What Happens in the Coffee Roasting Process?
Roasting is where green coffee beans are transformed into the fragrant, flavourful coffee we all know and love. It’s a process that blends art and science, requiring precision, sensory skill, and an understanding of the complex chemical changes that take place inside the bean. Whether you're a professional roaster or a dedicated coffee enthusiast, understanding what happens during the roast can deepen your appreciation and improve your results.
The Purpose of Roasting
Green coffee beans are hard, dense, and grassy-smelling. Roasting brings out the aromatic oils and volatile compounds responsible for the flavours we associate with great coffee. The goal is to unlock each bean’s potential by carefully controlling heat over time.
Phases of the Coffee Roasting Process
The roast can be broken down into distinct stages, each of which plays a critical role in shaping the final cup profile. Roasting is a complex chemical and physical transformation that involves multiple heat-driven reactions such as drying, Maillard reactions, caramelisation, and gas release. Environmental control, including airflow and cooling, also impacts the outcome significantly. Immediate post-roast cooling is essential to halt the roast and preserve aroma and flavour.
1. Drying Phase (Start to ~150°C)
- What Happens: Green beans contain around 8–12% moisture. The drying stage evaporates this water.
- Why It Matters: A stable drying phase sets the pace for an even roast. Rushing this step can lead to uneven development later on.
2. Yellowing & Maillard Reaction (~150°C to 180°C)
- What Happens: Beans turn yellow, and a bread-like aroma develops. The Maillard reaction begins—a complex chemical reaction between amino acids and sugars that produces hundreds of flavour compounds.
- Why It Matters: This is the heart of flavour development, contributing body, sweetness, and balance.
3. First Crack (~196°C)
- What Happens: Pressure from gases inside the bean causes it to crack audibly. The coffee expands in size.
- Why It Matters: This marks the transition from endothermic (absorbing heat) to exothermic (releasing heat). Roasters may choose to stop the roast here for lighter profiles.
4. Development Time / Post-Crack (~196°C to 210°C+)
- What Happens: Sugars caramelise, acidity rounds out, and the roast's character is finalised.
- Why It Matters: Managing this phase is crucial. Too short, and the coffee may taste sour or grassy. Too long, and it may lose complexity and taste flat or overly roasted.
5. Second Crack (Optional, ~224°C)
- What Happens: A second audible crack may occur, caused by the fracturing of the coffee's cellular structure and oil migration to the bean's surface.
- Why It Matters: This phase signals a transition into darker roast territory. Although second crack can produce deeper roast notes, it may also lead to bitterness and burnt flavours if not managed properly. Most speciality roasters avoid going too far into second crack to preserve the bean’s inherent characteristics.
The Role of Sample Roasting
Sample roasting is essential for evaluating the potential of green coffee beans before committing to a full production roast. It's used for green grading, quality control, and creating initial roast profiles. Consistency is critical here—roasters need tools that offer precision, repeatability, and control over time, temperature, and environmental variables.
A key part of sample roasting is using dedicated small-capacity machines that emulate the heat transfer and control of full-scale roasters. Equally important is tracking roast curves and cooling the beans immediately post-roast to preserve accurate sensory feedback.
Product Spotlight: Link by Nucleus Coffee Tools
As the roasting industry continues to evolve, precision and data-led tools are becoming indispensable. The Link sample roaster by Nucleus Coffee Tools represents a new standard in sample roasting. Purpose-built for commercial roasters and serious enthusiasts, it combines exceptional control with cutting-edge technology to help unlock the full potential of every bean.
What Is the Link?
The Link is a precision sample roaster that allows users to roast small coffee batches while mimicking the dynamics of a production roast. This is crucial for green coffee evaluation, roast development, and training.
Key Features & Capabilities:
- Infrared Heat Source: Unlike conventional convection or conduction heating, the Link uses an infrared element to deliver consistent, immediate thermal response. This ensures optimal heat transfer and reduces latency when adjusting roast variables.
- Real-Time Roast Profiling: The Link is equipped with intuitive roast tracking software. Users can monitor temperature curves, rate of rise (RoR), and time milestones, ensuring each roast can be accurately analysed and replicated.
- Airflow Control: Adjustable airflow settings allow users to simulate convection dynamics, giving deeper insight into how roast development responds to environmental manipulation.
- Compact Form Factor: Its small footprint makes it ideal for tight workspaces, training setups, and mobile use. This means quality control and sample evaluation aren’t restricted by location.
- Data Logging & Recipe Saving: Save roast curves and profiles to build a database of performance data, supporting repeatability across teams or locations.
Why the Link Matters
For roasters, every decision at the sample level influences broader production outcomes. With the Link, you're not just testing beans—you’re setting the foundation for scaling your roast profiles, reducing waste, and optimising sourcing decisions.
Whether you're assessing a microlot, trialling a new origin, or training a team, the Link gives you:
- Consistent cupping results across batches
- Improved quality control for import and purchasing decisions
- Detailed control and feedback for educational or R&D environments
Who It's For:
- Commercial roasters refining new coffees before scaling
- Quality control teams doing lab-based evaluation
- Trainers educating staff on roast development and sensory results
- Prosumers seeking professional-grade tools at sample scale
The Link is more than a roaster—it's a tool for discovery, refinement, and excellence.
Now available through Brew-It Group, it brings elite-level control into every roast lab, cupping table, or coffee classroom.
Final Thoughts
Coffee roasting is a journey of transformation. Every decision—from charge temperature to development time—shapes the aroma, flavour, and body of the final cup. Tools like the Link sample roaster give roasters the control they need to explore these variables with confidence.
Whether you're roasting at scale or trialling microlots, understanding each phase of the roast allows you to coax out the very best your beans have to offer.