1. Primary Roasting Equipment and Techniques
The three main types of commercial coffee roasters determine how heat is transferred to the beans:
| Roaster Type | Heat Transfer Method | Process Description | Flavor Profile Impact |
| Drum Roaster | Conduction & Convection | Beans are tumbled inside a rotating, pre-heated metal drum. Heat is transferred via conduction (contact with the hot drum walls) and convection (hot air circulating in the drum). | Tends to produce a more balanced, rounded, and full-bodied cup with classic roast-developed flavors (caramel, chocolate). The longer, gentler roast curve allows for complex flavor development. |
| Fluid-Bed Roaster (Air Roaster) | Convection Only | Beans are suspended and agitated in a column of intense, superheated air, similar to a hot-air popcorn popper. This is often called fluidized-bed roasting. | Tends to result in a cleaner, brighter, and more acidic cup, preserving the bean's origin characteristics (fruity, floral, specific terroir notes) due to the faster, more even heat transfer. |
| Infrared Roaster | Radiation | This modern technique uses infrared radiation as the primary heat source to directly heat the beans. | Offers highly precise control. Proponents suggest it can enhance sweetness and body while efficiently developing the roast. |
Coffee roasting is the process of applying heat to green coffee beans to transform their chemical and physical properties, creating the distinct flavors, aromas, and colors we associate with brewed coffee.
The techniques are generally categorized by the equipment used and the level/profile of the roast.
2. The Roasting Process Stages
A master roaster carefully controls the temperature and time through four main phases to create a "roast profile."
| Phase | Bean Temperature (Approximate) | Description | Key Chemical Reactions |
| 1. Drying Phase | Up to $160^\circ\text{C}$ ($320^\circ\text{F}$) | Green beans lose their moisture content (up to $10\%-12\%$) and change color from green to yellow. The beans begin to smell grassy or like hay. | Moisture evaporation. |
| 2. Maillard Reaction/Browning | $160^\circ\text{C}$ to $200^\circ\text{C}$ ($320^\circ\text{F}$ to $392^\circ\text{F}$) | Sugars and amino acids react, creating hundreds of flavor and color compounds called melanoidins. The beans turn light brown, developing the initial aroma and sweetness. | Maillard Reaction and the start of Caramelization. |
| 3. First Crack | $\approx 205^\circ\text{C}$ ($401^\circ\text{F}$) | The internal pressure from steam buildup causes the beans to audibly crack (like popcorn). This marks the transition from light to medium roast. Roast development begins here, where the coffee's flavor is finalized. | Exothermic reaction (the beans produce their own heat). |
| 4. Second Crack | $\approx 225^\circ\text{C}$ ($437^\circ\text{F}$) | A softer, quicker snapping sound. This signifies the cellular structure of the bean breaking down further and the release of oils. Stopping the roast after this point results in medium-dark or dark roast. | Full Caramelization and Pyrolysis (breakdown of organic material, leading to smoky/bitter notes). |
| 5. Cooling | End of Roast | Roasting is immediately halted by quickly cooling the beans (usually with forced air or a water quench) to prevent them from continuing to cook from residual heat. | Stops all chemical reactions. |
3. Roast Levels and Flavor Impact
The final roast level is determined by the end temperature and how far into the development phase the roast is taken.
| Roast Level | Color / Temperature | Acidity | Body | Flavor Profile |
| Light Roast | Light brown, no oil. Stopped near/at First Crack ($\approx 205^\circ\text{C}$). | High (Bright, Citrusy) | Light, Tea-like | Highlights the Origin Characteristics; Floral, fruity, complex, clean. |
| Medium Roast | Medium brown, non-oily surface. Stopped between the cracks ($\approx 210-220^\circ\text{C}$). | Medium (Balanced) | Medium, Rounded | Balanced between origin and roast flavor; Caramel, nutty, chocolate, sweet. The "all-rounder." |
| Medium-Dark | Darker brown, some oil spots. Started or just into Second Crack ($\approx 225^\circ\text{C}$). | Low | Full, Heavy | Sweetness is reduced; prominent roast flavors; Dark chocolate, spice, rich, slightly bitter. |
| Dark Roast | Dark, nearly black, very oily surface. Stopped well into Second Crack ($\approx 240^\circ\text{C}$). | Very Low | Heavy, Syrupy | Dominance of Roast Flavor; Bold, smoky, bittersweet, charcoal. Origin flavor is often completely masked. (Often used for espresso). |
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