Chris Bartlett explains how extraction rates affect the characteristics of coffee. Photo by Joshua Ness.
Without caffeine, coffee – both the plant and the drink – would surely never have made it out of Ethiopia. Its euphoric, energy-giving effects are what has caused coffee to be cultivated and consumed around the world for centuries. It keeps whole workforces going, stimulating both body and mind – indeed, it’s why you’re reading Caffeine magazine right now.
But caffeine is far from the only essential compound in your cup. The composition of any cup of coffee, from the most revered single estate to the harshest of dark-roasted blends, is complex. Each one may contain as many as a thousand distinct compounds, each with its own properties and its own effect on your drinking experience.
Caffeine belongs to a class of compounds known as purines. Its structure means it binds to adenosine receptors in the nervous system, and in doing so blocks the effects of adenosine – which is how it reduces feelings of drowsiness and increases alertness. The structure of a molecule also determines its physical behaviour in water and how we perceive that when drinking.
Whole structural classes – detected by chemoreceptors on the tongue and in the nose – are perceived similarly. So alkaloids taste bitter; sugars taste sweet; aromatic compounds containing oxygen, nitrogen and sulphur smell caramel-like, cereal-like and roasted, respectively.
The properties of molecular classes also affect the character of coffee, which is why it changes so significantly when adjusting the variables of extraction. Skilled roasters and baristas adjust their methods to extract appropriate levels of the different compounds. Achieving that balanced cup is what is known as optimal extraction.
The levels of extraction in your coffee can be described as under-extracted, optimal and over-extracted. The characteristics of each level are those all coffee drinkers will be familiar with. Broadly speaking, if it’s under-extracted it will be sour and salty; over-extracted and it will be bitter and harsh. Optimal extraction brings the acidic, sweet and complex flavours that most of us know and love.
Let’s delve a bit deeper into the science. The rate at which compounds are extracted in water is determined by two properties: size and polarity. Small, ionic compounds and those that contain an alcohol group are extracted before large, non-polar, fatty molecules. The compounds found in coffee are extracted in the following order, from quick to slow: salts, inorganic acids, organic acids, sugars, volatile organics, phenolics, large molecules.
So in an under-extracted cup, inorganic acids such as phosphoric acid may be 100 times stronger than organic acids like citric acid, giving a sharp or even metallic taste. When these acids and salts are the dominant compounds, the result is a sour cup. An over-extracted cup, meanwhile, will contain a dominant concentration of bitter and astringent compounds that overpower everything else. We have evolved to detect these compounds at extremely low concentrations because some are toxic. Quinine, for example, is detectable at 0.5 parts per million; sugar, by comparison, becomes detectable at around 5,000 ppm.
In an optimally extracted coffee, small organic acids like malic acid (think Granny Smith apples) are dissolved quickly by water at brewing temperature, developing a bright acidity. Sugars such as glucose have high solubility in water and their extraction is what balances the more bitter compounds. Small organic aroma molecules are well extracted, which means they are easily discerned. These include furaneol (caramel), damascenone (honey), 2-furfurylthiol (roasted) and 2-methylpropanal (fruit).
Of course, all the compound classes are present in any cup – it’s the concentration of each, which changes through extraction, that determines the flavour. And whatever the extraction level, the potential flavour of the brew is pre-determined by the quality, origin and species of the bean, followed by the quality and level of the roast.
But there’s no doubt that those who approach coffee with scientific rigour will be rewarded. The skills, knowledge and craft of those who cultivate, roast and ultimately brew coffee have been the driving force behind the third wave.