Several sulfur-containing compounds are of importance, including 2-furfurylthiol, with an aroma that on its own is actually commonly described as ‘roasted coffee’. 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.Ī number of families of compounds are significant contributors to coffee’s 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. 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. So, the more polar molecules in coffee beans are extracted in higher percentages during the brewing process than the non-polar molecules.Ī whole range of studies have been dedicated to discerning which of these extracted compounds contribute to the aroma of a cup of coffee. 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. Going back to our discussion of solubilities, polar molecules are more soluble in water than non-polar molecules. This results in the different ends of the molecule becoming slightly charged. 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. 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. 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. How well different molecules can be extracted depends on their solubilities, which in turn depends on a property known as polarity. The brewing part of the coffee-making process isn’t about chemical change – rather, it’s about extracting compounds from the roasted coffee beans.

In addition to this, degradation and decomposition of other compounds in the coffee beans can also produce aroma compounds. 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. 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. That leaves us with the slightly more modestly-sized molecules shown in the graphic. 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. Specifically, we’re talking volatile compounds – organic compounds that easily evaporate at room temperature and pressure. 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. 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.

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. 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.

Caffeine structure polarity