The transformation of coffee beans from green to roasted depends on roasting. The same bean, roasted to different degrees, will have completely different acidity, bitterness, sweetness, body, and aroma profiles. Today, Front Street Coffee systematically organizes the core knowledge of coffee roasting: what happens during the dehydration phase, browning phase, and flavor development phase; how to judge the drop time at first crack and second crack; which origins and brewing methods suit light, medium, and dark roasts; it will also explain why bean surface color can be misleading, and how to interpret Agtron values and Roaster % roasting percentages. At the end, Front Street's bean resting and grinding recommendations are included to help you get the best flavor after receiving the beans.

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Usually we talk more about how to brew coffee beans, but today Front Street takes a different angle and focuses specifically on coffee bean roasting. The transformation of green beans into roasted beans depends on the roasting process, and the results of roasting are usually divided into light roast, medium roast, and dark roast. So, what kind of beans should use what degree of roast, and what are the criteria for dividing light, medium, and dark roasts?

Coffee beans from every producing country and region carry the unique flavor imprint of their locale. To fully release these imprints, the roaster's operation is indispensable. Front Street roasters determine the roast level by comprehensively considering the coffee bean's producing country, region, variety, and flavor characteristics. Of course, when Front Street roasters receive a new bean, they first refer to the roasting curves of the major producing regions to make three curves and roast separately, then use cupping to select the one that best suits that bean.

Every time Front Street roasters roast a batch of beans, they record in detail: charge temperature, charge weight, turning point, boiler temperature per minute, dehydration point, first crack point, first crack time, drop time, dehydration rate, rate of rise, etc. The more detailed the record, the better. During roasting, subtle fluctuations in temperature and heat will change the taste of the beans, and different beans each have their own temperament. In addition, roasting itself is an operation completed quickly in a short time, so the process must be accurately predicted, and cupping must be used to calibrate the curve.

Before roasting, green coffee beans carry an earthy, raw smell; only after roasting do they give off a rich aroma. The degree of roasting affects both coffee flavor and caffeine content. The lighter the roast, the more prominent the fruit acidity and the higher the caffeine content; the darker the roast, the weaker the acidity, the more obvious the sweet-bitterness, and the lower the caffeine content.

So-called roasting means heating green beans to promote a series of physical and chemical reactions inside and outside the beans. In this process, various tastes such as acidity, bitterness, and sweetness are generated, body and color tone are formed, and the green beans are transformed into dark brown roasted beans.

The process of Front Street roasting coffee beans is mainly divided into the following stages:

Dehydration phase — coffee beans turn from green to yellow, with a faint grassy smell

At the beginning of charging, the temperature inside the roaster first drops because the green beans absorb heat, then rises again at the turning point. Green beans have a moisture content of 8%–14%. As the temperature rises, free water gradually becomes steam, the originally hard coffee beans soften and expand in volume, and the loss of chlorophyll turns the color from green to yellow. The color gradually changes from green to yellow or light brown, the silver skin begins to fall off, and a faint grassy smell can be detected.

Browning phase — the bean color becomes darker and darker, giving off the aroma of toast or caramel

After coffee beans turn yellow, chemical reactions that form flavor occur—the Maillard reaction and caramelization. During heating, the sugars and amino acids in coffee beans interact in complex chemical changes, producing hundreds of aromatic compounds. This process is called the "Maillard reaction," and it affects coffee's aroma and mouthfeel; the beans also turn light brown due to melanoidins. "Caramelization" refers to sugars dehydrating into brown caramel around first crack and producing aromatic compounds. Coffee's sweetness and bitterness come from here, the bean color also becomes darker, and the originally flavorless coffee beans release rich aromas blending caramel, cocoa, cream, and more. At this time, the heat should be reduced a little and the airflow increased a little to avoid roasting the beans too quickly, which can cause them to be cooked outside but raw inside.

Flavor development phase — a crisp, loud cracking sound is heard (first crack), the drop time for light-medium roasts

Caramelization and the Maillard reaction generate large amounts of steam and carbon dioxide inside the coffee beans, causing the beans to expand and pressure to increase. When the cell walls can no longer withstand the pressure, they crack—the center line of the bean splits open, the outer silver skin on the bean surface and the inner silver skin in the center seam fall off, and a sound like snapping twigs or popping popcorn is emitted. At this point, the coffee beans shift from absorbing heat to releasing heat, and the roaster temperature rises rapidly. The heat should be lowered in time and the airflow increased to avoid over-roasting the beans; if you want to present fruity acidity, you should seize the moment and drop the beans before first crack ends.

Flavor development phase — the cracking sound is duller and fainter (second crack), the drop time for medium-dark roasts

After first crack ends, there is a resting period of one to two minutes. The coffee beans change from releasing heat to absorbing heat, and the inside of the beans continues to expand under high temperature until the cell walls can no longer withstand the high temperature and pressure, producing the quieter second crack. When the second crack sounds, it indicates that the beans have entered the dark roast range. Various Italian dark roasts, French dark roasts, and Viennese dark roasts are all essentially wildly testing the edge of carbonization. At second crack, the beans again switch to releasing heat, and due to carbonization plus oils emerging on the surface, they become even blacker and more glossy.

