The luring smell of coffee can drag sleepers from beds and pedestrians into cafés, but this seemingly everyday beverage is actually very chemically complex. The myriad of compounds that affect taste, smell, and texture have been researched greatly in order to produce the extremely popular coffee that so many people wake up to. Food scientists must study the reactions that occur in coffee beans and in the beverage itself, starting from the roasting of the beans and ending with the final concoction.
A thousand volatile compounds have been found in coffee, but only a few dozen were shown to contribute to the smell. They include β-damascenone (which has an aroma like cooked apples), 2-furfurylthiol (sulfury, roasty), 2-isobutyl-3-methoxypyrazine (earthy), guaiacol (spicy), 2,3-butanedione (buttery), and 4-hydroxy-2,5-dimethyl-3(2H)-furanone (caramel-like). These chemicals react in a multitude of chemical reactions, including the Maillard reaction, caramelization, polyphenol degradation, polymerization of carbohydrates, and pyrolysis. The important thing to note here is that these compounds are necessarily volatile so that they can easily evaporate to quickly deliver the attractive aroma.
During roasting of the beans, the added heat input causes the decomposition of many compounds into more reactive subunits. Many of these contribute to the somewhat bitter taste of coffee. The aroma molecules in the beans are susceptible to degradation when exposed to heat, so fresh coffee that is heated for too long can have a noticeably different smell and taste only minutes after initial preparation. Some of the flavoring compounds are shown in the following diagram:
In expresso coffee, there is usually a reddish-brown froth on the surface called crema. These tiny gas bubbles are encased in thin films, and they keep in much of the flavor and aroma. These important substances are actually dispersed in emulsions, a type of solution composition, of tiny oil droplets.
The following diagram displays the optimal extraction time of expresso that will lead to the most aromatic coffee. Happy coffee making!
Author: Jonathan Yu