Lead Author JIEXIN LIANG shares findings of a recent paper, “An Equilibrium Desorption Model for Strength and Extraction Yield of Full Immersion Brewed Coffee,” published in Scientific Reports, that outlines a predictive model for the equilibrium strength and extraction of “full immersion brewed coffee” (cupping) between 80°C and 99°C (176°F and 210°F) and suggests we’re more easily able to control our total dissolved solids (TDS) via brew ratio instead of our extraction yield (E).

I have cupped coffee for many years, and in those three minutes while the coffee is steeping with a nice dome of crust on the surface, I often have the time to wonder what is going on inside the cup. When I was first taught to cup, I was told many things: that extraction mainly takes place during those three to four minutes when the crust is floating above the liquid, inside that slurry of water, gas, and coffee grounds; that one of the purposes of stirring the crust is to “cut down” the extraction process, as the grounds sink to the bottom and there is very little extraction happening henceforth. I was also told the changes in flavor as the coffee cooled down were mainly because of perceived changes with the increasingly cooler brew temperature, rather than an ongoing extraction process. Some of this made sense: a few minutes after pouring the water, the temperature of the brew dropped below 70°C, and I supposed the extraction rate would be much slower as the temperature continued to cool.

But then I started cupping Coffea canephora, and I noticed things that didn’t align with this understanding: the acidity of those C. canephora brews was very low when I first brewed them but became clearly more intense the longer you left the brews to “cool down.” As this held true almost every time I cupped a sample of C. canephora, I was puzzled. A hypothesis formed: C. canephora beans are much denser than C. arabica beans; they pack more fiber. Perhaps the water took a longer time to be absorbed by the denser coffee particles and to extract the acids? At any rate, in the case of robusta, it seemed to me that the 10–20 minutes of infusion until we finish tasting the brew are insufficient to extract those acids, making me think that maybe some of the things we think we know about cupping (that extraction is “cut down” when we stir the grounds and they sink to the bottom) may not be quite as true as we thought.

These and other similar observations I have made about cupping over the years did not make much sense until I started to learn about the work by Jiexin Liang et al. on full immersion brewed coffee. Liang’s initial findings about the extraction nature and equilibrium, which you’ll explore over the next few pages, suggest there are more accurate ways to describe what’s happening in the cup. Instead of saying, “you cut down the extraction when you stir the grounds and they sink to the bottom,” we could say, “equilibrium is reached after a few minutes,” or, “the extraction rate becomes very low when coffee cools down.”

Today, in light of Liang’s findings, I have a different hypothesis to explain the differences between arabica and robusta cuppings: it’s likely the brewing parameters of arabica cuppings were “optimized” by trial and error over many decades to achieve optimum extraction by the time we taste it. Maybe, with robusta’s high bean density, those 10–20 minutes are not enough to reach equilibrium, which is why the brew is so different after it “cools down.” If this hypothesis is correct, perhaps we should grind C. canephora finer or allow it to steep longer in order to taste the brew closer to equilibrium.

In any case, this is only the first piece of published research from this project, and—with a little insight from Jiexin on forthcoming work—we know there are more exciting insights to come. Coffee cuppers and French press enthusiasts alike will find much food for thought here!

Dr. Mario R. Fernández-Alduenda
Technical Officer

There are countless coffee cuppings performed by coffee professionals around the world, assessing coffee quality, every day.

But if you wish to evaluate the sensory qualities of coffee through cupping, it’s important to ask: How strong is the coffee in the cupping bowl? Cupping is one type of “full immersion” brew, a method of brewing that is also used in French presses, cold brew, and Japanese vacuum brewers (siphon). Since the sensory qualities of brewed coffee are known to be strongly correlated with the strength, measured as total dissolved solids (TDS), and the extraction yield (E) of the brew, a natural question to ask is: how can we predict the TDS and E of coffee in a full immersion brew?

To date, several modelling works have focused on flow-based brewing techniques, such as drip brew and espresso, but relatively little theoretical work has examined the strength and extraction of full immersion brews. Several key questions about full immersion brewing remain unanswered: How does the brew ratio affect the strength (TDS) and extraction (E) of full immersion coffee? What are the effects of brewing temperature, grind size, and roast level on TDS and E? Along with a UC Davis Coffee Center colleague, Ka Chun Chan, and Professor William D. Ristenpart, we developed a theoretical model for full immersion brewed coffee to answer these questions and then we evaluated it with a series of experiments under various brewing conditions.[1]

Developing the Model

The basic concept behind the model is simple. Imagine what happens to a sugar cube when it’s dropped in water: it dissolves! Some of the chemical substances in coffee grounds are also soluble in water—and some are more soluble than others—so we developed a model under an assumption of the “average” soluble substances present when brewing. Zooming into the brew at particle size level, the soluble coffee substances originally present in the coffee grounds (solid phase) will dissolve into the brew (liquid phase) when hot water is added. As full immersion brewing proceeds, coffee solubles are free to go through “adsorption” from the liquid to solid phase, and “desorption” from the solid to liquid phase. Notably, “adsorption” is different from “absorption”: Absorption is what water does in a sponge—it gets absorbed. Adsorption is the binding of molecules to a solid interphase, which is the case of coffee solubles here in full immersion brew.

