Coffee is bitter.
Bitterness is as much a part of the coffee flavor profile as water is necessary for its preparation. And nobody knows as much about it as the scientist Sara Marquart. Sara conducts research in the field of coffee at the Zurich University of Applied Sciences (ZHAW Wädenswil, Coffee Excellence Center) and has written a doctoral thesis on the subject of bitterness in coffee. In this article, she summarizes the most important aspects concerning bitterness in coffee.
Why does bitterness have a bad reputation?
Many people start their day with a morning cup of coffee, whether brewed as a pour-over or extracted as an espresso. Beyond the aroma—the scent of roasty, chocolatey, or fruity-berry notes—coffee also captivates us with its taste. But what actually defines the taste of coffee?
Alongside a distinct, sometimes fruity acidity, bitterness stands out above all else. Yet, bitterness in coffee does not always have a good reputation. This is understandable, as most people associate bitterness with an unpleasant taste sensation. This can be traced back to the fact that bitterness receptors represent an evolutionary protective mechanism for humans. In earlier eras, when there were no encyclopedias or Wikipedia, people did not know whether a fruit was edible or poisonous. In those times, sensitive bitter receptors helped our ancestors spit out potentially toxic fruit immediately (Fischer, et al. 2005). However, there are also many compounds in food that taste bitter but possess primarily health-promoting properties. And this brings us back to coffee: coffee contains a complex variety of different bitter-tasting compounds that have a range of different health effects.
Is all bitterness the same?
Bitterness is not always the same. There is a type of bitterness that humans perceive as pleasant, such as that found in beer, chocolate, tea, and some coffees. Besides that, there is also a bitterness perceived as very unpleasant in some medications or plants, like bitter gourd/melon. Depending on the bitter compounds present, they trigger a stimulus on our tongue and in our mouth at the bitter taste receptors. This stimulus travels through nerve pathways to our brain, where it is interpreted. In addition to the type of bitter compound, the interpretation by our brain is the deciding factor in whether we find the food we have just tasted pleasant or unpleasant.
The bitter taste is a sense of taste that is highly culturally dependent and relies primarily on our genetics and conditioning. People in Central Europe who, for example, come into contact with many bitter foods throughout their lives—like the aforementioned beer and coffee—are not just genetically, but also socio-culturally, less sensitive to bitterness compared to people in other parts of the world (Ong, et al. 2018). One could therefore say that, due to their genetics and cultural upbringing, people in Central Europe and North America are much more likely to choose bitter coffees than people in Southeast Asia, who might prefer a sweetened and less bitter coffee variant. This is particularly important regarding the alignment of product portfolios and customer-oriented roasting and preparation of coffee products. After all, roasters and coffee shop owners should always ask themselves: which coffee do my customers like, not which coffee do I like myself (Marquart 2018).
Why is coffee bitter?
The actual question, however, is what makes coffee bitter and is there any coffee that does not taste bitter? Fundamentally, coffee always tastes bitter. This is partly because coffee contains caffeine, which tastes bitter, but primarily due to the composition of coffee and the reactions that take place during roasting.
Acid vs. Bitter: Chlorogenic Acids and Chlorogenic Acid Lactones
The most important of these reaction cascades stems from a group of substances called chlorogenic acids. Depending on the species, variety, and country of cultivation, there are about 25–40 different types of these acid-tasting chlorogenic acids in coffee (Clifford, et al. 2003). During roasting, these chlorogenic acids break down into bitter-tasting chlorogenic acid lactones (Figure 1).
Figure 1: The dominant, acid-tasting chlorogenic acid (left) and the corresponding chlorogenic acid lactone (right), which tastes bitter.
Since there is a multitude of different chlorogenic acids, there is, in turn, an innumerable variety of different corresponding lactone molecules, all of which taste bitter. The most important of these lactones is 3-O-chlorogenic acid lactone (3-CGL), the formation and decay of which during roasting is well-documented. For instance, chlorogenic acid begins to break down after just five minutes of roasting, forming 3-CGL. 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 distinctly mild, pleasant, and almost velvety bitterness modality (modality describes the type of bitterness). They are what give coffee its incomparable bitterness.
Figure 2: Formation of 3-O-chlorogenic acid lactone during the roasting of Coffea Arabica cv. Bourbon.
From pleasant to harsh bitterness
As can be clearly seen from the curve, the bitter-tasting lactones react further. With longer roasting times and higher temperatures, phenylindanes can form from both chlorogenic acids and chlorogenic acid lactones. These phenylindanes represent another class of compounds that are very harsh and bitter-tasting.
Figure 3: A phenylindane isomer. There are many different & similar compounds among the phenylindanes, all of which taste very unpleasantly bitter.
