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The Science Behind Coffee Roasting
Curious about what actually happens during roasting? Here is a scientific explanation of the process.
Published October 15, 2021
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Joachim Estal
Coffee experts@The Coffee Lab
Coffee begins its journey as cherry seeds-turned beans in their hard, green form. They are generally tasteless and smell rather vegetal. Before farms bag these beans and ship them to consumers, they are first transferred to facilities where they experience a process of heating to bring out their aroma, flavor, and solubility– a process known as coffee roasting.
Roasting alters the physical and chemical properties of green coffee. The same or a higher level of sugars, acids, protein, and caffeine is present in unroasted beans compared to roasted ones. However, roasting is mainly the reason why you get the flavor in your cup of coffee every day.
Several chemical reactions are happening within the coffee beans at certain temperatures during roasting. Varied acids, aromatics, and flavor components are produced, changed, or balanced when this happens. And as a result, this will make your coffee achieve the perfect aroma, flavor, body, and acidity.
Maillard Reaction
The Maillard reaction is the chemical reaction between amino acids and reducing sugars present in “brown” food, giving it its distinctive flavor. This happens when we fry dumplings, grill steak, make beer, and of course, roast coffee. It is also a kind of non-enzymatic process which means that external energy is required in order to initiate the reaction, like heat.
People in the food industry call this phenomenon “browning,” which does not translate to the burning of food but the reaction between carbohydrates (sugars) and amino acids once the heat is applied.
In the case of coffee, when roasting reaches the 150-200 degrees Celsius, different carbonyl groups (derived from sugars) and amino acids will interact and form the aromas and flavors. Typical Maillard-induced flavors are meaty, toasty, burnt, nutty, and caramelly.
Caramelization
Both amino acids and sugars are present in green beans. However, this is not entirely the reason that makes your coffee dark brown in color. It is mainly because of caramelization.
Caramelization also takes place when the heat in roasting reaches 170-200 degrees Celsius. And while the Maillard reaction occurs in both amino acids and sugars, caramelization will only involve sugar.
Sucrose, the sugar present in green coffee, does not undergo the Maillard reaction. Instead, it will produce brown-colored, caramelized compounds. At the same time, this will release the aromatic and acidic compounds as well. Roasting your coffee too lightly will not degrade the existing bitter-tasting components.
As the temperature increases and as the roasting continues, other chemical reactions will occur. The brown hue will transition to a darker color since the sucrose and cellulose will then break down into carbon.
The First Crack
At around 205 degrees Celsius, the water present inside the beans will begin to vaporize. This will cause them to physically expand and crack and produce a popping sound. About five percent of the beans’ weight is lost due to water loss at this point in the process. Blonde or light roasts happens after this step.
Pyrolysis
Scientifically, pyrolysis is defined as the thermochemical process of decomposing organic material through heat application. This chemical reaction can happen with a minimal amount or no oxygen at all. During pyrolysis, solids (ash, char), liquids (oils), and gases (carbon dioxide) are produced with different compositions.
In the case of coffee roasting, pyrolysis occurs when the temperature reaches 220 degrees Celsius. By then, carbon dioxide will be released from the beans. This phenomenon is also where we begin to see a medium brown color and oils coming out of the beans. The coffee will also lose about 13% of its weight at this point.
Second Crack
Pyrolysis will still continue to occur as the roasting temperatures reach 225-230 degrees Celsius. And a second crack takes place. This means that the cellulose in each bean cell wall is already beginning to break apart. By this time, the roaster should have already achieved the desired taste and flavors for the coffee.
Final Thoughts
The science of roasting allows our master roasters to achieve many variations with very similar beans. With enough knowledge and skill, consistent results can be achieved in a very predictable manner. However, roasting is also a form of art. The roaster is given the freedom to explore everything that the coffee has to offer. And this is what really translates to the unique experience we get when we take a sip of our favorite coffee.
About the author
Joachim Estal
8-year barista for a world-class coffee chain from the Philippines. Passionate writer and coffee champion. "Coffee has always been one of my biggest love interests in life. And I am here to share it with all of you through this platform."
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