There are so many things that contribute towards a coffee’s flavor: the variety, the roast profile, the brew recipe, the processing method… Yet when you start breaking it down, its flavor profile is really the result of one thing – chemical compounds.
The way we roast coffee, the type of coffee we have, the altitude it was grown at, these all have an impact on these compounds, of course. But if we want to really optimise a coffee’s profile, we need to understand the science behind it. (This is particularly true for roasters, since your job is to manipulate heat to control several chemical reactions.)
I’m currently completing a PhD in Food Science, with a focus on how we can predict a coffee’s flavor, aroma, and quality through analyzing its chemical compounds. Let me talk you through the main points you should know.
Versión en Español: Ciencia del Café: Descifrando de Dónde Viene el Sabor
Your Coffee’s Flavor: A Matter of Chemistry
A coffee’s flavor, aftertaste, acidity, and body are all created in response to different chemical compounds. On the cupping table, we analyze them with our senses – but chemical reactions still come into play. We evaluate the dry aroma, followed by the aroma as we break the crust, followed by the taste. Different stages, different molecules.
As a Q-grader, I love the process of cupping coffee. But as a scientist, I also look directly to the chemical compounds and how they interact. There are many non-volatile compounds that have an impact on flavor and quality, from carbohydrates to caffeine. The amount of these will vary according to the quality of the green coffee – but remember that the chemical composition of green beans is entirely different from that of roasted beans.
And this brings me to the volatile compounds: these are mostly created by chemical transformations during the roasting process. They are essential in determining a coffee’s flavor and quality.
Coffee three ways. Credit: Elm Coffee Roasters
Non-Volatile Compounds Worth Knowing
Non-volatile compounds include alkaloids (caffeine and trigonelline), chlorogenic acids, carboxylic acids, carbohydrates and polysaccharides, lipids, proteins, melanoidins, and minerals. These all impact a coffee’s flavor.
Caffeine affects a coffee’s perceived strength, bitterness, and body. It’s soluble in water. Another essential alkaloid is trigonelline, which contributes to a roast and brewed coffee aroma.
Chlorogenic acids are formed by a trans-cinnamic acid (caffeic acid, ferulic acid, and p-coumaric acid) and quinic acid. Quinic acid creates bitterness and astringency. During roasting, chlorogenic acids degrade, increasing our perception of quinic acid and making very dark roasts taste bitter.
Organic acids are related to good acidity, which is a crucial attribute of quality correlated with sweetness. Arabica coffee is more acidic than Robusta, and a coffee’s acidity decreases with roasting. Dark roasts decrease a coffee’s acidic content and perceived acidity.
While some acids will degrade during roasting , others will increase in concentration.
The flavor wheel. Credit: Jemmy Wijaya Shalim
Carbohydrates & Polysaccharides: polysaccharides include arabinogalactans, mannans, and cellulose. These compounds retaining volatile compounds which contributes to aroma. They also create beverage viscosity and smaller ones like glucose and fructose contribute to perceived sweetness.
Lipids contribute to the texture of the brewed coffee. They’re extracted from the beans and, in espresso, creates the crema. The roasting process doesn’t affect their content levels but, while in the roaster, oils migrate to the bean surface (keeping the volatile compounds inside).
Finally, melanoidins are a product of Maillard reactions: reactions between amino acids and the carboxyl group of reducing sugars. These large molecules that create a bean’s brown color and add to a brew’s texture. Caramelisation also contributes to this colour. A coffee’s amino acid content is closely related to its quality. The more amino acids, the more reactions and the more melanoidins.
Brewing coffee with a Kalita Wave. Credit: Tyler Nix
What Are Volatile Compounds?
Now we get to the most exciting topic of all: volatiles in coffee. There are many, many volatile compounds in coffee, and as I said before, they are an essential determinant of a coffee’s flavor.
Volatile compounds are mainly created by chemical transformations during the roasting process – but that doesn’t mean the green beans have nothing to do with them. Volatile compounds characterize coffee varieties, farmers’ processing techniques, and also the geographical origins of the coffee.
We can easily say that there are over 1,000 volatiles compounds in coffee – after roasting. However, only a small number of these contribute to the perceived aroma. Several researchers suggest that there are around 20–30 individual volatiles at work in our beverage. When analyzing aroma, we should bear in mind that it’s probably more closely related to the quantity of one single compound, plus its synergy between other compounds and their “threshold,” than all 1,000 compounds. (Threshold means the minimum quantity of a chemical that the human nose is sensitive to).
Freshly roasted coffee.
Chemical Reactions That Create Volatile Compounds
So let’s take a look at the chemical reactions that lead to these volatile compounds. The primary chemical reactions are the Maillard reactions (non-enzymatic browning). We’ve already looked at them in terms of melanoidins; however, they also produce nitrogen and sulfur-containing heterocyclic compounds.
Another important reaction to take note of is phenolic acid degradation. This is a degradation of chlorogenic acids (remember those from the section on non-volatile compounds?) to form caffeic acid (or another trans-cinnamic acid), lactones, and quinic acids. These compounds contribute to bitterness and astringency in brewed coffee.
Strecker degradation is another critical reaction. It occurs mainly during the development stage of coffee roasting and refers to a breakdown of amino acids in aldehydes and ketones (good for aroma).
And, to be honest, there are at least seven more reactions that we could talk about during roasting – but that’s a topic for another article. In my mind, the roasting phase is like a big chemical party occurring inside the beans. There is so much going on.
Coffee cooling doing after being roasted. Credit: Crema Coffee Garage
Specific Compounds, Specific Flavor Notes
These volatile compounds include things like hydrocarbons, alcohols, aldehydes, ketones, carboxylic acids, esters, pyrazines, pyrroles, pyridines, furans, furanones, phenols, and more.
And while these names all sound very scientific and, perhaps, a little intimidating, they can all be traced to specific coffee attributes.
Have you used the Le Nez du Café aroma set? It contains 36 aromas that you can use to train your sensory skills. One of my favorites is apricot (number 16): a fresh, fruity aroma. Usually, floral and fruity aromas are the result of ketones and aldehydes. And if you look up the characteristics of number 16 in the book that comes with the Le Nez du Café set, you will see that this aroma is related to Benzaldehyde – an aldehyde. Beautiful!
Furan and furanones are what usually cause us to perceive caramel and roasted coffee notes. Pyrazines can be related to notes of nuts and roasted coffee. And, of course, the amount and interactions of some other compounds can create negative notes: guaiacol molecules (phenolic compounds) are responsible for phenolic and burnt notes.
Coffee cupping. Credit: Aveley Farms Coffee
I’m sorry, readers, but this world that I have begun exploring doesn’t seem to have any boundaries. This article is just an introduction to the wonders of coffee chemistry: there is so much more to learn. So let’s start talking about it together, whether over a science book or simply a cup of coffee.
Because while it all comes down to the profound and complex realm of science, we can always taste these compounds in the cup.
Perfect Daily Grind
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