Coffee is bitter.
Bitterness is as much a part of coffee's flavor profile as water is used for its preparation. And no one knows this better than scientist Sara Marquart. Sara researches coffee at the Zurich University of Applied Sciences (ZHAW Wädenswil, Coffee Excellence Center) and wrote her 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 start their day with a cup of coffee in the morning, whether brewed by hand filter or extracted as espresso. In addition to its aroma, the scent of roasty, chocolatey, or fruity-berry notes, coffee also captivates us with its taste. But what exactly makes up the taste of coffee?
Besides a distinct, sometimes fruity acidity, bitterness stands out most prominently. However, 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 attributed to the fact that bitterness receptors represent an evolutionary protective mechanism for humans. In earlier ages, when there were no encyclopedias or Wikipedia, people did not know whether a fruit was edible or poisonous. This is where our ancestors' sensitive bitter receptors helped them to immediately spit out potentially poisonous fruit (Fischer, et al. 2005). However, there are also many compounds in food that taste bitter but primarily possess 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 bitter always bitter?
Bitterness is not always the same as bitterness. For humans, there is a bitterness that is perceived as pleasant, such as that found in beer, chocolate, tea, and some coffees. However, there is also a very unpleasant bitterness in some medicines or plants, such as bitter gourd/melon. Depending on the bitter compounds they contain, these trigger a stimulus on our tongue and in our oral cavity at the bitter taste receptors. This stimulus travels via neural pathways to our brain, where it is interpreted by us. Besides the type of bitter compound, the interpretation by our brain 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 primarily depends on our genetics and conditioning. People in Central Europe, for example, who come into contact with many bitter foods such as the aforementioned beer and coffee throughout their lives, are not only genetically but also socio-culturally more 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 conditioning, would much more frequently opt for bitter coffees than people in Southeast Asia, who might prefer a sweetened and less bitter coffee variant. This is particularly relevant with regard to the alignment of the product portfolio and the customer-oriented roasting and preparation of coffee products. Because roasters and coffee shop operators should always ask themselves one question – which coffee do my customers like, and not which coffee do I like myself (Marquart 2018).
Why is coffee bitter?
The real question, however, is what makes coffee bitter, and is there a coffee that doesn't taste bitter? Basically, coffee always tastes bitter. This is partly because coffee contains caffeine, which tastes bitter, but mainly because of the composition of coffee and the reactions that occur during roasting.
Sour vs. Bitter: Chlorogenic Acids and Chlorogenic Acid Lactones
The most important of these reaction cascades originates from a group of substances called chlorogenic acids. There are about 25–40 different types of these sour-tasting chlorogenic acids in coffee, depending on the species, variety, and country of origin (Clifford, et al. 2003). During roasting, these chlorogenic acids decompose into 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 is a multitude of different chlorogenic acids, there is an innumerable variety of different molecules from 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 are well documented. Chlorogenic acid already decomposes after five minutes of roasting, and 3-CGL is formed. After about 7.5–10 minutes, depending on the roasting temperature and the chosen coffee, the formation of bitter lactones reaches its maximum (Figure 2, Farah, et al. 2005). These lactones are known for their very pronounced mild, pleasant, and almost velvety bitter 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 progression curve, the bitter-tasting lactones react further. With longer roasting times and higher temperatures, phenylindanes can form from both the chlorogenic acids and the chlorogenic acid lactones. These phenylindanes represent another class of very harsh, bitter-tasting compounds.
Figure 3: A phenylindane isomer. There are many different & similar phenylindane compounds, all of which taste very unpleasantly bitter.
Since phenylindanes can arise from both acids and lactones, there are also a number of different, structurally similar compounds (so-called isomers) among the phenylindanes, all of which taste very long-lasting and unpleasantly bitter. Many may be familiar with precisely this kind of bitter taste from very dark roasts, which were subsequently prepared as espresso with hot water. Phenylindanes are also, in this sense, the final stage in the formation of bitter taste during roasting. This is because phenylindanes can form even more complex structures through polymerization (i.e., a chain reaction of many individual phenylindanes into a large molecule) (Frank, et al. 2007). However, these, chemically speaking, large phenylindanes eventually become so large that they can no longer be absorbed by our bitter taste receptors and, accordingly, no longer have any taste for us humans.
And what does caffeine do now?
One last suspect in the array of bitter compounds remains, and that is caffeine. Although caffeine itself tastes bitter, it is barely degraded during roasting and accounts for only about 10% of coffee's bitterness. This is partly because both chlorogenic acid lactones and phenylindanes taste significantly more bitter, but also because chlorogenic acids "complex" the caffeine contained in the coffee beverage, meaning they bind to it and thus make it less perceptible to human receptors.
Bitterness in Coffee – Conclusion
In general, the findings of the research 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 further decompose during roasting into the harsh, metallic, and long-lasting bitter-tasting phenylindanes. And what about caffeine? It only degrades to a very small extent during roasting.
Figure 4: Progression curves of the formation of various bitter compounds during coffee roasting, depending on the degree of roast.
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 roasters are in charge of properly developing the bitterness through correct roasting. Through their experience, they understand how to control the invisible reactions in the coffee and adapt them to the desired end result – whether 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 coffee owes its characteristic coffee-like bitterness, which most people find pleasant, primarily to these compounds. Depending on the composition – one could almost say – the symphony of lactones, the roasted coffee develops a very finely balanced, mild, velvety bitterness when the drink is consumed. This bitterness is in a benevolent interplay with the already mentioned fruity acidity of the coffee, which is caused by compounds such as malic, citric, quinic, or the aforementioned chlorogenic acids.
If coffee is roasted too long or too dark, this pleasant bitterness and fruity acidity are thrown out of balance. On the one hand, the acidic compounds break down, thus depriving the coffee of a wonderful part of its flavor complexity, and on the other hand, harsh, metallic-bitter phenylindanes form. This is almost comparable to an over-salted dish: a proper dose of salt gives the food that certain extra, but too much of a good thing irrevocably ruins the dish.
In conclusion, it can perhaps be said that coffee holds a unique position due to the chlorogenic acids it contains. There are hardly any foods in the world that contain these compounds. This, combined with skillful roasting, gives coffee its very characteristic and delicate bitterness, which makes it and its taste experience so special. The balance of its acids, bitterness, and especially its aromatic finesse makes it something incomparably unique. Properly roasted, ground, brewed, or extracted, coffee becomes an act of enjoying balance.
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 and researched the roasting and bitterness of coffee in her doctoral thesis. She is currently working at the Coffee Excellence Center of the Zurich University on "Atomo Coffee", molecular coffee without the bean. Before that, 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 considered sensorially. However, the approach could go further, by describing the different qualities of bitterness, as we also do for acidity or body.
Sara also spoke about bitterness at the SCA's Coffee Expo. This resulted in a podcast in English, which you can listen to here.
