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Bitterkeit in Kaffee – Unterschiede und Qualitäten

Bitterness in coffee – differences and qualities

Coffee is bitter.

Bitterness is as much a part of the flavor of coffee as water is in its preparation. And no one knows this as well as scientist Sara Marquart. Sara conducts research in the coffee field at the Zurich University of Applied Sciences (ZHAW Wädenswil, Coffee Excellence Center) and has written a doctoral thesis on the topic of bitterness in coffee. In this article, she summarizes the most important aspects of bitterness in coffee.

Why does bitterness have a bad reputation?

Many people start their day with a cup of coffee in the morning, whether brewed by hand or extracted as an espresso. Besides the aroma, the roasted, chocolatey, or fruity-berry scent, coffee also captivates us with its taste. But what actually defines the taste of coffee?

In addition to a pronounced, sometimes fruity acidity, the bitterness is the most prominent. But bitterness in coffee doesn't always have a good reputation. This is understandable, as most people associate bitterness with an unpleasant taste. This can be traced back to the fact that bitterness receptors represent an evolutionary protective mechanism for humans. In earlier times, before encyclopedias or Wikipedia existed, people didn't know whether a fruit was edible or poisonous. Back then, sensitive bitter receptors helped our ancestors immediately spit out potentially poisonous fruit (Fischer et al. 2005 ). However, there are also many compounds in foods that taste bitter but primarily have health-promoting properties. And here we come back to coffee: coffee contains a complex variety of different bitter-tasting compounds that have a range of different health effects.

Is bitter the same as bitter?

Bitterness is not always bitterness. There is a bitterness that humans perceive as pleasant, such as in beer, chocolate, tea, and some coffees. However, there is also a bitterness that is perceived as very unpleasant in some medications or plants, such as bitter cucumber and bitter melon. Depending on the bitter flavors they contain, these trigger a stimulus at the bitter taste receptors on our tongue and in our mouth. This stimulus is transmitted via nerve pathways to our brain, where we interpret it. In addition to the type of bitter flavor, our brain's interpretation is the decisive factor in whether we perceive the food we have just tasted as pleasant or unpleasant.

Bitter taste is a very culturally dependent sense of taste, which depends primarily on our genetics and upbringing. People in Central Europe, for example, who come into contact with many bitter foods such as the aforementioned beer and coffee over the course of their lives, are not only genetically but also socio-culturally insensitive to bitterness compared to people in other parts of the world (Ong, et al. 2018 ). One could therefore say that people in Central Europe and North America, due to their genetics and cultural upbringing, would choose bitter coffees much more often than people in Southeast Asia, who may prefer a sweetened and less bitter coffee variety. This is particularly important with regard to the orientation of the product portfolio and the customer-oriented roasting and preparation of coffee products. Roasters and coffee shop owners should always ask themselves one question: which coffee does my customers like and not which coffee does I like myself (Marquart 2018 ).

Why is coffee bitter?

The real question, however, is: what makes coffee bitter, and is there any coffee that doesn't taste bitter? Basically, coffee always tastes bitter. This is partly because coffee contains caffeine, which makes it taste bitter, but also primarily due to the composition of coffee and the reactions that occur during roasting.

Sour versus bitter: chlorogenic acids and chlorogenic acid lactones

The most important of these reaction cascades originates from a group of substances called chlorogenic acids . Depending on the type, variety, and country of cultivation, there are approximately 25–40 different types of these sour-tasting chlorogenic acids in coffee (Clifford et al. 2003 ). These chlorogenic acids decompose during roasting into the bitter-tasting chlorogenic acid lactones (Figure 1).

Figure 1: The dominant, sour-tasting chlorogenic acid (left) and the corresponding chlorogenic acid lactone (right), which tastes bitter.

Since there are a multitude of different chlorogenic acids, there is also an innumerable variety of different molecules of the corresponding lactones, all of which taste bitter. The most important of these lactones is 3- O -chlorogenic acid lactone (3-CGL), whose formation and decomposition during roasting is well documented. After just five minutes of roasting, the chlorogenic acid decomposes and 3-CGL is formed. After about 7.5–10 minutes, depending on the roasting temperature and the coffee chosen, the formation of bitter lactones reaches its maximum (Figure 2, Farah, et al. 2005 ). These lactones are known for their very distinctive mild, pleasant and almost velvety bitter modality (modality describes the type of bitterness). They are what give coffee its incomparable bitterness.

salary 3 0 Figure 2: Formation of 3-O-chlorogenic acid lactone during roasting of Coffea Arabica cv. Bourbon.

