Just Published: Brewing Temperature and the Sensory Profile of Brewed Coffee

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A new study by researchers at the UC Davis Coffee Center, titled “Brew temperature, at fixed brew strength and extraction, has little impact on the sensory profile of drip brew coffee,” was published today in the prestigious journal Scientific Reports.

PETER GIULIANO, Chief Research Officer of the SCA and Executive Director of the Coffee Science Foundation, interviews Professor WILLIAM RISTENPART, Director of the UC Davis Coffee Center, about the newly published article, authored by Mackenzie E. Batali, William D. Ristenpart, and Jean-Xavier Guinard.

Published as a part of the three-year research project, “Towards a deeper understanding of brewing fundamentals,” supported by the Coffee Science Foundation, the SCA’s research arm, we were able to reach out to the principal investigator behind the paper, Professor William Ristenpart. Here, Prof. Ristenpart takes the time to answer questions about the research, which has significant implications for how we understand coffee brewing.

PETER GIULIANO (PG): First of all, the title of the paper is “Brew temperature, at fixed brew strength and extraction, has little impact on the sensory profile of drip brew coffee.” This seems to indicate that temperature isn’t important—which runs contrary to most coffee people’s intuition. Is that what your research shows?

Prof. William Ristenpart (WR): No, definitely not—the water temperature definitely affects how quickly the coffee is extracted! But what our research indicates is that what really matters is the final brew strength and final extraction yield at the end of the brew. If those metrics are the same in two different brews, it doesn’t matter what brew temperatures were used to achieve them because tasters can’t differentiate them: they taste the same. In other words, it’s the destination that matters, not the route you took to get there. So the brew temperature strongly affects how you get there, but the most important thing for coffee brewers to focus on is the final strength and extraction yield.

PG: I know this is a part of a multi-year investigation into filter coffee brewing, in collaboration with the Coffee Science Foundation. What is the history of this research, and why did you start researching brewing temperature?

WR: Our main motivation was the classic Coffee Brewing Control chart, originally developed by Earnest Lockhart in the 1950s and still widely used today, but which is outdated and needs to be updated using modern sensory science methodologies. Our initial publications focused on the effect of filter basket geometry,[1] how a brew develops versus time,[2] and how coffee tastes over the range of the control chart for different roast levels.[3] We wanted to investigate brew temperature because it features so prominently in the specialty coffee scene, both in terms of education and also certification of brewers. Also, in discussion with Breville who provided support for this research, we realized it was a key interest for them as a brewer manufacturer, and thus likely a key interest for many segments of the coffee industry. Our original hypothesis was that the brew temperature would matter a lot. We were surprised to find it matters very little, provided you control the brew strength and extraction yield.

PG: It seems that independent vs. dependent variables are an important concept here. Can you explain these concepts a bit?

WR: Sure. In experimentation, an “independent variable” is something that you can change, while a “dependent variable” is something that you are interested in measuring that varies as a consequence of whatever changes you make. A great example familiar to coffee experts would be cupping coffee at different roast levels. The independent variable would be the roast level (how dark you roasted it), and the dependent variables could be how bitter or how sour it tastes when cupped. Note that in this example, the independent variable of “roast level” itself depended on other parameters, like how long you roasted it for and how much heat you applied. Our brew temperature experiments were similar in the sense that our independent variables themselves depended on other parameters. For our study, the dependent variables of interest were the intensities of specific sensory attributes (how sour, how bitter, how much citrus, etc.), while our independent variables were the brew strength, the extraction yield, and the brew temperature.  For each of those independent variables we adjusted other parameters, like the grind size and brew time, to achieve the desired values for the independent variables, to allow those to be systematically varied to gauge the impact on the sensory attributes. 

PG: Your experiments evaluated coffee at three temperatures: 87°C, 90°C, and 93°C. How were those temperatures chosen? Also, can you tell us more about the coffee used?

