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A Brief Tour of Coffee’s Chemical Composition
Every day, millions of people around the world start their day religiously with a morning cup of coffee. Although today we easily recognize coffee in its drinking form, it was not always this way in the beginning. Throughout history, coffee has undergone many physical changes, while it served as a source of energy when nomadic tribes combined coffee beans with animal fat as an early form of energy wood. Later it was consumed as tea, then wine, and finally to the drink we have come to recognize today. From the beginning, coffee has always been a product of great mystery, discovered by chance in the jungles of Abyssinia (Ethiopia) and consumed in its native cherry form, then later, passed through fire to change its chemical state in important. And although coffee has existed for thousands of years, it was only in the last half century or so that scientists have been able to truly identify and understand what exactly is in this mysterious bean. To date scientists have identified more than 1,000 compounds in coffee, which when compared to products such as chocolate wine has a few hundred, pales in comparison to that of coffee. Fortunately through advances in technology, much of coffee’s chemical structure has been uncovered and we have a better perspective on the chemistry involved in this mysterious bean.
For many, drinking coffee is simply a delivery medium for a strong alkaloid that has come to be identified as caffeine or technically as 1,3,7 – trimethylxanthine. Although caffeine is associated with coffee, its production within the plant kingdom is not exclusive but is found throughout many other plant life forms. Mate, for example, which is traditionally eaten in parts of Uruguay and Argentina, contains less than one percent by weight. However, tea leaves (Camellia sinesis) which originated in China, have almost three times the concentration of caffeine than Arabica, with the Brazilian mate almost twice that of robusta coffee. Turns out that Mother Nature is quite generous when it comes to distributing coffee between government plants. But for humans, caffeine is very unique. So we are the only living forms on Earth that readily seek out caffeine for its stimulating and psychological effects. For all other life forms, caffeine is a strong poison capable of sterilization, phytotoxicity and antifungal properties. As such scientists believe that caffeine, with its strong bitter taste, has been present as a primitive defense mechanism in coffee ensuring its survival in the wild for thousands of years. It is not surprising then, that the caffeine content of the more “robust” Robusta variety is almost double that of the more delicate Arabica. The belief is that as the insects attack the coffee cherry, the bitter taste of the caffeine deters them, and they move on to the next fruit. Since Arabica is usually grown at higher altitudes than Robusta, where the attack of insects is less, Arabica tends to produce less caffeine.
Fat production and its subsequent survival after the roasting process play an important role in the overall coffee quality. In general, most of the lipids are in the form of a coffee wax and are found within the endosperm (bean) of the cherry, with a small portion left on the outside of the coffee wax. In the same way, scientists have examined and discovered that many of the chemicals used to make coffee oil are very similar to those of cooking oils. As such, much of the fat content in coffee does not change and is stable even at the high temperatures associated with roasting. In the green form both Arabica and Robusta coffee have a total of 15-17% and 10-11.5%, respectively. But because Arabica has about 60% more lipids than Robusta, many believe that this big difference is one reason responsible for the quality difference between the two species. Thus, the claim was confirmed, until French scientists discovered a direct correlation between the fat content and the quality of the cup as a whole. It turns out that as the fat content increases within the bean, so does the overall quality of the cup. It is a very plausible explanation when one considers that most of the major flavor compounds in coffee are also fat soluble.
Carbohydrates make up roughly fifty percent of the total dry weight of coffee by composition. After roasting, the carbohydrates remaining in the cup contribute to the mouthfeel or body, with some studies suggesting that they are also responsible for the quality of the foam common in espresso drinks. Although there are many different types of carbohydrates in coffee, perhaps the most important is that of sucrose. Sucrose, or more commonly known as table sugar, is 6-9% in Arabica with a smaller amount (3-7%) found in Robusta coffee. During burning, sucrose is readily degraded and studies have shown that up to 97% of the original sucrose content is lost even in light burning. Its effect during burning is great with a large proportion of available carbohydrates that participate in Maillard and many other secondary reactions. One class of important byproducts created during combustion are those of organic acids. In its native green form, coffee contains negligible amounts of formic, acetic and lactic acid. Although once roasted, there is a significant increase in aliphatic acid production, together with a similar increase in coffee acidity. Since acidity plays an important role in evaluating quality, it is not surprising why higher levels of subjective acidity are often found in Arabica coffee than Robusta, due in part to a higher sucrose concentration. Coincidentally, last year Brazilian scientists identified a single gene, sucrose synthase, which controls sucrose production in plants and may hold the key to producing superior coffee for years to come.
The protein content for both green Arabica and Robusta coffee varies between 10-13% and exists as free or bound proteins within the coffee matrix. Although the exact concentrations may vary, there are a number of factors that can affect free protein content, including improper storage which can increase free protein levels and lead to adverse effects on quality. During digestion, proteins combine with carbohydrates in what is the most important reaction for all processed foods – the Maillard taste. This set of reactions, discovered by a French scientist in 1910, is largely responsible for transforming the handful of compounds found in green coffee into the complex matrix that coffee is today. As temperatures reach 150C (302F), the Maillard reaction catalyzes the free proteins in the coffee to combine with reducing sugars, ultimately leading to the formation of hundreds of essential aromatic compounds. Of these, pyrazines and pyridines have the greatest aromatic contribution and are responsible for the distinct corn/nutty aromas found in coffee. The reaction also leads to the polymetric formation of brown pigment. melanoidins – the compounds responsible for coffee’s color.
Incidentally, this is the same reaction system that gives rise to the delicious aromas produced when baking a loaf of bread or grilling a steak. Although many of the products created during the Maillard reaction are beneficial to coffee, in other agricultural products, a set of browning reactions can be very detrimental to quality. In the cup, proteins also play a role in taste by forming secondary compounds during the roasting process. It turns out that most of the coffee’s “bitterness” is not only because of the caffeine, but rather the bitterness of the compounds produced during the Maillard taste. Caffeine, as bitter as it is, accounts for only 10-20% of coffee’s total bitterness.
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