Before coffee beans are brewed, they go through several processes—processing, drying, roasting, and grinding. Among these, roasting is perhaps the most critical. Roasting is the essential step that brings out the aroma and flavor of coffee.
Coffee roasting is the heating of green coffee beans to a specific temperature and time in order to bring out the desired aroma and flavor. Green beans smell green-earthy. But roasting triggers chemical changes in the beans, which results in the development of distinct flavors and aromas. The length and temperature of the roasting process can have a major impact on the final flavor and aroma of the coffee.
In industrial-scale, coffee beans are roasted using fast methods, usually at 392°F (200°C) for less than 10 minutes or the so-called “flash” methods, which is done at 1,472°F (800°C) for 90 seconds. These techniques do not allow the aromas and flavors to develop.
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Let’s further see how these desirable changes occur during roasting of coffee.
THE SCIENCE BEHIND ROASTING COFFEE BEANS
The aroma and flavor of coffee develop during the roasting of the green coffee beans. Roasting coffee beans is a complex chemical process involving two major processes: the Maillard reaction and caramelization. In the Maillard process, chemical reactions between amino acids and reducing sugars in beans occur. This reaction generates a variety of taste compounds such as caramel, hazelnut, and chocolate flavors. During caramelization, the sugars in the coffee beans break down into smaller molecules. The time of the roasting process and the temperature employed can have a significant impact on the balance of the flavor components, determining the final flavor and aroma of coffee.
Perhaps, the most significant occurrence between these reactions is the breakdown of chlorogenic acid, which results in a variety of fruity and flowery flavors. In general, more chlorogenic acid is broken down during longer, darker roasts. Some of these compounds undergo further transformations into other compounds, particularly melanoidins that gives coffee its brown color. Along with the formation of melanoidins is the breakdown of volatile aromatic compounds. Caffeic and quinic acid are both bitter tasting compounds that form during roasting. It is one reason why darker roasts are more bitter than lighter roasts.
Most other organic acids, such as malic and citric acid, achieve their maximum concentration with only a light roast and then gradually diminish. This explains why coffee beans with a fast, light roast have a higher level of acidity.
Compared to roasted cocoa beans, roasted coffee beans contain more compounds. Furans, thiophenes, pyrroles, thiazoles, oxazoles, and phenols are compounds that contribute to the distinctive flavor of coffee. More than a thousand molecules that contribute to the flavor and aroma of coffee have been found in recent studies. The most significant of these include ketones, furans, pyrazines, pyrroles, and pyrazines.
TYPES OF COFFEE ROAST
Light, medium, medium-dark, and dark roast are the four types of coffee roast. The length and temperature of the roasting process, which determine the coffee’s flavor, aroma, and strength, are what differentiate them.
Light roast
Light roasted coffee beans are roasted for a shorter period of time than medium or dark roast coffee, often 5 to 7 minutes at temperatures ranging from 356°F (180°C) to 205°C (401°F). It’s important to note that these time and temperature limits may vary depending on the coffee roaster’s equipment and personal preferences.
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The low roasting time of the beans contributes to the lighter hue. These beans have a milder flavor than beans that have been roasted for a longer period of time. The roasting process permits the bean’s oils to rise to the surface. However, because light roast beans are not roasted for an extended period of time, this does not occur. Lightly roasted beans have a more acidic flavor profile and less of a “roasted” flavor profile than heavier roasts. Light roast coffee is frequently used in pour-over or drip coffee brewing techniques because of the lighter flavor profile produced during roasting of coffee beans.
Medium roast
Medium roast is derived from medium brown coffee beans. These beans are roasted for a longer period of time than light roast beans, typically between 8-10 minutes at a temperature between 410°F (210°C) to 428°F (220°C). This results in a richer flavor, but less acidity than light roast. Medium roasted coffee offers more caramelization of the sugars within the bean, and its flavor profile has more substance than lighter roasted beans.
Most coffee drinkers find medium roast coffee to possess the peak sweetness.
Medium roast coffee is distinguished by its balanced flavor profile. It includes notes of chocolate and caramel, as well as a moderate level of acidity. It is typically medium brown in color with a slightly oily surface and a relatively smooth finish. It’s a popular choice among coffee enthusiasts who want a well-rounded, tasty cup of coffee that’s neither too strong nor too mild.
This is the most common type of roast in the United States.
Medium-dark roast
The roasting time for medium-dark roast coffee varies. This depends on various factors, including the type of coffee beans used, the roasting equipment used, and the desired flavor profile. Medium-dark roast coffee is roasted for a longer period of time than medium roast, but not as long as dark roast. Roasting time for medium-dark roast coffee is normally between 10-12 minutes at 437°F (225°C) to 446°F (230°C).
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Medium-dark roast is somewhere between darkened and black beans in the dark roast category and non-oily beans in the medium roast category. These beans have no oils on the outside because they were not roasted long enough to release the oils. However, these beans contain a tinge of the bitterness that is more common in dark roasts.
Depending on the origin of the beans, medium-dark roast coffee has a rich, chocolaty flavor with traces of nuts and caramel. It may also have some fruit or floral overtones. Medium-dark roast coffee is for drinkers who desire a robust, powerful flavor without the overwhelming bitterness of a dark roast.
Dark roast
Dark roast coffee takes longer to roast than light and medium roasts, often 12-15 minutes at a temperature between 464°F (240°C) and 482°F (250°C). These temperatures allow the release of oils within the beans and rise to the surface. For this reason, dark roasted beans are black in color and have a lustrous and an oily surface. Furthermore, the coffee beans are more bitter. There is a lot of variability in the dark roast bean category; some beans can be pure black, while others can simply be past the medium-dark roast range.
Darker roasted beans have a deeper roasted flavor that leaves less of the bean’s original traits left. Some intensely roasted beans can even taste scorched to an expert palate. People who want a strong, bold coffee with noticeable bitterness and low acidity frequently prefer dark roast coffee. It is frequently used in espresso blends as well because of the robust flavor and oils that contribute to a thick and creamy espresso shot.
The particular time and temperature ranges, as with other roast kinds, might vary based on the roaster’s equipment and tastes.
References:
J. Frisks (2016). Coffee: Everything You Ever Wanted to Know About Coffee. CreateSpace Independent Publishing.
J. deMan, J. Finley, W. Jeffrey Hurst, C. Y. Lee (2018). Principles of Food Chemistry (4th edition). Springer.
S. Racineux, C. Tran (2018). Coffee Isn’t Rocket Science: A Quick and Easy Guide to Buying, Brewing, Serving, Roasting, and Tasting Coffee. Black Dog Publishing.
H. Belitz, W. Grosch, P. Schieberle (2009). Food Chemistry (4th Edition). Springer
