Espresso is the pinnacle of all coffee drinks and is the basis for almost every recipe. In all third-wave coffee shops (shops which use only the highest quality beans), espresso extraction is a precise and consistent process. But before we get into the actual extraction, we must first understand the delicate chemistry of coffee.
Unroasted (or green) coffee is typically flavourless and must undergo the ‘Maillard reaction’ before the complex flavours come out. This is a process in which amino acids and reducing sugars interact in the presence of heat and is the reason food goes brown when cooked. Roasted coffee contains over one thousand chemical compounds which contribute to its distinct characteristics like acidity, sweetness and bitterness. It is important to note that coffees from different regions require different roasting profiles due to variation in soil composition, resulting in a wide range of distinct flavours.
When extracting espresso, we must think about the many compounds present in roasted beans. Although it is the best-known bitter compound in coffee, caffeine only makes up about 15% of coffee’s chemical composition. It is the caffeine which binds to adenosine receptors in the brain to produce coffee’s stimulating effect. During extraction, acidic compounds dissolve first, followed by sugars and most of the organic compounds. The last to dissolve are bitter tasting compounds like chlorogenic acid.
A balanced shot of espresso must contain complimentary amounts of all these compounds. Dark roasted coffee (cheaper and lower-quality coffee that must be burned to hide unpalatable characteristics) contains the highest number of bitter compounds and is therefore not used in high-end coffee shops. Light to medium roast coffees are preferred as these are the most flavoursome and can be used to create extraordinarily fruity and sweet espressos.
A shot of espresso is made with about 18g of coffee extracted to 36g of espresso. The water must be between roughly 90-95 degrees Celsius. The process must also happen under 9 bars of pressure, which is about the same as the pressure 90 meters underwater. The high temperature is required to dissolve the desired organic compounds and the pressure ensures that the process takes no more than 35 seconds (as opposed to filter-brewing under gravity, which can take minutes).
This is just a brief overview of the science that goes on behind the bar in a coffee shop and is in no way exhaustive. There are still many more areas to discuss like milk-steaming and filter-brewing but for now let’s just sit back and appreciate the overwhelming complexity of one of the world’s most common beverages.
Dorian Dederko – Science Writer