When a discussion weighs in on the merits of one of the world’s most widely consumed beverages, I think that a modicum of science is rightly called for. Then again, I am quite liberal with my modicums of science, so I would say that, wouldn’t I? Whether you enjoy a lively French press, a robust pot of moka or a silky latte, there is an imperceptible microcosm of science inside the tastes, smells, and textures that are pushing you out of bed or into that next meeting. Everyone knows that caffeine is the keystone of coffee chemistry, but there is much more going on than just 1,3,7-trimethylpurine-2,6-dione (that’s the really, really, really long way of saying ‘caffeine’). Interested in getting up close and personal with your favourite beverage? Don’t forget to check out the companion article on milk and foam when you finish! Now, read on.
Freshly picked coffee fruit (also called berries or cherries) is specially dried to give seeds (beans) that are green in colour, and definitely do not make for welcome drinking. Almost all of the flavour in a cup of espresso is created during the roasting process, when the green beans are heated to up to around 250ºC, depending on the type of roast desired. At the top end of the temperature scale lie the burnt, smoky, and bittersweet Italian and Vienna roasts, whereas mild, more acidic, caramel flavoured coffee that still retains a part of the green bean character results from roasts that reach a maximum of 210ºC. The impressive range of tastes and aromas that result from dutiful application of fire to a tiny plant seed are the product of an astounding number of different physical processes and chemical reactions occurring. It is impossible to list them all, but we can definitely take a peek at a few of the most important.
The Maillard reaction and caramelisation are two of those biggies, responsible for a large fraction of the flavour and colour of the drink. Melanoidins, large brown-coloured molecules formed during the Maillard reaction, give the characteristic tawny hue to frothy crema. Smaller compounds such as pyrazoles, pyridines, and furans are created when sugars and proteins react with themselves and each other under the high heat of roasting. All of them have different smells and properties; many pyrazoles, for example, give burnt, caramel flavours. Though they have strange names that sound more like things you would expect to find in a Cthulhu novel than in your food, you would be surprised how many you have already encountered. Trigonelline, the little chemical with the big name, degrades during coffee roasting to give many sweet and earthy aromatic compounds along with one very familiar pyridine – niacin, also known as vitamin B3.
Chlorogenic acids are other components that also break down during roasting to give smaller, more aromatic compounds, some of which go a long way to contributing to the taste and also bitterness of brewed coffee. 4-vinyl guaiacol is one of these, and has been described as a clove-like blend of smoky, spicy, and sweet. Don’t believe me? Order up some 4-vinyl guaiacol and let the smells and flavours roll across your palate and olfactory glands, triggering a cascade of sensations linked to memories of aromas of yore. Are you getting queasy flashes of sausages, lederhosen, and a 24 hour hangover? It’s highly possible, because this compound is also found in German wheat beers. Mmmm…beer.
As the temperature increases during roasting, more and more compounds are formed. One important by-product of all of this decomposition is carbon dioxide. Along with water vapour, this gas swells the beans and contributes to the recognizable “first crack” of coffee roasting. A vital consequence of the carbon dioxide production is the formation of an oxygen free environment inside the roasted bean. The precious cargo carried within, in the form of delicious, but fragile molecules, is protected from destruction when guarded by the hard, brown shell. Once the coffee is ground they are vulnerable to oxidation by air, meaning that the shelf life of ground coffee sitting on your counter is no more than a day or two. Espresso contains at least one thousand different chemicals, and even these are only a fraction of the total – the fraction that dissolves into water.
Think of a cup of espresso as a rock ballad being crushed out by a trio of musicians. On guitar/vocals is the flashy, headline grabbing roasted coffee bean, pumping its chest at the front of the stage. On bass guitar, occasionally rewarded with a soulful solo, is the espresso machine. Hidden behind these two frontmen, perched against the backdrop, is the hard-working but largely invisible drummer, water. Yes, water is the Neil Peart of the espresso act.
Just as Rush would be nothing without Neil Peart, espresso without water would be – well, it would be a messy espresso machine. A very dry, very messy espresso machine. Think about how much is spent on fresh, premium roasted arabica espresso beans or a Rocket double pump, chromed piece of sexy, espresso machinery. Then think about how much people invest in water. Pretty much nothing at all beyond a fraction of the cost of their water bill. Still, if there is one thing marketing has taught us it is that cost is never an indication of quality, and it appears that when it comes to the best quality of water that can be used to make coffee, cost is less of a factor than dumb, blind luck.
That said, what type of water is best for espresso? Researchers have very recently examined this question in some detail and have made a number of crucial observations. Magnesium, a component of “hard” water, is found to aid the extraction of coffee into water and improve the taste. High levels of bicarbonates have the opposite effect, being detrimental to the taste of coffee, probably because they decrease the acidity (an increase in the pH value). High levels of sodium also have a negative impact on coffee taste, but there are many more factors that alter how the water interacts with coffee grinds during the extraction process. The authors noted that for hard water a dark roast may result in an over extracted coffee, while a medium roast might be the best fit. Conversely, with softer water a medium roast could well be the ticket.
Overwhelmingly, people are reliant on local water supplies to produce their coffee, and just as the composition of the air or soil can vary from place to place, so too will the water composition. Levels of different chemicals in municipal water can change daily for any number of reasons. Some households may have softened water in which “hard” elements such as calcium and magnesium are swapped for others such as sodium. Unless you are using bottled, distilled water it is extremely difficult to consistently get the same water quality. Still, the researchers found that if you are prepared to test your water to determine what its composition is, then you can choose the kind of coffee that forms a perfect match.
So coffee is more than just caffeine! All this is only a splash in the pot of coffee chemistry; the amount of research done on this little seed could fill a library. Questions? Feel free to ask and I will do my best to answer!
Check out this excellent graphic for an in-depth look at a variety of the hundreds of chemicals contributing to the exuberant aroma of a cuppa java. The Specialty Coffee Association of America (SCAA) has also has its own standards for coffee water quality available on the internet.