From pleasant to harsh bitterness

As can be clearly seen from the curve, the bitter-tasting lactones continue to react. With longer roasting times and higher temperatures, both the chlorogenic acids and the chlorogenic acid lactones can form phenylindanes. These phenylindanes represent another class of very harsh, bitter-tasting compounds.

Figure 3: A phenylindane isomer. There are many different and similar phenylindane compounds, all of which have a very unpleasant bitter taste.

Since phenylindanes can be formed from both acids and lactones, there are also a number of different, structurally similar compounds (so-called isomers) among phenylindanes, all of which have a very long-lasting and unpleasantly bitter taste. Many people may be familiar with exactly this type of bitter taste from very dark roasts that were then prepared with hot water as an espresso. In this sense, the phenylindanes are the final stage in the development of the bitter taste during roasting. This is because the phenylindanes can form even more complex structures through polymerization (i.e. the chaining of many individual phenylindanes to form a large molecule) (Frank, et al. 2007 ). However, these chemically large phenylindanes eventually become so large that they can no longer be absorbed by our bitter taste receptors and therefore no longer have any taste for us humans.

So what does the caffeine do?

One final suspect in the list of bitter flavorings remains, and that is caffeine. While caffeine itself tastes bitter, it is barely broken down during roasting and only accounts for about 10% of the bitterness in coffee. This is partly because both chlorogenic acid lactones and phenylindanes have a much stronger bitter taste, and partly because chlorogenic acids "complex" the caffeine contained in coffee, binding it to themselves, making it less perceptible to human receptors.

Bitterness in coffee – Conclusion

The research findings can be summarized in Figure 3. Initially, the chlorogenic acids are broken down into the pleasant, coffee-like, bitter-tasting chlorogenic acid lactones. Both the chlorogenic acids and the lactones decompose upon further roasting into the harsh, metallic, and long-lasting bitter-tasting phenylindanes. And the caffeine? It decomposes only to a very small extent during roasting.

trend curve Figure 4: Curves of the formation of various bitter flavors during coffee roasting, depending on the degree of roasting.

How do you roast mild, non-bitter coffee?

The art of roasting lies in adapting the roast to the raw coffee. Science can only provide a starting point; the roasters are responsible for achieving the right bitterness through proper roasting. Through their experience, they understand how to control the invisible reactions in the coffee and adapt them accordingly to the desired end result—a dark espresso roast or a light filter coffee (Marquart 2019 ).

Good coffee = balance between acidity and bitterness

Chlorogenic acid lactones represent a very special group of bitter-tasting substances, as it is primarily these compounds that give coffee its coffee-like bitterness, which most people find pleasant. Depending on the composition—one could almost call it a symphony of lactones—roasted coffee develops a very finely balanced, mild, velvety bitterness when consumed. This bitterness interacts harmoniously with the aforementioned fruity acidity of coffee, which is produced by compounds such as malic, citric, cinchona, or the aforementioned chlorogenic acids.

If you roast the coffee too long or too dark, you'll disrupt the balance between this pleasant bitterness and fruity acidity. On the one hand, the acidic compounds break down, thus depriving the coffee of a wonderful portion of its flavor complexity; on the other, the harsh, metallic-bitter phenylindanes form. This can almost be compared to an oversalted dish: a good dose of salt gives the dish that certain something, but too much of a good thing ruins it irreparably.

In conclusion, it might be said that coffee occupies a unique position due to its chlorogenic acids. There are hardly any foods in the world that contain these compounds. This, combined with skillful roasting, gives coffee its very characteristic and subtle bitterness, which makes it and its taste experience so special. The balance of its acids, bitterness, and, above all, its aromatic finesse makes it something incomparably unique. Properly roasted, ground, brewed, or extracted, coffee becomes a balancing act of enjoyment.

About the author Sara Marquart

Sara Marquart is a food chemist who researched coffee roasting and bitterness in her doctoral thesis. She is currently working on " Atomo Coffee ," molecular coffee without the bean, at the Coffee Excellence Center at the University of Zurich. Prior to that, she curated the special exhibition "Cosmos Coffee" at the Deutsches Museum in Munich.

Further links on bitterness

We've described more about the sensory evaluation of Fine Robusta in an article. When evaluating Fine Robusta, at least the balance of bitterness and sweetness is examined sensorially. However, the approach could be taken even further by describing the different qualities of bitterness, as we do with acidity or body.

Sara also spoke about bitterness at the SCA Coffee Expo. The result was a podcast in English, which you can listen to here.