Why are no two cups of coffee ever the same?

Who doesn't like a good cup of coffee? Whether we use it to wake us up or as an excuse to spend a bit of quality time with friends or family, coffee is an indispensable daily drink for many people. But have you ever wondered why a cup of coffee made in a coffee shop tastes so different from coffee made at home, even when the same brand is used?

The answer is because coffee is a unique artisanal drink that we don't buy as a finished product, like we do beer for instance, but rather it is one that we prepare ourselves. And the brewing process has a major influence on the drink's chemical composition, and therefore on its final taste and aroma.

The process of making coffee begins with the harvesting of the beans. Once this has taken place, they must be immediately dried in the sun, the husk removed, and any substandard beans separated to prevent them from spoiling the entire batch.

The coffee is then roasted, which produces a series of chemical reactions and is key to ensuring that it develops good flavour and aroma. The most characteristic of these reactions is caramelisation, also known as the Maillard reaction, which causes the sugars to combine with amino acids, peptides and proteins. The resulting coffee beans not only change colour, from green to brown, but also the number of volatile compounds that they contain triples (to around 700), thus giving it its characteristic aroma. The coffee beans' characteristics will vary according to the temperature and time for which they are roasted. Too high a temperature will produce a more bitter coffee because it leads to the formation of compounds called polyphenols. Conversely, if the temperature is too low, reactions such as the Maillard reaction will not occur properly, and the final product will be more acidic.

Once we have our roasted coffee beans, we can prepare a cup for sampling. First, the coffee needs to be ground and then brewed with hot water to extract all the chemical compounds responsible for its flavour and aroma. This is when the differences emerge between homemade coffee and that produced in a coffee shop. Basically, three factors come into play: coffee tamping, water temperature and the contact time between the water and the coffee. When all three factors combine correctly, the hot water comes into contact with the coffee for just long enough to extract all the chemical compounds and make the perfect cup of coffee. We can also tell if the process has worked well or not by looking at the coffee crema. If it is too light, it means the coffee was not sufficiently well tamped, the temperature was too low, or it was in contact with the water for too short a time. On the other hand, if it's too dark, it means it was over-tamped.

And what if we want a coffee without caffeine? Well, chemistry, once again, has the answer. Generally, the green coffee beans are put in contact with methylene chloride or carbon dioxide, which separates the caffeine from them. They are then washed and dried in the manner already described. To check that the coffee is genuinely decaffeinated, it is analysed by chemists using a technique called chromatography. Want to learn about this technique by doing an experiment at home? Then visit the Repta Experimenta website!

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