Did you know that a cup of coffee contains around 1500 chemicals? Many of them with antioxidant properties and health benefits. Like any natural product the coffee is loaded with many chemical structures, and hundreds of these are produced by chemical transformations during the roasting process at temperatures exceeding 400 °F and a pressure of 25 bar.

I don’t think we can deny the fact that the fragrance of freshly brewed coffee in the morning is the wakeup call for “Carpe Diem”.   No wonder why so many people drink it around the world and more than 6 billion Kg of coffee are traded every year, ranking second in value after petroleum.  Coffee is a very reliable friend, the kind of friend who plays an important role in the various activities we perform during the day. The flavor (bitter-acidic balance) and aroma of the coffee is a matter of personal preference, and the science behind it goes way beyond the brewing process. 

The coffee roasting process reminds me of the “Goldie locks storybook” because the roasting temperature has to be “just right” to allow all the thermal and chemical reactions of decarboxylation, dehydration, isomerization and polymerization to occur.  It is worth to mention that due to the unusual thickness of the cell walls of the coffee plant, the internal pressure in which these reactions occur is ~25 bar.  The ideal environmental temperature for a best reaction ratio is anywhere from 383 ^oF (195 ^oC) for a light “cinnamon” roast which is the lightest drinkable roast, to the “Spanish” roast done at 482 ^oF (250 ^oC). The temperature should never exceed 520 ^oF (271 ^oC) because this would destroy the structure integrity of the cell walls of the bean which as described above are of great importance.  One thing worth to mention here is that the higher the temperature, the more oil comes to the outside of the bean so the dark roast coffee beans are shiny with oil. My personal favorite with the best bitter-acidic balance for my taste is the “American roast” done at 410 ^oF (~210 ^oC) and has a medium light brown color.

There are two kinds of coffee plants: a) Arabiga (Coffea arabiga) (which is the one I am used to drink) is cultivated in South America and Ethiopia and provides a very delicious and mild cup of coffee and b) Robusta coffee (Coffea canephora) which tribes in harsh environment like the African rainforest.   In general Arabiga coffee has less caffeine and higher lipid content than the Robusta variety. 

In my recent trip to El Salvador, I had the chance to visit my cousin’s coffee plantation (Finca Los Amates) at the precise moment where workers were in the tedious process of eliminating by hand the “defective” beans (brown in color) from the desired “green” beans (see small picture above). This process is called “picking” and the very best coffees are actually “triple-picked”.  These dry beans are the result of eliminating the pulp and all the layers from the “red cherry” that comes from the tree and that have been dried in the sun until the internal humidity content reaches ~12% (check out the coffee processing in Wikipedia).  Although the difference between green and brown beans is almost invisible to the “naked eye” it is very important to remove the brown and other defective beans because they have a different chemical content that would provide and undesired taste and aroma to the final product.  Among the bad smelling compounds present in “defective” beans are the “stinky guys”: 2,4,6-trichloroanisole and 2-methylisoborneol which are products from fungi metabolism and they will definitely be responsible for reduction in value of the affected lots.  The selected green beans are called “parchment coffee” and are only covered with a very thin and crumbly skin and are basically ready to be chipped to the strictly controlled roasting facilities.  The green beans do not have the color or aroma of the roasted beans and if you were going to prepare a drink out of them, it would have a grassy and astringent taste, which makes me wonder, how the coffee drink was invented in the first place.  Perhaps people started chewing the beans to get the stimulant effects of caffeine, however the discovery of the roasting experience must have been a mere culinary experiment.

Chemically speaking there is a big difference in the content of a raw “green” bean and the commercially available “roasted bean” which is finally converted into the ground coffee we brew.  The green beans contain caffeine (0.9%-2.4%), lipids (9%-18%) and minerals (3%-4.5%).  These three components remain almost unchanged during the roasting process.  The remaining ingredients: Carbohydrates (including oligo and polysaccharides, ~60%), chlorogenic and phenolic acids (5-8%) carboxylic acids (1-2%), proteins (11-13%) and trigonelline (0.6-1.2%) undergo amazing transformation reactions.

Carbohydrates (sugars) as one can imagine, suffer major transformation during the roasting process and the color light brown to dark brown will depend upon the degree of roasting.  Sugars and nitrogenated compound undergo a condensation called “Maillard reaction” (caramelization) where the unstable intermediates further decomposed into low molecular weight compounds (which provide aroma) and many others that are so reactive that polymerize to products called “melanoidins” which are pigments that impart the roasted coffee its traditional brown color.  Melanoidins are so soluble in water that will for sure end up in your cup of coffee.  Lots of CO₂ and water are released during this process, producing the bean to “puff” up. 

Trigonelline is another component of coffee beans and perhaps the most significant contributor to bitterness.  About 85% of the trigonelline degrades to other compounds during the “medium” roasting process, a “light” roast bean will have more trigonelline but will also be bitterer and the “dark” roast will have less trigonelline but will have a less bitter flavor.  Trigonelline is one of the components that may be beneficial to human health.  It is degraded during the roasting process to several aromatic compounds including nicotinic acid (also known as niacin, PP pellagra preventing and also vitamin B₃) and methylpyridinium, a known anticancer compound.

Finally chlorogenic and phenolic compounds including the chlorogenic, caffeic, p-coumaric and ferrulic acids, have been shown to display potent antioxidant activity in vitro and it has been speculated that they play a protective role in several pathologies in the human body.  Quinic acid and caffeic acids are a result of the hydrolysis of chlorogenic acid.  Quinic acid is slightly sour adding sharp quality, character and complexity to a cup of coffee.

As I sip on a cup of coffee, it just shocked me the fact that I am actually consuming around 1500 chemical compounds.  The nice thing is that although there are many conflicting reports of the effects of coffee on health, the vast majority of scientific publications show a very positive heath benefit associated with drinking coffee.  Raw coffee can be store for up to 2 years, however roasted coffee with its wonderful chemical compounds (including antioxidants) are somewhat unstable to air and moisture and that is why is recommended to grind, brew, and consume the coffee quickly in order to obtain the best flavor.

There is so much more to learn about of coffee but these information at least makes me appreciate even more the sensorial properties (vision, touch, taste and olfaction) and chemistry behind a good cup of coffee.