Researchers from the UC Davis Coffee Centre published a study in October 2020 in Nature, a prestigious scientific journal, that examined the impact of brew temperature on drip brew coffee. What follows is a summary of the original study’s takeaways. Read the original article on nature’s website here…
History of the Research
Research for this paper is part of a multi-year investigation into filter coffee brewing, together with the Coffee Science Foundation associated with the SCA. The motivation for the research was to update the classic Coffee Brewing Control chart, developed initially by Earnest Lockhart in the 1950s, using modern sensory science methodologies. Previous publications focused on the effect of filter basket geometry, how a brew develops versus time, and how coffee tastes over the range of the control chart for different roast levels. This study was more sensory-based.
The Importance of Independent vs Dependent Variables
The study involved independent and dependent variables. In experimentation, an “independent variable” is something that you can change. In contrast, a “dependent variable” is something you are interested in measuring that varies as a consequence of whatever changes you make. For the study, the dependent variables of interest were the intensities of specific sensory attributes, while the independent variables were the brew strength, the extraction yield, and the brew temperature. For each independent variable, the researchers adjusted other parameters, like the grind size and brew time, to achieve the desired values for the independent variables. As a result, allowing a systematically varied to gauge the impact on the sensory attributes.
The Methodology of the Study
Experiments evaluated coffee at 87 °C, 90 °C, and 93 °C. Researchers wanted to use a commercial brewer with a programmable flow rate to precisely control the total brew time to achieve their desired brew strength and extraction yield at the specified temperature. For their brewer, 93 °C was the highest they could reproducibly achieve for the large volumes of coffee needed for the trained panel assessment. They 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/ The original belief was that going several degrees below would provide a noticeable impact on the sensory profile.
The researchers intended to reflect a more-or-less “standard” coffee by using a representative “clean” wet-processed Arabica from Honduras.
Other Variables in Coffee Brewing
The study found that total dissolved solids (TDS) and percentage extraction (PE) —two dependent variables in coffee brewing—seem more correlated to sensory attributes than the independent variables like temperature. Researchers plan to continue investigating other variables in coffee brewing, such as brew time or grind, and their impact on sensory attributes.
Small differences detected
While the study did not find any significant differences in overall taste based on brew temperature, when looking at individual flavour attributes, only “Nutty” had a noteworthy difference based on the temperature at which it was brewed. On the other hand, when looking at other factors, such as total dissolved solids (TDS) and percentage of extraction (PE), eight flavour attributes, including “Bitter”, “Astringent”, and “Smoky”, were significantly impacted by TDS alone, and one attribute, “Black Tea”, was affected by PE alone. Additionally, five flavour attributes, including “Sour”, “Berry”, and “Citrus”, were significantly impacted by both TDS and PE.
What the Study Showed
The study essentially found that brew temperature does affect the rate of coffee extraction. However, the most crucial thing for coffee brewers to focus on is the final strength and extraction yield, rather than the route taken to get there. When the final brew strength (or total dissolved solids – TDS) and final extraction yield (or percentage extraction – PE) are the same in two different brews, it doesn’t matter what brew temperatures were used to achieve them. Essentially, tasters could not differentiate them: they tasted the same.
- Brew temperature, at fixed brew strength and extraction, has little impact on the sensory profile of drip brew coffee – Read article here…
- Filter stories podcast Ep2 – Coffee Extraction – listen here…
- SCA website: Just Published: Brewing Temperature and the Sensory Profile of Brewed Coffee – read here…