A generation ago, most coffee lovers recognised two kinds of coffee: regular and decaf. Now we’re concerned about every last detail from the beans’ country of origin and whether they are organic, have been shade-grown and traded fairly to the degree of roasting and how much pressure was applied during brewing. Yet for all of this connoisseurism, how much do we understand about the science of coffee?
Know your joe
According to Ronald Pegg, a food science professor who teaches an introductory course on coffee at the University of Georgia, Coffee’s main attributes are bitterness, acidity and body, also known as mouth-feel. "The key is to get these three attributes in balance," says Pegg.
As coffee makes its way from tree to mug, producers must keep this balance in mind. It’s not an easy task, because coffee contains more than 1,000 chemical compounds, according to research by Thomas Hofmann of the Technical University of Munich and Oliver Frank of the University of Muenster.
Consider the first element, the bean. There are 70 species of coffee, but two dominate the market: coffea arabica, the more common variety native to Ethiopia, and coffea canephora, also known as coffea robusta, a more disease-resistant plant. Arabica is almost universally acknowledged as the superior variety, although blends often contain robusta because it’s cheaper and adds a strong bitterness, enjoyed by many.
To understand why a sugary bean makes a better brew, we have to consider roasting. Freshly-picked coffee beans have virtually no flavour. Roasting makes coffee palatable. The roasting process triggers a long list of chemical reactions.
One is caramelisation, the breaking-down of sugar, which contributes to coffee’s brown colour. Roasting also creates compounds that contribute to coffee’s unique flavour and aroma, including diacetyl, which adds a butterscotch flavour, and furans, which contribute a nutlike taste.
Roasting is a touchy business, though. High heat can also overproduce bitter compounds and ruin the drink. For many years, food scientists believed that caffeine was the main source of bitterness in coffee. While it’s true that pure caffeine has a bitter flavour, it’s present in such low proportions in an average cup of coffee that it contributes only a small amount of flavour.
How much of the coffee ends up in the cup depends on the brewing method chosen. Percolators and drip machines are extremely inefficient at transferring chemicals from bean to beverage. French-press (or plunger pot) coffee is similarly mild, but it is high in oil content, providing a body that some drinkers prefer. Espresso is more than 5% coffee bean. These differences affect not only flavour but also the perception of body and mouth-feel.