Welcome to this month’s issue of coffee science. Last month we briefly discussed the role of two alkaloids: caffeine and trigonelline, and briefly their role in coffee composition. This time we’ll explore some of coffee’s more common components, namely carbohydrates and protein, and discuss how these seemingly ordinary compounds react to form coffee’s alluring aroma.
Carbohydrates make up roughly fifty percent of coffee’s total dry weight by composition. After roasting, remaining carbohydrates in the cup contribute to mouth-feel or body, with some studies suggesting they are also responsible for the quality of the foam common in espresso beverages.
Although there are numerous types of carbohydrates in coffee, perhaps the most important is that of sucrose. Sucrose, or more commonly known as table sugar, makes up six to nine percent in Arabica with a slightly less (three to seven percent) amount contained in Robusta coffee. During roasting, sucrose is readily decomposed, and studies have shown that up to 97 percent of the initial sucrose content is lost even in light roasts. Its role during roasting is enormous with a large portion of the available carbohydrates participating in the Maillard and numerous others secondary reactions. One class of important byproducts created during roasting are those of organic acids. In its native green form, coffee contains negligible amounts of formic, acetic, and lactic acid. Though once roasted, there is an exponential increase in acid production, along with a paralleled increase in coffee acidity. Since acidity plays an important role in assessing quality, it’s no surprise why we see typically higher levels of perceived acidity in Arabica coffee than Robusta, due in part, to its higher sucrose concentration.
Protein content for both green Arabica and Robusta coffee varies between 10–13 percent and exists as free or bound proteins within the coffee matrix. Although actual concentrations vary within the bean, there are a number of factors that affect protein content. Factors such as level of maturation, variety, and storage conditions all have an effect on protein byproducts during roasting.
During roasting, proteins combine with carbohydrates in what is perhaps the most important reaction for all thermally processed foods – the Maillard Reaction. This set of reactions, discovered by a French chemist in 1910, is what is largely responsible for transforming the mere handful of compounds found in green coffee to the complex matrix that coffee is today.
As temperatures reach 150oC (302oF), the Maillard reaction react free proteins with sugars ultimately leading to the formation of hundreds of important aromatic compounds. Furans, for example, impart sweet, caramel-like aromas, while more complex molecules, such as pyrazines, impart more nuttier complex flavor notes. Ketones, or smaller molecules, also play a role with diacetyl (butanedione) imparting buttery-butterscotch notes reminiscent to fresh popcorn. There are literally hundreds and hundreds of aromatic compounds being created during the roasting process, each contributing a small portion to coffee’s complex aroma.
If you’ve ever wondered why coffee is brown in color, it’s due to the very same reaction that creates flavor. During roasting large molecular weight compounds combine with proteins to form complex brown colored melanoidins, which ultimately give coffee its characteristic color. Until recently, very little was known of these compounds, but over the years science has elucidated many of their structural properties associated with them. Perhaps the most promising is that many of these compounds have potent antioxidant, antimicrobial, and anti-inflammatory properties associated with them. This is great news considering that coffee is the second to third most popular beverage consumed in the world, just after water and tea. It’s just another reason to enjoy another cup of coffee at home or your favorite café. Cheers!
Joseph A. Rivera holds a degree in food chemistry and was formerly the Director of Science & Technology at the Specialty Coffee Association of America (SCAA). He’s the creator of coffeechemistry.com and newly developed Coffee Science Certificate (CSC) program. He can be reached at firstname.lastname@example.org