How Hot is Hot Enough? Brew Temperature, Sensory Profile, and Consumer Acceptance of Brewed Coffee | 25, Issue 15
From farm to cup, a wide range of variables influences the final flavor of a brewed cup of coffee.
ANDREW COTTER considers the results of recent scientific publications focused on the impact of coffee’s brewing temperature on consumer preferences of brewed coffee.
Many of these variables—such as growth altitude, processing method, and roast level—can be controlled on a large scale by farmers and roasters in an effort to produce many bags of coffee that are nearly identical to one another. However, whole coffee beans in bags have not yet finished their process. Additional steps, such as grinding and brewing, are needed to make the final beverage that people will enjoy as their coffee. With these steps, a host of variables—that may influence the flavor of the brew and, in turn, how consumers like it—are in the hands of baristas and home brewers alike.
Brew temperature is widely considered to be one of these key variables affecting the final quality of coffee, with a temperature near 93°C believed by most to be best. And, while it is no surprise then that drip brewers that can’t reach 92°C fast enough fail their certification, it turns out there is little empirical (and even less scientific) evidence, however, to support any particular range of brew temperatures for optimal sensory quality and consumer acceptance.
Measuring Temperature’s Impact
One of the most useful ways to monitor the results of a given brewing process is to measure the physical properties of the finished beverage and the grounds from which it was extracted. Within the industry, two of the most commonly used measures are total dissolved solids (TDS) and percent extraction (PE).
Brewed coffee is a liquid that is mostly water with a small fraction of actual coffee materials dissolved or suspended in it, which comes courtesy of the ground coffee. Prior to brewing, these grounds were dry and full of all sorts of hydrophilic or water-loving components such as acids, sugars, and caffeine. Hot water is passed through the grounds, and many of these components are extracted from the grounds into the water. TDS refers to the total concentration of this coffee material in the water at the end of brewing, which can be expressed as a percentage (for example, a cup of coffee with a TDS of 1% is 99 parts water and roughly 1 part coffee material.)
Percent extraction is a related metric, but instead of focusing on the “water + coffee” solids mixture, we are now focusing on the spent coffee grounds. If we were to take spent coffee grounds, dry them, and then weigh them, we would find that the weight of the spent grounds is less than the weight of the grounds prior to brewing. This is due to the grounds giving up some of their matter to the water. The percentage change in the weight of the dry grounds from pre-brew to post-brew is the percent extraction.
There are a variety of brewing related variables that can ultimately impact the brewing process to the point that one would observe changes in these physical measures, and therefore in the flavor of the brewed coffee (an idea originally proposed by Ernest Earl Lockhart in the 1950s.) For example, TDS and PE can both be increased by using a more finely ground coffee. Another way to impact TDS and PE is by changing the temperature of the brewing water. It is reasonable to think that a higher temperature water would extract coffee solids more quickly than a lower temperature water, but it is less clear whether changes in water temperatures alone will result in flavor changes, or if these changes are simply a result of water temperature exerting its influence on TDS and PE. What happens if we adjust our brewing parameters enough to achieve similar TDS and PE profiles at different brewing temperatures?
Most coffee consumers have a general sense of a rather extreme example of this. Cold brew coffee, often brewed at room or refrigerator temperature, has a flavor that is very different than that of hot-brewed then chilled iced coffee (the former of which usually being described as less acidic, less bitter, and sweeter). However, there is lively debate about water temperature even within the realm of drip brewed hot coffee. A recent series of studies out of the University of California Davis Coffee Center, in partnership with the Specialty Coffee Association’s Coffee Science Foundation and supported by Breville, investigated the effects of water temperature, as well as TDS and PE, on the flavor profile and consumer acceptability of drip brewed coffee.
Building an Understanding
Figure 1. An example of how similar TDS and PE profiles were achieved at different brewing temperatures by adjusting other parameters such as grind size, brew time, and duty cycle.
The studies employed two different panels of coffee tasters. One panel consisted of a small group of expert judges (12) who were highly trained to evaluate individual flavor descriptors (think “roasted,” “citrus,” “bitter,” etc.) and rate their intensities using linear scales. A second panel was made up of a larger group of consumers (118) who regularly consumed black coffee. These panelists were asked questions more focused on how much they enjoyed the flavor of the various coffees. Both panels evaluated the same set of coffees: a medium roast single origin coffee sourced from Honduras brewed 27 different ways to cover a wide range of TDS, PE, and brew temperatures. In total, this study had 433 different batches of coffee brewed and 4,158 cups of coffee tasted.