WR: We wanted to use a commercial brewer with a programmable flow rate so that we could precisely control the total brew time to achieve our desired brew strength and extraction yield at the specified temperature – the colder temperatures required longer brew times. For our brewer (a Curtis G4), 93°C was the highest we could reproducibly achieve for the large volumes of coffee needed for the trained panel assessment. We then wanted to test brew temperatures evenly spaced out at lower temperatures. Given that the current “Gold Cup” standards specify 92°C as the minimum allowed temperature, we though going several degrees below that would provide a noticeable impact on the sensory profile… and we were surprised when it didn’t. Regarding the coffee, we used a representative “clean” wet-processed Arabica from Honduras that was intended to reflect a more-or-less “standard” coffee.

PG: So the main insight seems to be that total dissolved solids (TDS) and extraction percentage—two dependent variables in coffee brewing—seem to be more correlated to sensory attributes than the independent variables like temperature. What about other variables, like brew time or grind size?

WR: We have to be careful in our terminology: the brew strength (as measured by TDS) and the extraction yield can be either dependent or independent variables, depending on what experiment you’re talking about. For example you can change the grind size for several different brews, then measure the resulting TDS in each. In that experiment the grind size is the independent variable and the TDS is the dependent variable. For our work, we carefully changed several parameters, including the grind size and flow rate, to achieve a specific TDS, and we then measured the resulting sensory profile at that TDS. In this case, the independent variable is the TDS, and the sensory attributes are the dependent variables. Brew time, grind size, and brew temperature are all tremendously important for coffee brewing—but our data strongly indicate that they are important because they affect what TDS and extraction percentage you get, not because they intrinsically alter the coffee (at least over the range of temperatures tested here). Again, it’s the destination that matters, not the exact route you took to get there.

PG: What’s next in research on temperature in coffee?

WR: Our experiments to date have focused on hot drip brew, and as mentioned above we focused on a representative wet-processed coffee. It’s entirely possible that coffees with more exotic sensory profiles (such as blueberry notes in a natural processed Yirgacheffe) might exhibit a more pronounced brew temperature dependence; more research would be necessary to test that hypothesis. Also, in this specific study we only examined a narrow range of temperatures for hot drip brew. We’re currently doing experiments on cold brew coffee, with exciting preliminary data so far. We’re excited to share more results in the near future! 

PG: Anything else you would like to mention?

WR: I’d like to emphasize that none of this research would be possible without the support of the Coffee Science Foundation and especially Breville Corporation for providing underwriting. The funds directly support our graduate students, including Ms. Mackenzie Batali who performed this study as part of her PhD dissertation research. We thank them for the support!

The scientific paper discussed above is available for download from Scientific Reports. A plain-language version of the work will be included in an upcoming issue of 25, a quarterly publication of the Specialty Coffee Association.

Find plain-language versions of the other articles related to “Towards a deeper understanding of brewing fundamentals” here:

See all the latest publications and outputs related to the Brewing Fundamentals research project here.

Footnotes

[1] Scott C. Frost, William D. Ristenpart, and Jean-Xavier Guinard, “Effect of Basket Geometry on the Sensory Quality and Consumer Acceptance of Drip Brewed Coffee,” Journal of Food Science, Volume 84, Issue 8 (August 2019): 2297-2312. https://onlinelibrary.wiley.com/doi/abs/10.1111/1750-3841.14696  

[2] Mackenzie E Batali, Scott C Frost, Carlito B Lebrilla, William D Ristenpart, and Jean-Xavier Guinard, “Sensory and monosaccharide analysis of drip brew coffee fractions versus brewing time,” Journal of the Science of Food and Agriculture, Volume 11, Issue 7 (May 2020): 2935-2962. https://onlinelibrary.wiley.com/doi/abs/10.1002/jsfa.10323

[3] Scott C. Frost, William D. Ristenpart, Jean-Xavier Guinard, “Effects of brew strength, brew yield, and roast on the sensory quality of drip brewed coffee,” Journal of Food Science, Volume 85, Issue 8 (August 2020): 2530-2543. https://onlinelibrary.wiley.com/doi/abs/10.1111/1750-3841.15326

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