The longer we brew the coffee, the stronger it will become. When the full immersion brew goes on for a long time, TDS approaches a steady value. When this happens, the brew reaches equilibrium—the desorption and adsorption of coffee solubles are still happening, but the TDS doesn’t increase further (i.e., the rate of change of concentrations of desorption coffee solubles is zero).

Our model equations (see page 42) predict that the TDS varies inversely with brew ratio, but that the E at equilibrium is independent of the brew ratio. Our model also yields a more complicated equation indicating that the extraction yield measured by a standard oven drying method underestimates the actual extraction yield due to the retained liquid coffee trapped in moist spent coffee grounds.

Evaluating the Model

Evaluating the model with experiments was straight-forward (figure 1). We ground up some coffee, put it into a beaker, added hot water, and let it brew until equilibrium was reached. Then, we separated the moist spent grounds and liquid coffee through a filter paper. We measured the TDS of liquid coffee using a digital refractometer, then calculated E using the TDS measurement. As some of the spent grounds are still wet, we need to remove the liquid before we can measure the mass of the dried, spent coffee grounds. To do this, we returned the filter paper with the grounds back into the beaker so that we could bake off the water in an oven. The oven drying extraction can be calculated using the mass of fresh coffee grounds and the mass of dried coffee grounds. Because there are some dissolved coffee substances trapped inside the coffee grounds’ retained liquid, left inside the dried spent grounds after oven-drying, the resulting Eoven underestimates the true Eoven at equilibrium!

**Figure 1.** Experimental scheme for full immersion outlining the steps we followed to test the model, in order from left to right.

Overall, a total of 99 individual experiments were performed following this procedure under various brewing conditions. Furthermore, we also performed experiments following a standard cupping procedure in collaboration with Peet’s, where we examined the TDS and E with caffeinated coffee at different roast levels including light, medium, dark, and extra dark roasts, and with a decaffeinated coffee blend.

Findings

Overall, our experimental results strongly agree with our model predictions of the TDS and E of full immersion brews at equilibrium. Moreover, the equilibrium TDS varies inversely with brew ratio, and the equilibrium E is approximately 21% over the wide range of brew ratios tested. In other words, the equilibrium TDS increases as the brew ratio decreases (figure 2), but equilibrium E stays relatively the same even when the brew ratio is varied (figure 3).

Insensitivity to Temperature and Grind Size

A surprising result we found is that the equilibrium TDS and E extraction is insensitive to the brew temperature within the range we tested. Although the brew temperature will affect the dynamics of the brew, where the hotter brews reach equilibrium more quickly than the lower temperature ones, the variation of brew temperatures from 80°C to 99°C (176°F to 210°F) does not affect the final equilibrium TDS or E. This finding aligns with other work recently published by scientists at UC Davis Coffee Center as a part of the brewing fundamentals research project with the Specialty Coffee Association (SCA) Coffee Science Foundation, supported by Breville, on brewing temperature and the sensory profile of drip brew coffee. A paper by Batali et al. demonstrated that drip brew coffees prepared to identical TDS and E using different brewing temperatures didn’t result in significantly different sensory profiles.[2]

Similarly, the particle size of the coffee grounds has only a minor effect on the strength and extraction. Over the range of median particle sizes from 579 μm to 1311 μm, coffee brewed with finer coffee grounds resulted in slightly higher TDS and E, but these small TDS differences are negligible under the major effect of brew ratio over the range 2–25 tested. However, we emphasize that these results do not mean that the brews prepared at different temperatures and different grind sizes will necessarily taste the same: It is entirely possible that two coffee brews with the same TDS and E would have different sensory profiles due to the difference in underlying chemical compositions.

