Coffee is bitter.
Bitterness is as much a part of coffee's flavor profile as water is needed for brewing. And no one knows more about it than scientist Sara Marquart. Sara conducts research at the Zurich University of Teacher Education in the field of coffee (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 regarding bitterness in coffee.
Why Does Bitterness Have a Bad Reputation?
Many start their day with a cup of coffee in the morning, whether brewed as a hand filter or extracted as an espresso. Along with the aroma, the scent of roasty, chocolatey, or fruity-berry aromas, coffee also enchants us with its taste. But what exactly makes up the taste of coffee?
In addition to a pronounced, sometimes fruity acidity, bitterness stands out especially. However, bitterness in coffee has not always had 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 protection mechanism for humans. In earlier times, when there were no encyclopedias or Wikipedia, people didn't know whether a fruit was edible or poisonous. Our ancestors' sensitive bitter receptors helped them immediately spit out the potentially poisonous fruit (Fischer, et al. 2005). However, there are also many compounds in foods that taste bitter but primarily possess health-promoting properties. And here we arrive back at coffee: coffee has 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. There is a type of bitterness that is perceived as pleasant by humans, such as that found in beer, chocolate, tea, and some coffees. Beside that, there is also a type of bitterness perceived as very unpleasant in some medications or plants, such as bitter cucumber or bitter melon. Depending on the bitter-tasting compounds they contain, these trigger a stimulus on our tongue and in our mouth at the bitter taste receptors. This stimulus travels via nerve pathways to our brain and is interpreted there by us. In addition to the type of bitter-tasting compound, the interpretation of 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 that depends primarily on our genetics and conditioning. People in Central Europe who, for example, come into contact with many bitter foods like the aforementioned beer and coffee throughout their lives are not only genetically but also socio-culturally rather insensitive to bitterness, compared to people in other parts of the world (Ong, et al. 2018). One could say, then, that people in Central Europe and North America, due to their genetics and cultural conditioning, would much more frequently reach for bitter coffees than people in Southeast Asia, who might prefer a sweetened and less bitter coffee variant. This is especially important with regard to the orientation of the product portfolio and customer-oriented roasting and preparation of coffee products. After all, roasters and coffee shop operators should always ask themselves one question – what coffee do my customers like, not what 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 primarily because of 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 comes from a group of substances called chlorogenic acids. There are about 25–40 different chlorogenic acids in coffee, depending on the type, variety, and country of origin (Clifford, et al. 2003). During roasting, these sour-tasting chlorogenic acids break down into the bitter-tasting chlorogenic acid lactones (Figure 1).
Abbildung 1: The dominating, sour-tasting chlorogenic acid (left) and the corresponding chlorogenic acid lactone (right), which tastes bitter.
Since there are many different chlorogenic acids, there are correspondingly countless varieties of the different molecules among the lactones, which all taste bitter. The most important of these lactones is 3-O-chlorogenic acid lactone (3-CGL), whose formation and breakdown during roasting is well documented. Already after five minutes of roasting, the chlorogenic acid breaks down and 3-CGL forms. After approximately 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 silky bitter modality (modality describes the type of bitterness). They are what give coffee its incomparable bitterness.
Abbildung 2: Formation of 3-O-chlorogenic acid lactone during roasting of Coffea Arabica cv. Bourbon.
From Pleasant to Harsh Bitterness
As you can see clearly from the curve, the bitter-tasting lactones continue to react. Because 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 compounds with very harsh, bitter-tasting compounds.
Abbildung 3: A phenylindan isomer. There are many different and similar phenylindan compounds, which all taste very unpleasantly bitter.
Since phenylindanes can form from both the acids and the lactones, there are also many different structurally similar compounds among the phenylindanes (so-called isomers), all of which taste very long-lasting and unpleasantly bitter. Many may recognize exactly this type of bitter taste from very dark roasts that are subsequently prepared with hot water as an espresso. The phenylindanes are also, in this sense, the endpoint in the development of bitter taste during roasting. Because phenylindanes can still form increasingly complex structures through polymerization (i.e., a linking of many individual phenylindanes to form one large molecule) (Frank, et al. 2007). These chemically speaking large phenylindanes eventually become so large that they can no longer be absorbed by our bitter taste receptors and accordingly have no taste for us humans.
And What About Caffeine?
One last suspect in the array of bitter-tasting compounds remains, and that is caffeine. Caffeine itself tastes bitter, but it is hardly broken down during roasting and accounts for only about 10% of coffee's bitterness. This is partly because both chlorogenic acid lactones and phenylindanes taste considerably more bitter, but also because the chlorogenic acids «complexize» the caffeine contained in the coffee beverage, i.e., bind to it and thus make it less perceptible to human receptors.
Bitterness in Coffee – Conclusion
In general, the findings of research can be summarized in Figure 3. First, chlorogenic acids are broken down into the pleasantly, coffee-like bitter-tasting chlorogenic acid lactones. Both chlorogenic acids and the lactones break down further during roasting into the harsh, metallic, and long-lasting bitter-tasting phenylindanes. And caffeine? It only breaks down to a very small extent during roasting.
Abbildung 4: Progression curves of the formation of various bitter-tasting compounds during coffee roasting, as a function of the degree of roast.
How Do You Roast Mild, Non-Bitter Coffee?
The art of roasting lies in adapting the roast to the green coffee. Here, science can only provide a starting point; the correct development of bitterness through proper roasting is in the hands of the roasters. Through their experience, they understand how to control the invisible reactions in the coffee and adapt them to the desired end result – dark espresso roast or light filter coffee – accordingly (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 coffee-like bitterness primarily to these compounds, which most people find pleasant. Depending on the composition – one might almost say – symphony of the lactones, the roasted coffee unfolds a very finely balanced, mild, silky bitterness when enjoying the beverage. This bitterness interplays pleasantly 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 you roast the coffee too long or too dark, you throw this pleasant bitterness and fruity acidity out of balance. On one hand, the acidic compounds break down, and thus you deprive the coffee of a wonderful part of its flavor complexity; on the other hand, the harsh, metallic-bitter phenylindanes form. This can almost be compared to an over-salted dish: the right amount of salt gives the food that certain something extra, too much of a good thing ruins the dish irretrievably.
To conclude, one might say that coffee, through the chlorogenic acids it contains, occupies a unique position. There are few foods in the world that contain these compounds. This and the combination with skillful roasting gives coffee precisely its very characteristic and refined bitterness, which makes it and its taste experience so special. The balance of its acids, the bitterness, and especially its aromatic finesse makes it something uniquely incomparable. 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 and conducted research on coffee roasting and bitterness in her doctoral thesis. Currently, she is working at the Coffee Excellence Center of the Zurich University of Teacher Education on "Atomo Coffee", molecular coffee without the bean. Previously, as a curator at the German Museum in Munich, she realized the special exhibition Cosmos Coffee.
Further Links on Bitterness
More on the sensory evaluation of Fine Robusta, we have described in an article. In the evaluation of 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, just as we do with acidity or body.
As part of the SCA Coffee Expo, Sara also spoke about bitterness. This resulted in a podcast in English, which you can find nachhören here.