Since phenylindanes can be formed from both acids and lactones, there is a series of different, structurally similar compounds (known as isomers) among phenylindanes as well, all of which taste very long-lasting and unpleasantly bitter. Many might recognize exactly this type of bitter taste from very dark roasts that were subsequently prepared as espresso with hot water. In this sense, phenylindanes are also the final destination in the development of the bitter taste during roasting. This is because phenylindanes can form even more complex structures through polymerization (the chaining together of many individual phenylindanes into one large molecule) (Frank, et al. 2007). However, these chemically large phenylindanes eventually become so large that they can no longer be detected by our bitter taste receptors and consequently no longer have any taste for us humans.
And what does the caffeine do?
One final suspect in the lineup of bitter-tasting substances remains, and that is caffeine. Although caffeine itself tastes bitter, it is hardly degraded during roasting and only accounts for about 10% of the coffee's bitterness. This is partly because both chlorogenic acid lactones and phenylindanes taste significantly more bitter, and partly because the chlorogenic acids "complex" the caffeine contained in the coffee beverage—meaning they bind to it, making it less perceptible to human receptors.
Bitterness in coffee – Conclusion
In general, research findings can be summarized in Figure 3. First, the chlorogenic acids are broken down into the pleasant, coffee-like bitter-tasting chlorogenic acid lactones. Both chlorogenic acids and lactones break down further during continued roasting into the harsh, metallic, and long-lasting bitter-tasting phenylindanes. And caffeine? It only decomposes to a very small extent during roasting.
Figure 4: Development curves of the formation of various bitter substances during coffee roasting, depending on the roast degree.
How to roast mild, non-bitter coffee?
The art of roasting lies in adapting the roast to the green coffee. Science can only provide a starting point here; the correct shaping of bitterness through proper roasting is in the hands of the roaster. Through their experience, they know how to control the invisible reactions in the coffee and adapt them to the desired end result—dark espresso roast or light filter coffee (Marquart 2019).
Good coffee = balance between acidity and bitterness
The chlorogenic acid lactones represent a very special group of bitter-tasting substances, as coffee owes its coffee-like bitterness—which most people find pleasant—primarily to these compounds. Depending on the composition—one could almost call it a symphony of lactones—the roasted coffee unfolds a very finely balanced, mild, velvety bitterness when the beverage is consumed. This bitterness exists in a favorable interplay with the aforementioned fruity acidity of the coffee, which is caused by compounds such as malic, citric, quinic, or the mentioned chlorogenic acids.
If you roast the coffee for too long or too dark, you throw this pleasant bitterness and fruity acidity out of balance. On one hand, the acidic compounds break down, depriving the coffee of a wonderful part of its flavor complexity; on the other, harsh, metallic-bitter phenylindanes form. This is almost comparable to an over-salted dish: the right dose of salt gives the food that certain something extra, but too much of a good thing ruins the dish irretrievably.
In conclusion, one might say that coffee holds a unique position due to the chlorogenic acids it contains. There are hardly any other foods in the world that contain these compounds. This, and the combination with skillful roasting, gives coffee its very characteristic and fine bitterness, which makes it and its taste experience so special. The balance of its acids, bitterness, and especially its aromatic refinement makes it something incomparably unique. When properly roasted, ground, brewed, or extracted, coffee becomes a balancing act of enjoyment.
Further reading:
Clifford, et al. Hierarchical Scheme for LC-MSn Identification of Chlorogenic Acids, J. Agric. Food Chem. 2003, 51, pp. 2900–2911.
Farah, et al. Effect of roasting on the formation of chlorogenic acid lactones in coffee, J. Agric. Food Chem. 2005, 53, pp. 1505–1513.
Fischer, et al. Evolution of bitter taste receptors in humans and apes, Molecular biology and evolution. 2005, 22, pp. 432–436.
Frank, et al. Structure determination and sensory analysis of bitter-tasting 4-vinylcatechol oligomers and their identification in roasted coffee by means of LC-MS/MS, J. Agric. Food Chem. 2007, 55, pp. 1945–1954.
Marquart The Rainbow of Taste. 2018, Kultur & Technik.
Marquart Roasting – a story of technical innovations. In Cosmos Coffee, 1st ed.; Marquart, S.; Jahreis, M.; Möllers, N., Eds.; Deutsches Museum: München, 2019.
Ong, et al. Understanding the role of bitter taste perception in coffee, tea and alcohol consumption through Mendelian randomization, Scientific reports. 2018, 8, p. 16414.
About the author Sara Marquart
Sara Marquart is a food chemist who researched the roasting and bitterness of coffee for her doctoral thesis. She currently works at the Coffee Excellence Center at the Zurich University of Applied Sciences on «Atomo Coffee», molecular coffee without the bean. Prior to this, as a curator at the Deutsches Museum in Munich, she realized the special exhibition Cosmos Coffee.
Further links on bitterness
We have described more about the sensory evaluation of Fine Robusta in an article. When evaluating Fine Robusta, at least the balance of bitterness to sweetness is looked at from a sensory perspective. However, the approach could go even further by describing the different qualities of bitterness, just as we do for acidity or body.
Sara also spoke about bitterness during the SCA Coffee Expo. This resulted in a podcast in English, which you can listen to here.
