The science behind a perfect cup of coffee

There is something about the smell of a good cup of coffee in the morning that just cannot be beaten. Unless you prefer tea of course, in which case it should be gently highlighted that coffee is now the most popular drink worldwide and it’s probably time to give it a go. But how do you make a perfect cup of coffee?

Coffee beans are harvested from tropical evergreen coffee trees, which flourish in temperatures between 18−21°C and in rich soils. Some coffee plants do best in high-altitude mountainous areas and most prefer plenty of rain mixed with dry periods. This means many coffee plantations can be found in the fertile soils on the slopes of volcanoes. The beans that you will most likely see in supermarkets and coffee shops are arabica or robusta beans, which have slightly different tastes and caffeine contents.

Coffee beans don’t start out brown and crunchy. Red cherries grow on the coffee plant, and within them reside green-ish coffee beans (meaning that they are technically a seed). Once harvested, the coffee beans are roasted which is when the flavour of the coffee is formed. The amino acids and sugars in the beans combine in a chemical reaction, creating aromatic compounds that produce the delicious flavours of coffee. Once the beans are ground, they are then ready to create a cup of coffee.

The perfect cup of coffee is rather a hot topic of discussion and can be very subjective.

Just add water
The first step to a hot cup of coffee is adding water. The water breaks the chemical bonds of those aromatic compounds within the coffee grounds, releasing the flavours and its distinctive aroma via volatile oils. A major part of taste is actually dependent on our sense of smell, which is why the aroma of coffee is important – without the volatile oils coffee would only taste sour or bitter. There are actually around 800 different compounds that can potentially be created during the roasting process, although we are not able to smell all of them.

Unless you’re making a ‘cold brew’ the optimum temperature for the water is between 90−96°C. If the temperature is too low, not as much flavour will be extracted. If it is too high it can cause a loss of quality in flavour as the volatile oils are boiled off. The particular flavour of each brand of coffee bean, such as those nutty, chocolatey, or citrus overtones, is dependent on the location where they were grown. Climate and elevation play a role, and surprisingly, so does the soil. The coffee tree extracts nutrients from the soil, and the specific quantities of elements such as nitrogen and phosphorous ultimately affect the flavour of the bean.

Extraction methods
The perfect cup of coffee is rather a hot topic of discussion and can be very subjective. From the bean type and extraction method (the chosen technique to add water to coffee), to whether we should add milk, ice cream, or whiskey, there is a huge range of ways to drink it. The size, or coarseness, of the coffee grind also has implications for the extraction technique.

A finely ground coffee, such as one for making an espresso, has a large surface area. This means that the aromatic compounds along with the caffeine content are extracted very rapidly. A coarser grind, such as one for a cafetière, can take up to four minutes to brew. The perfect cold brew must at least be left overnight for proper extraction. However you prefer to drink it, the world is not short of ways to experiment with the perfect coffee!

Ruth Kirk is a science writer based in the UK.