Aroma Chemistry - The Smell of Coffee

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Whether you’re a coffee connoisseur or completely unfussy about the manner in which you get your caffeine fix, there’s no denying that the smell of freshly-brewed coffee in the morning is an invigorating one. The chemistry behind this aroma, though, is far from simple; a complex collection of chemical compounds are responsible, and this graphic takes a look at a selection of these.

We’ve taken a look at chemical compounds found in coffee beans previously, but then we were primarily concerned with what causes the bitter notes in the flavour of coffee, as well as looking at some of the more obvious compounds present, such as caffeine. Generally, the majority of the molecules mentioned in that post aren’t big players when it comes to coffee’s aroma. Kick-starting your day with caffeine might be the goal of the morning coffee, but caffeine itself is odourless as well as essentially tasteless, at least as far as coffee’s concerned, and instead it’s a range of other compounds that contribute to the scent.

Specifically, we’re talking volatile compounds – organic compounds that easily evaporate at room temperature and pressure. Compounds have to be airborne in order for our nose to be able to detect their smell, so it follows that any compounds that are particularly large (for example, the melanoidins that contribute to coffee’s colouration) will have low volatilities, and won’t contribute to the aroma. That leaves us with the slightly more modestly-sized molecules shown in the graphic. But where do these come from?

There are a number of different ways in which coffee’s aroma compounds are created, but they’re all commonly produced as a consequence of the roasting process. The Maillard reaction, the complexities of which were discussed on the site recently, is a big contributor here, the reaction between proteins and sugars in the coffee beans producing a range of products. In addition to this, degradation and decomposition of other compounds in the coffee beans can also produce aroma compounds.

The brewing part of the coffee-making process isn’t about chemical change – rather, it’s about extracting compounds from the roasted coffee beans. How well different molecules can be extracted depends on their solubilities, which in turn depends on a property known as polarity. Different types of atoms exert more of a ‘pull’ on the electrons in chemical bonds than others; oxygen exerts more of a pull on bonding electrons than carbon, for instance. A bond between a carbon and oxygen atom is what we would refer to as a polar bond, as the bonding electrons are pulled closer to the oxygen atom, giving it a slight negative charge.

The presence of polar bonds in a molecule can lead to the molecule being polar as a whole if the polar bonds are not distributed evenly. This results in the different ends of the molecule becoming slightly charged. Going back to our discussion of solubilities, polar molecules are more soluble in water than non-polar molecules. This is because water itself is a polar molecule – the oxygen atom exerts more of a pull on bonding electrons than the hydrogen atoms – and interacts with and surrounds other polar molecules, allowing them to dissolve. So, the more polar molecules in coffee beans are extracted in higher percentages during the brewing process than the non-polar molecules.

A whole range of studies have been dedicated to discerning which of these extracted compounds contribute to the aroma of a cup of coffee. Whilst over a thousand different chemical entities have been identified in coffee beans, and a significant number of these will be extracted during brewing, it’s a comparatively small subset of chemicals that impact on the aroma. The studies often consider two main factors when discerning a compounds’ aroma impact: the concentration of the compound, and the compound’s odour threshold, or the minimum concentration at which we can detect its smell. The ratio of a compound’s concentration to its odour threshold gives the compounds ‘odour activity value’ (OAV), which gauges its importance to the overall aroma.

A number of families of compounds are significant contributors to coffee’s aroma. Several sulfur-containing compounds are of importance, including 2-furfurylthiol, with an aroma that on its own is actually commonly described as ‘roasted coffee’. There are also some compounds which on their own might smell pretty unpleasant, but in chorus with the other compounds add nuances to the aroma; for example methanethiol, which has a smell described as like that of rotten cabbage, and which is also a significant contributor to the smell of flatulence. Another sulfur-containing compound, 3-mercapto-3-methylbutyl formate, is brilliantly described as having a ‘catty’ odour in isolation.

Other contributing families of compounds in include aldehydes, which generally add a fruity, green aroma, furans, which contribute caramel-like odours, and pyrazines, which have an earthy scent. Guaiacol and related phenolic compounds offer smoky, spicy tones, and pyrroles and thiophenes are also present in low concentrations.

As it happens, coffee’s aroma might even have a little more to it. A 2008 study found that the smell of coffee beans affected gene and protein activity in rat brains, some of which were linked to stress relief. Whilst rat brains and human brains have their differences, it might suggest that the lift from your morning coffee isn’t solely the consequence of its caffeine content!



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