Specialty Coffee Science: Understanding Coffee Roasting Levels and How to Read Roasting Percentages

Coffee Bean Roasting Knowledge: From Green Beans to Perfect Roast

We often talk about coffee bean brewing, but today FrontStreet Coffee wants to share knowledge about coffee bean roasting. We know that the process from green beans to roasted beans is the roasting process, which includes light roast, medium roast, and dark roast. So how should different coffee beans be roasted, and how do we distinguish between light, medium, and dark roasts?

The Art and Science of Coffee Roasting

Coffee beans from different origins and regions each have their own local flavor characteristics. To highlight these unique flavor profiles, it requires the skilled craftsmanship of a roaster. FrontStreet Coffee's roasters determine the degree of roast based on the coffee bean's country of origin, region, variety, and flavor characteristics. Of course, when FrontStreet Coffee's roasters receive a new bean, they will roast three batches according to the general regional roasting curves and conduct cupping tests to select the most suitable curve for that particular coffee bean.

FrontStreet Coffee's roasters must keep detailed records for every batch of coffee beans, including drop-in temperature, bean weight, temperature recovery point, drum temperature per minute, dehydration point, first crack point, first crack time, discharge time, dehydration rate, temperature rise rate, and more (the more detailed, the better). Since subtle changes in temperature and heat during the roasting process can alter the beans' flavor, and different beans have different characteristics, and roasting is a rapid operation that must be completed in a short time, it requires accurate prediction of the roasting process and curve correction through cupping tests.

Coffee green beans have an earthy smell before roasting, but after roasting, they emit a rich aroma. The degree of roast (roasting degree) also affects coffee flavor and caffeine content. The lighter the roast, the stronger the fruit acidity and the higher the caffeine content. The darker the roast, the milder the acidity, the more prominent the sweet-bitter taste, and the lower the caffeine content.

Coffee roasting refers to the process of heating green beans to promote a series of physical and chemical reactions inside and outside the coffee beans, generating multiple flavors such as acidity, bitterness, and sweetness during this process, forming body and color, and transforming green beans into dark brown original beans.

The Coffee Roasting Process

FrontStreet Coffee's coffee bean roasting process is mainly divided into the following stages:

Dehydration Stage - Coffee beans turn from green to yellow, with a faint grassy aroma

During the initial stage of dropping beans, the temperature inside the drum will first drop due to the green beans absorbing heat, then rise again at the temperature recovery point. The moisture content of green beans is 8%-14%. As the temperature increases, the free water gradually turns into water vapor, the originally hard coffee beans soften and expand in volume, and the loss of chlorophyll causes the color to change from green to yellow. The color gradually changes from green to yellow or light brown, the silver skin begins to peel off, and you can smell a faint grassy aroma.

Yellowing Stage - Bean color deepens, emitting toasted bread or caramel aroma

After the coffee beans turn yellow, flavor-forming chemical reactions occur—the Maillard reaction and caramelization reaction. During the heating process, the sugars and amino acids in the coffee beans interact to produce complex chemical changes, generating hundreds of aromatic compounds. This process is called the "Maillard reaction," which affects the coffee's aroma and mouthfeel, and the beans turn light brown due to melanoidins. The "caramelization reaction" involves sugars dehydrating into brown caramel and producing aromatic compounds before and after the first crack. The sweetness and bitterness of coffee come from this, and the bean color becomes darker, allowing the originally tasteless coffee beans to emit rich aromas blending caramel, cocoa, cream, and more. At this point, the heat should be reduced slightly and the airflow increased to avoid the beans roasting too quickly, resulting in beans that are cooked on the outside but raw on the inside.

Flavor Development Stage - Hearing crisp, loud cracking sounds (first crack), the timing for light to medium roast discharge

Caramelization and the Maillard reaction generate large amounts of water vapor and carbon dioxide inside the coffee beans, causing the beans to expand in volume and increase in pressure. When the cell walls can no longer withstand the pressure, they crack—the bean's center line splits, the outer silver skin on the bean surface and the inner silver skin in the middle seam peel off, emitting sounds similar to breaking branches or popcorn. At this point, the coffee beans transition from endothermic to exothermic state, causing the drum temperature to rise rapidly. Heat should be reduced promptly and airflow increased to avoid over-roasting the beans. To present fruity acidity, seize the opportunity to discharge the beans before the first crack ends.

Flavor Development Stage - Cracking sounds become muffled and fine (second crack), the timing for medium to dark roast discharge

After the first crack ends, there will be a one to two-minute rest period. The coffee beans transition from exothermic to endothermic, and the beans continue to expand internally at high temperatures 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 espresso dark roasts, French dark roasts, and Vienna dark roasts are essentially madly testing the boundaries of carbonization. During the second crack, the beans become exothermic again, becoming darker and more oily due to carbonization and oils floating to the surface.