Cooling: Once the beans leave the roaster, they must be cooled immediately to avoid the self-roasting effect causing the beans to become darker than the appropriate roast level already reached. There are three ways to cool the beans:

1) Water cooling: spray water onto the freshly roasted hot beans to cool them. Because coffee easily absorbs water, this process significantly increases the beans' density.

2) Cool with ordinary air.

3) Cool with refrigerated air.

The changes brought by roasting are very complex. Although scientists continue to research and analyze them, the full picture still cannot be fully understood. Broadly speaking, the changes are as follows:

Weight loss: moisture content drops from about 13% to 1%, with weight loss of roughly 12%–21%. The darker the roast, the greater the weight loss.

Volume expansion: after roasting, coffee bean volume increases by more than 60%. Cell pores enlarge: green bean cell walls are hard and the cell cavities are closed, so they are not easily deteriorated. But after roasting, the cell walls become very fragile and the cell pores enlarge, making it very easy for internal substances to be lost.

Formation of carbon dioxide: high-temperature decomposition causes the carbohydrates inside coffee beans to break down and combine with other substances to form large amounts of carbon dioxide, which remains inside the coffee beans.

Changes in tissue structure: after roasting, carbohydrates drop sharply from 58.9% to 38.3%, and acidic substances (fatty acids, tannic acid, chlorogenic acid, etc.) drop from 8.0% to 4.9%. Under high-temperature cracking, these substances reorganize and transform into caramel, carbon dioxide, and some volatile substances. Among them, caramel accounts for 25% of roasted bean mass and forms coffee's sweetness. Fats originally accounted for 16.2% of green beans and increase to 17% after roasting, and are the source of mellow taste and body. Caffeine content barely changes. The bitterness of dark-roasted coffee does not come from having more caffeine.

Coffee bean roast levels

Light roast — the bean color turns an appealing cinnamon color, so it is also called cinnamon roast or half-city roast. Acidity dominates the flavor of light-roasted beans, such as Ethiopian Yirgacheffe coffee.

Medium roast — coffee beans present an elegant brown. New Yorkers like to use medium-roasted coffee beans at breakfast, with rich milk and sugar, to open each day, so this roasting method is also called breakfast roast or city roast. Medium roast preserves the original taste of coffee beans while also moderately releasing aroma, so coffee beans from producing countries such as Blue Mountain, Costa Rica, Honduras, and Panama all use medium roasting.

Dark roast — the darker the coffee bean color, the sweeter, more fragrant, and mellower the flavor. At this point, when the oils have turned into caramel, bitterness gives way to sweetness and the aftertaste is endless, making it most suitable for brewing strong Espresso, so it is also called Italian roasting. For example: coffee beans from Brazil and Colombia.

Sometimes the coffee beans you see are relatively dark but are actually medium roast or even light roast, and some are dark roast but look like medium roast. This is because some coffee beans absorb color more easily, creating the so-called color difference problem. To address this issue, the Specialty Coffee Association of America (SCAA) and the American food technology pioneer Agtron Company established in 1996 a universally applicable set of roast degree data, known as the famous Agtron test. The measurement principle is to irradiate roasted coffee powder with infrared rays emitted by an analyzer, then reflect back and summarize the values for light and dark roasts.

The darker the roast, the more obvious the caramelization (or carbonization); the blacker the bean surface, the weaker the reflected light, and the lower the measured value. Conversely, the lighter the roast, the lower the carbonization; the less black the bean surface, the stronger the reflected light, and the higher the measured value. In other words, the Agtron Number is inversely proportional to roast degree; the higher the value, the lighter the roast, and the lower the value, the darker the roast.

According to Front Street's understanding, some roasting companies use Roaster % to indicate roast degree. In fact, this percentage % is similar to Agtron. For example, Roaster 95% indicates light roast, Roaster 90% indicates cinnamon roast, Roaster 85% indicates medium roast, Roaster 80% indicates medium-dark roast, and Roaster 70% indicates dark roast.

Front Street's coffee brewing recommendations:

For coffee beans just after roasting, Front Street does not recommend brewing them immediately, because the carbon dioxide inside the coffee beans has not yet been fully released, and the flavor of the coffee beans has not yet been fully expressed. Therefore, the coffee beans need to be rested. The resting period for coffee is about 4–7 days. The coffee beans shipped by Front Street Coffee (FrontStreet Coffee) are all within 5 days of the roast date. Front Street's roasting principle is "freshly roasted good coffee," so that every customer who places an order receives the freshest coffee. Therefore, when customers receive it, it is exactly at its best flavor.

For friends who need ground coffee, Front Street warmly reminds you: if the coffee beans have already been ground in advance, there is no need to rest the beans, because during transportation the pressure generated by carbon dioxide inside the package can also make the coffee flavor rounder, so once you receive the ground coffee you can immediately brew a cup and drink it. However, ground coffee needs to be brewed promptly, because after ground coffee contacts air it oxidizes relatively quickly, meaning the coffee's flavor will dissipate relatively quickly and the coffee will not taste as good. Therefore, Front Street recommends buying whole beans and grinding fresh before brewing, so you can better taste the coffee's flavor.

The above content is compiled by CoffeeHunters, a coffee news website.