With the same bag of green beans, a roaster can produce a cup of coffee that’s tangy like fruit tea or a bitter, burnt cup, differing by less than two minutes at the end of the batch. The roasting level is the variable: it determines how much acid remains, how intense the caramel sweetness is, and whether the beans release oil to the surface. This article will cover each temperature threshold, the two crucial crackles, how to read the Agtron scale, and suggest roast levels suitable for your brewing method.
1. What are Coffee Roasting Levels and How Are They Measured Using the Agtron Scale?
Roasting levels describe the degree of transformation a bean undergoes in the oven, often named after the finished product’s color: light, medium, medium-dark, and dark. The problem is that visual perception of color is very subjective, so the coffee industry uses a reflectance spectrophotometer to produce the Agtron index, also known as the Gourmet color scale. The higher the index, the lighter the bean: around 90–75 corresponds to a light roast, 65–55 to a medium roast, 45–35 to a medium-dark, and below 30 to a very dark roast. The Specialty Coffee Association standardizes this color range using a set of sample plates so that all parties speak the same language during transactions. A technical detail often overlooked is that both whole beans and ground coffee must be measured: a large difference between the two indices indicates that the outer shell is overcooked, a sign of a roast that was heated too quickly.
2. What Happens Inside the Bean When Roasted
A batch of roasts goes through three consecutive stages. The drying stage, lasting from the moment the beans are added until about 150°C, accounts for about one-third of the total time, its sole purpose being to evaporate most of the water in the beans. The second stage begins when the beans turn yellow then brown, the aroma shifting from fresh grass to toasted bread and then roasted nuts. The third stage, called development, from the first crack until the batch is discharged, largely determines the character of the cup. Simultaneously, the internal pressure of the beans gradually increases to about 20–25 bar due to the water vapor and CO2 gas not escaping quickly enough, causing the cell structure to crack and the bean volume to expand by 50–100%. It is this newly formed porous system that allows water to penetrate and extract the soluble substances when you brew, so beans that haven’t fully expanded always result in a weak and empty cup.
3. The first explosion occurred at approximately 196 degrees Celsius.
The first crack is an audible sound, similar to the snapping of a small stick or the popping of popcorn. It usually occurs around 196°C (the temperature of the grain mass), ranging from 192–200°C depending on the machine, grain size, and initial kernel moisture content. This is caused by the accumulation of water vapor and CO2 exceeding the cell wall’s capacity, causing the grain to crack and burst. From this point onward, the beans are ready to drink; subsequent operations simply involve deciding where to stop. The time from the first crack to the discharge of the batch is called the development time, usually accounting for 15–25% of the total batch; below 15% can result in a harsh sour taste and raw grain smell, while above 25% the fruity aroma fades. Immediately after the crack, the reaction becomes exothermic, with the grains generating their own heat, so the roaster must reduce the heat to prevent the batch from exploding too quickly.
4. The second explosion occurred at approximately 224 degrees Celsius.
If heating continues, the beans enter a second crack around 224°C, fluctuating between 220–230°C. This time, the sound is quieter, crispier, and denser, like the crunching of cereal in milk. The mechanism is also different: it’s no longer steam pushing the cracks, but the cellulose structure of the cell walls begins to break down, releasing oil from the inside to the surface. That’s why beans always have an oily sheen after the second crack. At this temperature range, the flavor compounds associated with the variety and growing region are almost completely broken down, replaced by the aroma of the combustion process itself: dark chocolate, smoke, burnt wood. If the temperature is increased by another 8–10°C, the beans turn black and glossy, the bitter, burnt taste becomes dominant, and the risk of burning in the drum increases. For most quality beans, stopping before or just as the second crack begins is a reasonable limit.
5. Maillard Reactions and Caramelization Create Flavor
Two groups of reactions make up almost all the flavors you taste. The Maillard reaction is between reducing sugars and amino acids, clearly initiated at around 150°C, producing hundreds of aromatic compounds such as pyrazines, furans, and aldehydes – the source of baked goods, roasted nuts, malt aromas, and the characteristic brown color. Caramelization is a separate process involving sugar, requiring higher temperatures, clearly evident from around 170°C and above, resulting in a rich sweetness, caramel, and buttery flavor. Prolonging the Maillard reaction will produce a thick, layered cup of coffee, but if too long, the acidity and fruity notes will fade. Simultaneously, chlorogenic acid gradually breaks down into quinic and caffeic acids, which are the main sources of bitterness, not caffeine, which is quite heat-stable and remains virtually unchanged across roast levels. Therefore, a darker roast does not necessarily mean a cup with more caffeine.