Cooling: Immediately Necessary After Roasting

Once the beans leave the roaster, they must be cooled immediately to avoid the carry-over baking effect causing the beans to become darker than the appropriate roast degree they have already reached. There are three methods for cooling beans:

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

2) Cooling with ambient air.

3) Cooling with refrigerated air.

Changes During Roasting

The changes caused by roasting are very complex. Although scientists continue to research and analyze, the full picture cannot yet be fully understood. The main changes include:

Weight loss: Moisture content drops from about 13% to 1%, with weight loss of approximately 12%-21%. The darker the roast, the greater the weight loss.

Volume expansion: After roasting, coffee bean volume increases by more than 60%. Cell pore enlargement: The cell walls of green beans are tough and the cell pores are closed, making them less prone to deterioration. However, after roasting, the cell walls become very fragile, the cell pores enlarge, and internal substances are easily lost.

Formation of carbon dioxide: High-temperature decomposition causes carbohydrates inside the 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 organizational structure: After roasting, carbohydrates drop dramatically from 58.9% to 38.3%, and acidic substances (fatty acids, tannins, and chlorogenic acids, etc.) drop from 8.0% to 4.9%. Under high-temperature pyrolysis, these substances reorganize and transform into caramel, carbon dioxide, and some volatile substances. Among these, caramel accounts for 25% of roasted bean weight and forms the sweet taste of coffee. Fat, which originally accounted for 16.2% in green beans, increases to 17% after roasting and is the source of body and richness. Caffeine content shows almost no change. The bitterness of dark-roasted coffee does not stem from higher caffeine content.

Coffee Bean Roast Levels

Light roast: The beans change to an appetizing cinnamon color, hence also called cinnamon roast or half-city roast. Acidity dominates the flavor of light roast beans, such as Ethiopian Yirgacheffe coffee.

Medium roast: Coffee beans display an elegant brown color. New Yorkers like to start their day with medium roast coffee beans during breakfast time, adding rich milk and sugar, so this roasting method is also called breakfast roast or city roast. Medium roast can preserve the original flavor of coffee beans while appropriately releasing aromatics, so coffee beans from origins like Blue Mountain, Costa Rica, Honduras, and Panama all use medium roasting.

Dark roast: The darker the color of coffee beans, the sweeter and more mellow the flavor. At this point, when the oils have caramelized, the bitterness gives way to sweetness with endless aftertaste, making it most suitable for brewing strong Espresso coffee, hence also called Italian roast. For example: coffee beans from Brazil and Colombia.

Sometimes you might see coffee beans with darker colors but they are actually medium or even light roasts, while some dark roasts might look like medium roasts. This is actually because some coffee beans take on color more easily, creating what's known as color variation issues. To address this problem, the Specialty Coffee Association of America (SCAA) and food technology pioneer Agtron Inc. established a universally applicable roast degree data system in 1996, known as the famous Agtron test. The measurement principle involves infrared light emitted by an analyzer irradiating roasted coffee powder, with the reflected light being returned and categorized into numerical values for roast depth.

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

According to FrontStreet Coffee's understanding, some roasting manufacturers use Roaster % to indicate roast degree. This percentage is actually 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.

FrontStreet Coffee's Brewing Recommendations:

For freshly roasted coffee beans, FrontStreet Coffee doesn't recommend brewing them immediately because the carbon dioxide inside the beans hasn't been completely released, and the coffee bean flavors haven't fully developed. Therefore, the beans need to go through a resting period. The coffee resting period is about 4-7 days. The coffee beans shipped by FrontStreet Coffee are all roasted within 5 days. FrontStreet Coffee's roasting philosophy is "freshly roasted good coffee," ensuring that every customer who places an order receives the freshest coffee possible. Therefore, when customers receive their coffee, it's at its peak flavor.

For friends who need ground coffee, FrontStreet Coffee kindly reminds you: once coffee beans are ground in advance, there's no need for a resting period, because during transportation, the pressure created by carbon dioxide inside the packaging can also help the coffee flavor become more rounded. So you can brew a cup immediately upon receiving the ground coffee. However, ground coffee needs to be brewed promptly because coffee grounds oxidize quickly when exposed to air, meaning the coffee flavor will dissipate more rapidly and won't be as good. Therefore, FrontStreet Coffee recommends purchasing whole beans and grinding fresh for each brew to better experience the coffee's flavor.

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Important Notice :

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