How does the brew ratio affect the strength of full-immersion coffee

This is a summary of the article “An equilibrium desorption model for the strength and extraction yield of full immersion brewed coffee" published in Scientific Reports by Liang, et al.:

Liang, J., Chan, K. C., & Ristenpart, W. D. (2021). An equilibrium desorption model for the strength and extraction yield of full immersion brewed coffee. Scientific Reports, 11(1), 1-13.

https://www.nature.com/articles/s41598-021-85787-1

Introduction and Context

Coffee brewing is a delicate balance of science and art. Two key metrics used to assess the quality of brewed coffee are:

1. Total Dissolved Solids (TDS): This represents the concentration of coffee solubles in the beverage, expressed as a percentage.

2. Extraction Yield (E): The percentage of coffee solubles extracted from the grounds relative to their original dry mass.

These parameters are influenced by factors like water temperature, grind size, brew ratio, and time. While these have been extensively studied, the mechanisms governing these interactions—particularly in full immersion brewing methods like the French Press—remain less understood.

Liang et al. aim to address this gap by proposing a new equilibrium desorption model to predict TDS and E for full immersion brewing. Their model provides a theoretical basis for understanding how variables like the brew ratio (water-to-coffee mass ratio) influence these metrics.

Theoretical Model

Key Assumptions

The authors based their model on the following principles:

1. Coffee grounds and water reach an equilibrium state during full immersion brewing.

2. At equilibrium, solubles desorb (detach) from the coffee grounds and dissolve into the surrounding water.

3. The equilibrium concentration of solubles depends on the partitioning coefficient K, which describes how solubles distribute between the coffee grounds and the water.

Predictions

The model predicts:

1. TDS decreases with increasing brew ratio. This means diluting the coffee with more water leads to a weaker concentration.

2. E remains constant across brew ratios. The amount of solubles extracted from the coffee grounds does not change, even if more water is added.

Experimental Methods

Brew Setup

• Full Immersion Method: The authors used the French Press brewing method.

• Variables Tested: Brew ratio (R), grind size, roast level, and water temperature (80–99°C).

• Control Parameters: Fixed steeping time of 5 minutes with uniform stirring for consistency.

Measurements

• TDS was measured using a refractometer.

• E was calculated using the TDS value and the weight of the coffee grounds before and after brewing.

• Additional experiments tested the drying method bias, showing retained liquid in spent grounds could skew E values.

Key Findings

TDS and Brew Ratio

The experimental results matched the model's predictions: TDS was inversely proportional to the brew ratio. Higher water content diluted the concentration of coffee solubles, leading to weaker coffee.

Extraction Yield Independence

E was approximately constant (around 21%) across all brew ratios, confirming the model’s prediction. This finding highlights that brew ratio affects concentration (TDS) but not the efficiency of soluble extraction.

Partitioning Coefficient Insights

The equilibrium constant K was:

• Largely insensitive to grind size, roast level, and temperature (80–99°C).

• This suggests the desorption and equilibrium dynamics are robust against minor brewing variations.

Retained Water Bias

The authors identified a limitation in traditional methods for calculating E. Spent grounds retained water, leading to underestimations of E by up to 6%. Correcting for this retention is crucial for accurate yield assessments.

Implications for Brewing

1. Precise TDS Control:

   • Full immersion brewing offers reliable control over coffee strength (TDS) by adjusting the brew ratio.

   • This is ideal for maintaining consistency across batches.

2. Limited E Control:

   • While TDS can be fine-tuned, E is inherently limited by the solubility dynamics and remains constant across brew ratios.

3. Practical Brewing Recommendations:

   • Use a consistent brew ratio to achieve predictable TDS levels.

   • Focus on grind size, temperature, and steeping time for minor flavour adjustments, as they have limited impact on the overall extraction yield.

Limitations and Future Research

The study acknowledges:

1. Simplified Assumptions: The model assumes ideal conditions and may not account for complex chemical interactions during brewing.

2. Unexplored Variables: Factors like agitation intensity, steeping duration, and water chemistry were not fully explored.

Future studies could investigate:

• The impact of water composition (e.g., mineral content).

• Applicability to other brewing methods, such as drip or espresso.

Conclusion

Liang et al. provide a robust theoretical framework for understanding full immersion coffee brewing. Their equilibrium desorption model successfully predicts the relationship between brew ratio, TDS, and E, offering valuable insights for both scientists and baristas.

By highlighting the balance between brew ratio and extraction dynamics, the study underscores the importance of precise brewing practices in achieving a consistently high-quality cup of coffee.