Water temperatures ranged from 87°C–93°C, and similar TDS and PE values were achieved at each temperature to evaluate the effects of brewing temperature alone on flavor quality and consumer acceptance. Figure 1 shows how other brewing metrics such as grind size, dose, and duty cycle (water on/off pattern) were adjusted to achieve similar extraction profiles at different water temperatures.
What we found was surprising: A variety of sensory attributes showed significant variation with TDS and PE. For example, the intensity of sour taste was highest in coffees that were low in PE (around 16%) and high in TDS (around 1.5%), while the intensity of brown roasted flavor was strongest at higher PE (around 24%), high TDS (1.5%) coffees. These results validated some of the principles that were proposed by E.E. Lockhart in his development of the original Coffee Brewing Control Chart and expanded on them with more modern methods of evaluating flavor. Similar trends were also observed when these physical measures were investigated in a study looking at the effect of roast level on the flavor profile of a single origin coffee. The TDS and PE sensory results have been discussed in detail previously in 25, Issue 13 (“Towards A New Brewing Control Chart”).
The flavor modulating effects of TDS and PE were large enough to be noticed even by the untrained consumer panel, and they impacted how much the individual coffees were liked. It was found that acceptability of the coffee was most sensitive to changes in TDS, and that there were two distinct clusters of consumers formed based on TDS. Some consumers tended to prefer TDS values around 1% while others enjoyed stronger coffees with TDS values above 1.25%. PE played a secondary role in consumer acceptance—its effects were most evident at high TDS values where a very low PE can result in a cup of coffee with a stronger sour taste.
Figure 2. The intensities of taste and flavor descriptive attributes as a function of (A) total dissolved solids, (B) percent extraction, and (C) brewing temperature.
Figure 3. Average overall liking of the two preference clusters as a function of (A) total dissolved solids, (B) percent extraction, and (C) brewing temperature.
But what about water temperature? Did we find any differences in flavor when we changed water temperature at a fixed TDS and PE? In short, no. Relative to TDS and PE, any changes in flavor that were a result of changing brewing temperature alone were so small (Fig. 2) that they were considered mostly insignificant when statistical tests were ran on the data. Brew temperature also did not seem to matter to the consumer panel either. Despite the consumers noticing slight differences in the consumption temperature based on the various brewing temperatures, this variable did not turn out to be an important predictor of acceptability and no strong segmentation in preferences was found to be driven by this variable (Fig. 3).
What do these results mean? On the one hand, it was demonstrated that changing brewing parameters to influence physical measures such as total dissolved solids and percent extraction can lead to a wide variety of flavor profiles, even when one is working with the exact same coffee. On the other hand, there is no “one size fits all” solution to which TDS and PE values are the best in the eyes of consumers. When we think about brewing temperature, we first need to consider how raising or lowering the brewing temperature is going to impact the extraction. If you can adjust your process to achieve the desired TDS and PE at different temperatures (at least within the range of 87°C–93°C), we will expect to see little to no change in the flavor profile delivered by the brewed coffee or to the consumer’s response to them. ◇
ANDREW COTTER is a sensory scientist and former researcher at the University of California, Davis focusing on the Coffee Brewing Control Chart. This feature is based on two scientific publications: “Consumer Preferences for Black Coffee are Spread Over a Wide Range of Brew Strengths and Extraction Yields,” published in the Journal of Food Science, and written by Andrew Cotter, Dr. Mackenzie Batali, Professor William D. Ristenpart, and Professor Jean-Xavier Guinard; and “Brew Temperature, at Fixed Brew Strength and Extraction, Has Little Impact on the Sensory Profile of Brewed Coffee,” published in Scientific Reports, and written by Dr. Mackenzie Batali, Professor William D. Ristenpart, and Professor Jean-Xavier Guinard.
These were published as a part of the Coffee Science Foundation research project, “Towards a Greater Understanding of Coffee Brewing Fundamentals,” underwritten by the Breville Corporation. Learn more at sca.coffee/brewing-research.
References
Batali, M.E., Ristenpart, W.D. and Guinard, J.-X. (2020), Brew temperature, at fixed brew strength and extraction, has little impact on the sensory profile of drip brew coffee. Scientific Reports 10, 16450. https://doi.org/10.1038/s41598-020-73341-4
Cotter, A.R., Batali, M.E., Ristenpart, W.D. and Guinard, J.‐X. (2021), Consumer preferences for black coffee are spread over a wide range of brew strengths and extraction yields. Journal of Food Science, 86: 194–205. https://doi.org/10.1111/1750-3841.15561
Frost, S. C., Ristenpart, W. D., and Guinard, J.-X. (2020), Effects of brew strength, brew yield, and roast on the sensory quality of drip brewed coffee. Journal of Food Science, 85(8), 2530–2543. https://doi.org/10.1111/17503841.15326
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