Strength and Extraction Control via Brew Ratio

Although the brew temperature range we tested does not appreciably affect the equilibrium brew strength or extraction, our results clearly demonstrate that the brew ratio has a huge influence on the equilibrium TDS. Both our model and experimental results indicate that the TDS varies inversely with the brew ratio. Our traditional cupping experiments with various roast levels also align with this finding, suggesting that roast level has only a minor effect on equilibrium TDS. Since the sensory profile is strongly correlated with the TDS,[3] this result strongly suggests that the simplest way to modify the sensory profile for full immersion brewing is to modify the brew ratio. Again, variations in brew temperature, grind size, roast level, and agitation will affect how quickly the brew approaches equilibrium, but these changes will not alter the TDS of the final brew itself; in contrast, the brew ratio directly controls the final TDS. So, our results suggest that you should pay careful attention to the measurements of mass of water and coffee grounds for the precise control of brew strength if you’re preparing coffee using full immersion brewing techniques.

On the other hand, our results also indicate that full immersion brewing offers little control over the extraction yield. To good approximation, the E is predicted to be independent of the brew ratio. Specifically, based on our measurements, it appears that full immersion brews at equilibrium invariably result in an E near 21% over the range of brew ratios from 3 to 25. In addition, variations in the brew temperature, grind size, and brew ratio we tested do not effectively alter the final E. Moreover, this “equilibrium E” is what matters because it directly corresponds to the coffee beverage to be consumed, whereas the “effective E” (Eoven) measured using a standard oven drying method is potentially misleading because it includes dissolved but retained coffee liquid in the spent coffee grounds, which underestimates the actual E in the beverage.

Implications

In terms of practical implications, full immersion brewing techniques effectively remove a “knob” available for fine-tuning the desired sensory profile due to the unvaried equilibrium E. In terms of the classic Coffee Brewing Control Chart, full immersion brews at equilibrium only allow a brewer to move vertically with respect to TDS by changing the brew ratio, but not horizontally with respect to E. Previous work with drip brews has shown that coffee brews with the same TDS but different E have significantly different sensory profiles.[4] Maximal amounts of flavor attributes like sourness, sweetness, and tea/floral—respectively associated with combinations of high TDS&low E, low TDS&low E, and low TDS&high E—are hard to obtain using full immersion brew techniques. Low E can be produced by cutting the brew short and not allowing it to approach equilibrium, but higher E values cannot be achieved with a single full immersion brew. In contrast, flow-based brewing methods like the drip brew increase the flexibility in controlling strength and extraction, but are offset by the complexity and increased expense of drip brew equipment. These results confirm what many coffee professionals may already know: cupping provides good consistency—which is what we want when we’re using it as a tool to evaluate many kinds of coffees!—but it doesn’t showcase the broadest range of a coffee’s flavor that might be accessible through different brewing methods and conditions at the point of consumption.

The fact that cupping is widely preferred by coffee industry professionals because of its simplicity and robustness also highlights the merit of full immersion brews. Overall, there is a tradeoff between flexibility and simplicity when choosing between flow-based and full immersion brewing techniques, and coffee brewers should use these characteristics of brewing methods as tools in achieving their coffee brewing goals. ◇

JIEXIN LIANG is a PhD candidate in food science at the UC Davis and was lead author on the paper, “An Equilibrium Desorption Model for Strength and Extraction Yield of Full Immersion Brewed Coffee,” published in Scientific Reports. KA CHUN CHAN, a former undergraduate student at UC Davis studying chemical engineering, and Professor WILLIAM D. RISTENPART co-authored the aforementioned paper and this feature.

This Coffee Science Foundation (CSF) project was made possible thanks to generous funding from Breville and Toddy through their support of the CSF’s Brewing Fundamentals and Cold Brew research projects, respectively. Learn more at coffeescience.foundation.

References

[1] Jiexin Liang, Ka Chun Chan, and William D. Ristenpart, "An equilibrium desorption model for the strength and extraction yield of full immersion brewed coffee," Scientific Reports 11, 6904 (2021): https://doi.org/10.1038/s41598-021-85787-1

[2] Mackenzie E. Batali, William D. Ristenpart, and Jean-Xavier Guinard, "Brew temperature, at fixed brew strength and extraction, has little impact on the sensory profile of drip brew coffee," Scientific Reports 10, 16450 (2020): https: //doi.org/10.1038/

[3] Scott C. Frost, William D. Ristenpart, and Jean-Xavier Guinard, “Effects of brew strength, brew yield, and roast on the sensory quality of drip brewed coffee,” Journal of Food Science 85, no. 8 (July 2020): 2530–2543.

[4] Frost, Ristenpart, and Guinard.

We hope you are as excited as we are about the release of 25, Issue 17. Both the print edition and the availability of these features across sca.coffee/25 wouldn’t have been possible without our generous underwriting sponsors for this issue: Tempesta Barista Attitude, BWT water+more, and TODDY. Thank you so much for your support! Learn more about our underwriters here.

How Strong is the Coffee You’re Cupping? New Model Captures the Equilibrium Extraction Nature of Full Immersion Brewing | 25, Issue 17