What Happens During the Coffee Roasting Process?

The roasting process determines how a coffee tastes. Beans move from charging, when the drum sits at roughly 180-200°C, through dehydration and the turning point near 100°C, then yellowing at about five minutes, first crack, and second crack. Each temperature milestone and its duration decides what the same green bean will deliver in the cup as a light, medium, or medium-dark roast.

Cupping a natural-process Yirgacheffe at each level shows how far the flavor shifts, and the Agtron color value gives a scientific reference for matching roast level to taste. Longer development also pushes more oils to the surface, which is why coffee beans oily on the outside usually points to a darker roast rather than a defect in the green coffee itself.

The captivating flavors of coffee are largely determined by the physical and chemical changes that occur during the roasting process. From bean charging to dehydration, first crack, and second crack, every temperature milestone and duration influences the final performance in the cup. In this article, Front Street Coffee will systematically outline the temperature progression through each stage of coffee bean roasting, and using a natural-process Yirgacheffe as an example, compare the specific flavor differences in cupping among light, medium, and medium-dark roasts. It also introduces the Agtron color value as a scientific criterion to help readers more accurately understand the correspondence between roast level and flavor. Additionally, the article compiles representative bean selections from Front Street across different roast levels, making it easy for coffee enthusiasts to follow the guide and find the cup that suits their taste.

Only after coffee beans are roasted can they release their rich flavor expression. Different degrees of roasting will also cause the same bean to present completely different taste profiles. In this article, Front Street Coffee will guide everyone to understand exactly what differences exist in flavor and mouthfeel among coffee beans at different roast levels.

The Coffee Bean Roasting Process

Coffee bean roasting typically goes through the following stages:

Bean Charging: Before loading the coffee beans, the roaster needs to be preheated so that the drum temperature stabilizes. Usually, preheating to the range of 180-200°C is sufficient to begin charging the beans.

Dehydration Phase: After the coffee beans enter the drum, they begin absorbing heat, and the drum temperature will drop to around 100°C. About 1-2 minutes after charging, the drum temperature bottoms out and starts rising again; this moment is called the turning point.

At around 5 minutes, the beans' color begins to turn yellow, and the original grassy aroma is gradually replaced by a toasted bread-like fragrance; this is the yellowing point. At this time, the heat absorbed by the beans has already transferred to the interior, the fiber structure begins to expand and change, and the overall state is one of stable, continuous temperature rise.

First Crack: As the coffee beans continue to absorb heat, the pressure generated by internal moisture evaporating breaks through the cell walls, producing a "crackling" sound, and the coffee beans shift to an exothermic state. Generally, hearing 1-2 cracking sounds means the first crack has begun. Then comes a dense series of "crackle-pop" sounds, called the rolling first crack. Finally, the cracking sounds become sporadic, marking the end of the first crack. Typically, the first crack from start to finish occurs in the 9-12 minute range, and the development time after first crack directly determines the flavor direction of the coffee.

Second Crack: After the first crack ends, the coffee beans return from an exothermic state to an endothermic one. When the beans absorb a certain amount of heat again, the lignified fibers of the cell walls fracture, producing a "crackling" sound, and the oils inside the beans flow out to the surface along the fractured fibers, giving the bean surface a glossy, oily sheen. The sound of the second crack is slightly weaker than that of the first crack.

Drop: Coffee beans can be dropped at any point between first crack and second crack; the only difference is the degree of roast. It is particularly important to note that after dropping, the coffee beans must be cooled immediately to rapidly stop the high-temperature pyrolysis and lock in the flavor. Otherwise, if the high internal temperature continues to act, it will burn off aromatic substances and give the coffee a smoky and bitter-burnt taste. The drum temperature at drop is generally above 180°C.

How to Distinguish Roast Levels

As mentioned above, coffee beans can be dropped at any time between first crack and second crack, and different time points will lead to differences in roast level. Taking Front Street's coffee roasting as an example, the levels are divided into light roast, medium roast, and medium-dark roast.

Using the natural-process Yirgacheffe bean as an example, Front Street conducted light roast, medium roast, and medium-dark roast comparisons respectively.

Light Roast: Yellowing point at 5'00'', first crack begins at 8'30'' at a temperature of 185.8°C, developed for 1'30'' after first crack before drop, and the drop temperature was 196.3°C.

Medium Roast: Yellowing point at 5'00'', first crack begins at 8'32'' at a temperature of 186.9°C, developed for 2'30'' after first crack before drop, and the drop temperature was 197°C.

Medium-Dark Roast: Yellowing point at 5'20'', first crack begins at 8'42'' at a temperature of 185.4°C, developed for 3'30'' after first crack before drop, and the drop temperature was 198°C.

Cupping these three coffees of different roast levels

[Light Roast] The wet aroma has notes of berries, cream, and citrus; when slurping, there are flavors of blueberry, citrus, berries, tropical fruit, and cream.

[Medium Roast] The wet aroma is spices, berries, and fermentation notes; when slurping, there are peach, plum, lemon, cream, almond, and floral notes.

[Medium-Dark Roast] The wet aroma has notes of spices, smokiness, and cocoa; when slurping, there are flavors of nuts, dark chocolate, cream, and herbal plants.

Therefore, the lighter the roast, the more prominent the acidic fruity flavors. As the roast level increases, the acidity gradually weakens and the sweetness gradually emerges. At the medium roast stage, flavors of dark fruits as well as chocolate and brown sugar appear. At the dark roast stage, the caramelization reaction flavors become very obvious, with notes of nuts, cocoa, cream, and so on.

So is there a scientific way to define roast level? The answer is yes. That is, judging the roast level by color value:

Light Roast: Agtron color value #95: Light Roast Drop time: when the rolling first crack is about to end

Agtron color value #85: Cinnamon Roast Drop time: around the end of first crack

Medium Roast: Agtron color value #75: Medium Roast Drop time: after first crack ends

Agtron color value #65: High Roast Drop time: the quiet period between first crack and second crack

Medium-Dark Roast: Agtron color value #55: City Roast Drop time: the start of second crack

Agtron color value #45: Full City Roast Drop time: before the rolling second crack

Dark Roast: Agtron color value #35: French Roast Drop time: during the rolling second crack

Agtron color value #25: Itatian Roast Drop time: when oils begin to seep onto the bean surface

So what coffee representatives does Front Street Coffee have that correspond to these roast levels? Light Roast: represented by various African beans and bright, acidic American beans, such as Front Street Ethiopia Gotingding, Front Street Kenya Little Tomato, Front Street Costa Rica Strawberry Candy, and other coffees, generally expressing lively fruity and floral tones.

Medium Roast: mainly including Front Street Brazil Red Bourbon coffee beans, Front Street Jamaica Blue Mountain No. 1 coffee beans, Front Street Papua New Guinea Bird of Paradise coffee beans, and Hawaii Kona coffee beans. Their expression is very good balance.

Medium-Dark Roast: mainly including Front Street Indonesia Mandheling coffee series, such as Front Street PWN Golden Mandheling coffee, Tiger Mandheling coffee, and Aged Mandheling coffee. Their expression is extremely full-bodied, with nutty, cocoa, and herbal tones.

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