6. Light Roast: Preserves the Character of the Growing Region
Light roasting finishes immediately after the first crack, with a discharge temperature of approximately 196–205°C and an Agtron value of around 75–90. The beans are light brown like cinnamon, with a completely dry surface, free of any oil, and if broken in half, you’ll still see a lighter color in the core. Weight loss after roasting is only 12–14% compared to green beans because most of it is simply evaporated water. This level retains the most organic acids, resulting in a bright, clear acidity, floral, citrus, and tea aromas, and a clean aftertaste—the kind that specialty roasters strive for. However, the body is thin, the sensation is light, and it’s not suitable to drink with condensed milk as it can easily be overwhelmed. Lightly roasted beans are also harder, requiring a good grinder and a higher water temperature, around 93–96°C, to extract all the nutrients; using water that’s too cold will result in a harsh, empty drink.
7. Medium Roast is the most balanced and easy-to-drink roast.
Medium roasts stop at around 210–219°C, a noticeable gap after the first but not yet the second, with Agtron reaching 55–70. The beans are a uniformly brown color, the surface is still dry or only slightly moist, and the weight loss is 14–16%. At this level, the Maillard reaction has run long enough to create a sweet caramel base and milk chocolate flavor, while some fruit acidity remains, resulting in a balanced drink with a balance of sour, sweet, and bitter. This is why medium roasts account for the largest share of the retail market: they appeal to a wide range of tastes, are easy to brew correctly, are less dependent on skill, and tolerate variations in grind size or water temperature. If you’re unsure where to start, buy a 250g bag of medium roast as a benchmark and compare it to other levels around it.
8. Medium-Dark Roast and Dark Roast for a Full-Body Flavor
Medium-dark roasts are typically roasted to around 220–227°C, right at the first crack, with an Agtron of 35–50. The beans are dark brown, beginning to show oil spots, with a loss of 16–18%. Acidity decreases sharply, the rich caramel sweetness gradually transitions to dark chocolate, with a full-bodied flavor and a long finish. Dark roasts are further increased to 228–240°C, with an Agtron below 30. The beans are dark brown, glossy, and oily, with a loss of 18–22% and almost no trace of the growing region remaining. The resulting cup is distinctly bitter, smoky, and has a lingering finish, suitable for those accustomed to traditional tastes. It’s important to note that the surface oil is a double-edged sword: it provides a rich flavor but also oxidizes quickly, causing dark-roasted beans to become rancid within two to three weeks if stored exposed. That’s why you should buy dark-roasted beans in small bags and consume them within a month.
9. Appropriate Roasting Levels for Arabica and Robusta
The two most common varieties react differently to the same heat curve. Arabica beans are more porous, contain more sugar and flavor precursors, so a light to medium roast is enough to reveal all the layers of flavor; pushing it too far past the second crack is almost certainly a waste of money as the delicate flavors are burned away. Robusta beans are denser, harder, and less sugary; roasting them too lightly will reveal bitterness and raw grain notes, so a medium to dark roast usually yields a more well-rounded result. Robusta also needs a longer development time for the heat to reach the core, so roasters often lower the temperature in batches and extend the roasting period after the first crack. High-quality Fine Robusta tends to be roasted lighter than average to retain the aftertaste and hazelnut notes, showing that the above rule has exceptions.
10. Choose the Coffee Roast Level According to Your Brewing Style
The general principle is that the longer the contact time with water, the lighter the roast level should be. Pour-over V60, Chemex, and cold brew methods work well with light to medium roasts and water at 92–96°C, resulting in a clear and flavorful drink. Traditional espresso requires medium to dark roasts for good crema and sufficient flavor to complement milk; if using an automatic machine, a medium-dark roast is more stable because overly oily beans can easily clog the grinder. Vietnamese drip coffee, with its slow extraction time of 4–6 minutes, is best suited to medium to dark roasts, achieving the familiar strong and bitter taste. Moka pots are a safe bet between espresso and drip coffee. Finally, consider the roasting date: light-roasted beans need 7–14 days to release their energy, while dark-roasted beans only need 3–7 days for optimal flavor.