Understanding Full Immersion Coffee Brewing: Insights from an Equilibrium Desorption Model
Liang, J., et al. An equilibrium desorption model for the strength and extraction yield of full immersion brewed coffee. Sci Rep 11, 6904 (2021). https://doi.org/10.1038/s41598-021-85787-1
The short version:
Full immersion brewing, such as French press and cupping, is a popular method for extracting coffee’s rich flavours. A study by Liang et al. (2021) introduces a desorption-based model to predict how brew ratio, grind size, and temperature affect extraction. Their research confirms that brew ratio is the key factor controlling coffee strength (TDS), while extraction yield (E) remains stable at ~21% across different conditions. Temperature influences the speed of extraction but not the final yield, and finer grinds only slightly enhance extraction. This study provides a scientific foundation for optimizing full immersion brewing, helping coffee enthusiasts refine their recipes for the best possible flavour.
Introduction
Coffee brewing is an intricate process that affects the final taste, aroma, and strength of the beverage. Among the various brewing techniques, full immersion brewing (such as the French press or cupping) is widely used due to its simplicity and ability to extract a full range of coffee flavours. However, until recently, little theoretical work had been done to model the fundamental extraction process of full immersion brewing.
A recent study by Jiexin Liang, Ka Chun Chan, and William D. Ristenpart, published in Scientific Reports, introduces a pseudo-equilibrium desorption model to predict how total dissolved solids (TDS) and extraction yield (E) behave in full immersion brewing. Their model, supported by experimental results, offers important insights into how brew ratio, grind size, and temperature influence coffee extraction.
The Science Behind the Model
In this study, the authors developed a desorption-based model, which assumes that the chemical components of coffee dissolve from the grounds into the liquid until a pseudo-equilibrium state is reached. They derived equations to predict:
TDS (brew strength): the total concentration of coffee solubles in the brewed liquid.
Extraction yield (E): the percentage of the coffee grounds' mass that is dissolved into the final brew.
A key assumption of the model is that each coffee compound has its own solubility, but instead of tracking each individual compound, they use an average equilibrium constant (K) to describe the behaviour of all dissolved coffee species.
The model predicts two key relationships:
TDS is inversely proportional to the brew ratio (i.e., the more water used, the weaker the brew).
Extraction yield is largely independent of the brew ratio and remains at approximately 21% under equilibrium conditions.
Experimental Validation
To test their model, the researchers performed a series of experiments using different brew ratios, temperatures, and grind sizes. Their setup involved:
1-L beaker immersion brews with controlled water-to-coffee ratios.
Cupping-style brews to compare traditional industry practices.
Measurement of TDS using a digital refractometer and E using oven-drying mass measurements.
Key Findings
TDS decreases as brew ratio increases
A lower brew ratio (more coffee, less water) results in a stronger cup.
A higher brew ratio (less coffee, more water) leads to a weaker, more diluted brew.
The inverse relationship predicted by the model was confirmed experimentally.
Extraction yield remains constant (~21%)
Regardless of the brew ratio, E was observed to stay within a narrow range of 20–22%.
This means that while brew strength can be adjusted via brew ratio, the total amount of coffee extracted from the grounds remains stable.
Temperature has little effect on final extraction yield
While higher temperatures speed up extraction, they do not increase overall yield.
TDS stabilised after ~20 minutes, regardless of temperature.
Grind size has only a minor effect on TDS and E
Finer grinds resulted in slightly higher TDS and extraction, but the difference was small.
Brew ratio remains the dominant factor in determining coffee strength.
Oven-drying underestimates true extraction yield
Traditional methods of measuring E (drying spent grounds) give lower values because some coffee solids remain trapped in the used grounds.
The model accounts for this by incorporating a retained liquid correction factor.
Practical Implications
1. Control Brew Strength with Brew Ratio
If you want a stronger cup of coffee, reduce the brew ratio (use more coffee per unit of water). Conversely, to create a lighter brew, increase the brew ratio.
2. Full Immersion Has Limited Control Over Extraction Yield
Unlike percolation methods (e.g., V60, espresso), which can significantly alter extraction yield by adjusting flow rate and grind size, full immersion brewing always leads to a similar E (~21%).
3. Temperature Changes Extraction Speed, Not Final Yield
While hotter water brews coffee faster, it does not significantly increase the total extraction. This means that if you’re in a hurry, brewing at a higher temperature can speed things up, but won’t necessarily result in a stronger or better-tasting cup.
4. Traditional Oven-Drying Methods Can Be Misleading
If measuring E in a scientific or commercial setting, standard drying techniques underestimate extraction yield. Practitioners should account for retained liquid in spent grounds when assessing coffee extraction.
Conclusion
This study provides a solid theoretical and experimental foundation for understanding full immersion brewing. It confirms that brew ratio is the primary lever for controlling coffee strength, while extraction yield remains remarkably stable across different brewing conditions.
For coffee enthusiasts and professionals alike, these insights can help refine brewing techniques and optimize flavour profiles by fine-tuning brew ratio, temperature, and grind size.
For further reading, you can access the full study here: Scientific Reports, Liang et al. (2021).
