee. Chlorogenic acids are by far the most common, followed by citric, malic and succinic acids. These are the products of the Krebs Cycle, an important chemical reaction responsible for releasing stored energy in the coffee plant.

The roasting process also has its own impact on acid development. Chlorogenic acids are decomposed by heat during roasting. Furthermore, as a result of the roasting process, acetic, lactic, quinic and gluconic acids are formed.

 

Acids and their tastes

Like other sensory aspects in coffee, it is quite challenging to describe and quantify the sensory perception of acids. In general, we found that citrus acidity presents itself as lemon or citrus notes; malic acid tends to have a more acidic or apple-like flavor; and acetic acid may present as a vinegar-like aroma. Some acids even contribute to bitter notes.

There is a lot of research at the moment around the interaction of the acids present and sensory perception, but in general, acidity reflects the complexity of the sensory interaction found between other coffee attributes as well.

The challenge is that it is not always clear which acid leads to specific tasting notes in a coffee. For example, you might have a coffee that contains large amounts of citric acid but does NOT have citrus notes. At the same time, you can have a coffee with low amounts of citric acid, but with strong citrus flavor notes.

 

Perception of Acidity in Coffee

Just because these acids are present in large amounts does not mean that coffee will be perceived as acidic.

You may have heard "that Kenya's brilliant coffee has phosphoric acid to thank" for its award-winning acidity. However, if that were the case, then Robusta coffee, which tends to have higher levels of phosphoric acid than Arabica, should have a similar characteristic, shouldn't it? In fact, it may be the higher malic acid content in Kenyan coffee that contributes to its uniqueness. But wait! Just because the acid is present, does that mean we notice it? Possibly not.

A 1987 study by HG Maier showed that, although the total amount of acids present in green Robusta coffee is slightly higher than in Arabica, it is roasted Arabica that appears to have a greater perception of acidity.

This could be due to the higher level of acids developed during the roasting of Arabica, the type of acids present or the sensory impact of the types of acids found in Arabica. Be that as it may, there is clear evidence that the presence of acidic compounds in green coffee (or roasted coffee for that matter) does not necessarily directly correlate with perceived acidity. The same study concluded that acetic, citric and malic acids have the greatest sensory impact on the perception of acidity, even while other acids such as quinic, formic and chlorogenic acids tend to be present in greater amounts.

 

Extended Fermentation and Acids

The conversation around acidity in coffee seems to have been taken to a new level with the recent madness in post-harvest processing innovations. It is often asked: "how much acidity in the cup is too much"? "How do you get very strong and acidic coffee?"

While it's not always a good way to respond with "it depends," the reality is that there's a lot going on. The growth of coffee consumption in new markets has created a new frontier for flavor exploration and acceptance. Combined with the global nature of coffee and the complexity of sensory science and perception, this new frontier is constantly evolving and changing.

There seem to be some markets that love these coffees and are paying a dollar for them, while others believe they are of inferior quality (and we won't even touch on attitudes towards this from a terroir and "coffee purity" point of view). One thing that is clear is that we continue to see the myths surrounding acids in coffee being challenged as science shines a spotlight on these mysteries.

Our tongues don't lie (although they can be deceived!). Acidity in coffee is real and is an aspect that contributes to the overall sensory perception of the coffee brewed. However, we must be careful in the way we describe and understand its development as we seek to produce coffees with specific flavors and characteristics. One thing is for sure though...acidity in coffee will continue to be a topic of great interest, and I look forward to more science and research to come!

 

*We want to thank Dr. Samo Smrke and his recent Instagram series for providing insights for this article. In addition, Clarke and Vitzthum's "Coffee" (2001): Recent Developments" and Flament (2002) Coffee Flavor Chemistry also provided great insight!

 

AUTHOR:-Tim Heinze is our partner's Director of Education and Director of the Center. Since 2009, he has been working in many areas of coffee, focusing on growing, producing, processing and trading coffee. He is an SCA AST and CQI instructor for sensory and processing programs and holds a Master of Agriculture from Texas A&M. His passions include training and education across the coffee supply chain. In his spare time, he can often be found on the baseball field or enjoying time with his family.