Why Resting Freshly Roasted Coffee is Crucial for Flavour Extraction
Freshly roasted coffee is an intricate product of chemistry, with carbon dioxide (CO₂) playing a surprisingly central role in the brewing process. While the CO₂ generated during roasting is vital to the coffee’s freshness and flavour, it can also inhibit proper extraction when brewing. Understanding this interaction sheds light on why resting coffee after roasting is crucial to unlocking its full potential.
The Role of CO₂ in Brewing
1. Gas Release During Brewing
When hot water meets freshly roasted coffee, the trapped CO₂ begins to rapidly escape in a process known as degassing. This release creates visible bubbles during brewing, particularly with methods like pour-over. These bubbles disrupt the water’s ability to uniformly saturate the coffee grounds, preventing water from fully dissolving the coffee’s soluble flavour compounds. This can result in uneven extraction, where some areas of the coffee grounds are under-extracted while others are over-extracted.1
2. CO₂ as a Chemical Influence
CO₂ also impacts brewing chemistry. The gas is acidic in nature and can lower the pH of the water during extraction, altering solubility rates and influencing how certain compounds are dissolved. High concentrations of CO₂ can mask delicate flavours, overemphasise sharp acidity, and disrupt the overall balance of the cup.2 While a small amount of CO₂ contributes to freshness, excessive amounts can negatively impact the coffee’s taste profile.
Why Resting is Important
1. Degassing Timeline
Resting coffee after roasting allows the trapped CO₂ to gradually escape. Research shows that the majority of CO₂ dissipates within the first 48–72 hours, although degassing can continue for up to two weeks, depending on factors such as roast level, storage conditions, and bean density1. Lighter roasts, due to their denser structure, tend to retain CO₂ longer than darker roasts.
2. Improved Extraction
Once excess CO₂ has escaped, water is better able to penetrate the coffee grounds during brewing. Without the interference of gas release, water can uniformly extract soluble compounds, resulting in a more balanced and consistent cup.3 The absence of rapid bubbling also ensures a smoother brewing process, particularly in manual methods like pour-over and AeroPress.
3. Flavour Stabilisation
Resting not only reduces CO₂ levels but also allows volatile aromatic compounds to stabilise. These compounds are essential for the coffee’s flavour complexity, but when freshly roasted, they can be unstable or overpowering. Resting allows these compounds to reach equilibrium, creating a more cohesive and refined flavour profile.4
Why Not Grind Fresh and Just Wait?
Some might wonder, “Why not simply grind freshly roasted coffee and allow the CO₂ to escape before brewing?” While this might seem like a logical solution, it introduces more problems than it solves. Grinding exponentially increases the surface area of the coffee, which not only accelerates the release of CO₂ but also causes the rapid loss of volatile aromatic compounds essential to the coffee’s flavour. These aromatic compounds are highly sensitive and begin to dissipate as soon as the coffee is ground, leading to diminished complexity and a flat, stale flavour profile.
Moreover, controlling the rate at which CO₂ escapes from ground coffee is nearly impossible. While degassing whole beans happens gradually, ground coffee releases gas at an inconsistent rate due to the varied particle sizes and uneven exposure to air. This makes it difficult to achieve the balanced flavour and controlled extraction that resting whole beans provides.
By resting coffee first, then grinding and brewing immediately, you maximise the chances of achieving a harmonious balance in the cup. Resting allows the beans to stabilise, ensuring the delicate aromatic compounds remain intact and ready to shine. Grinding just before brewing preserves these volatiles while enabling consistent extraction, delivering a brew that captures the full complexity and vibrancy of the coffee’s flavour profile.
A Fun Analogy
Think of freshly roasted coffee as a sponge filled with gas; when you try to push water through it, the trapped air creates resistance and prevents even saturation. Resting the coffee is akin to squeezing the sponge first, allowing it to absorb water evenly and efficiently when needed.
By resting coffee, we optimise both the physical and chemical conditions for water to properly interact with the coffee’s soluble components. This simple yet critical process unlocks the coffee’s full potential during brewing.
The Science Behind It
Key take-aways of what the science tells us:
- The rapid release of CO₂ during brewing disrupts water flow and prevents even extraction.
- Excess CO₂ lowers pH levels, masking delicate flavours and affecting solubility.
- Resting stabilises volatile compounds, enhancing flavour consistency and complexity.
Freshly roasted coffee holds incredible potential, but its flavours are often masked by excessive CO₂. Allowing coffee to rest after roasting provides a simple yet critical way to reveal its true character. Whether you’re a professional roaster or an at-home enthusiast, understanding the science behind resting coffee gets you one step closer to an exceptional brew.
References
- Anderson, B. A., et al. (2003). The diffusion kinetics of carbon dioxide in fresh roasted and ground coffee. Journal of Food Engineering, 59(1), 71–78. ↩︎
- De Santis, D., et al. (2023). Optimization of Coffee Oil Extraction from Defective Beans Using a Supercritical Carbon Dioxide Technique. Foods, 12(13), 2515. ↩︎
- Supanivatin, P., et al. (2023). Comparative Analysis of Phenolic Content, Antioxidant Activity, and Chemical Profile of Coffea robusta Extracts. Foods, 12(18), 3443. ↩︎
- Hunsub, P., et al. (2023). Production of Coffee Oil and Bioactive Peptides from Spent Coffee Grounds via Supercritical Carbon Dioxide Extraction and Enzymatic Hydrolysis. Waste and Biomass Valorization, 15, 2061–2070. ↩︎
Considered as one of the region’s pioneers in specialty coffee education and Dubai cafe culture development, Ryan Godinho is an Australian entrepreneur who is accredited as the country's first SCAA AST and National Coffee Championships Coordinator. He is a frequent contributor to Forbes and Entrepreneur Magazines and also holds a postgraduate Certificate of Advanced Studies in Coffee Excellence from Zurich University (ZHAW).
