Director of the University of California Davis Coffee Center, Professor WILLIAM RISTENPART shares a summary of a four-year-long project on cold brew.

At the 2016 Re:co Symposium, one of our session titles was a question: “Is Cold Brew a Category, or a Craze?” That year felt like a turning point. Cold brew consumption was growing fast (in the US at least) and was driving coffee sales, especially during hot summer months. Coffee companies were releasing new cold-brew products left and right. But questions abounded: How should we define “cold brew?” Are we talking coffee brewed in the refrigerator or simply at room temperature? And how could we characterize the difference between coffee brewed with hot water versus cooler water? Was there a chemical difference between the two beverages? What about a sensory difference?

One of the speakers in that session was Julia Leach, CEO of Toddy, a leader in the cold brew world. She observed the rapid growth of the cold brew segment, but pointed out that there was still a lot to learn about cold brew. There was a lot of focus on cold brew’s reduced acidity, for example, but this hadn’t yet been thoroughly researched. And, though many people had strong intuitions about cold brew’s flavor characteristics, very little sensory science had been done on the brewing technique. At that conference, we discussed the possibilities of doing real science on cold brew.

At the 2016 Re:co Symposium, one of our session titles was a question: “Is Cold Brew a Category, or a Craze?” That year felt like a turning point. Cold brew consumption was growing fast (in the US at least) and was driving coffee sales, especially during hot summer months. Coffee companies were releasing new cold-brew products left and right. But questions abounded: How should we define “cold brew?” Are we talking coffee brewed in the refrigerator or simply at room temperature? And how could we characterize the difference between coffee brewed with hot water versus cooler water? Was there a chemical difference between the two beverages? What about a sensory difference?

One of the speakers in that session was Julia Leach, CEO of Toddy, a leader in the cold brew world. She observed the rapid growth of the cold brew segment, but pointed out that there was still a lot to learn about cold brew. There was a lot of focus on cold brew’s reduced acidity, for example, but this hadn’t yet been thoroughly researched. And, though many people had strong intuitions about cold brew’s flavor characteristics, very little sensory science had been done on the brewing technique. At that conference, we discussed the possibilities of doing real science on cold brew.

PETER GIULIANO
Executive Director, Coffee Science Foundation

Over the course of four years, the UC Davis Coffee Center researched a never-ending topic of debate: how does brewing temperature affect the sensory qualities of coffee? Now that the project is coming to an end, we’re sharing a recap of what we’ve learned.

It’s easy to find people in the coffee industry with strong opinions about the impact of temperature on coffee brewing. One school of thought is that coffee brewed for many hours at low temperatures tastes intrinsically different from hot brew coffee; many claim that the cold brew coffee is less acidic and smoother. Other people argue that chilled hot brew, made by hot brewing coffee and chilling it in a fridge or over ice, quickly yields coffee identical to cold brew made over many hours at lower temperatures.

Starting in 2020, researchers at the UC Davis Coffee Center, including me, Professor Jean-Xavier Guinard, graduate students Mackenzie Batali, Sara Yeager, and Jessie Liang, and many undergraduate research assistants, geared up to research cold brew. With support from the Coffee Science Foundation and underwriting from Toddy, we performed several series of experiments to answer the question “Is cold brew coffee different from chilled hot brew?” Our results indicate that the short answer to this question is an unqualified yes. The longer answer involves several lines of evidence.[1]

Beverage Color

First, it turns out that cold brew and chilled hot brew don’t even look the same. In our early experiments, an observant undergraduate student (Ashley Thompson) noticed qualitatively that the cold brew coffees (brewed at 4°C) had a more reddish color than the chilled hot brews (originally brewed at 92°C and chilled to fridge temperature). There is a lot of published research on the color of roasted coffee beans, but surprisingly little investigation of the color of beverage coffee. To investigate the impact of brew temperature on beverage color, we set up a careful series of experiments with three different origin coffees (a washed coffee, a honey-processed coffee, and a wet- hulled coffee), each roasted to three different roast levels (light, medium, dark), and each then brewed using either 4°C, 22°C, or 92°C water (yielding a total of nine experimental conditions, each done in triplicate for adequate statistics). Importantly, we brewed each coffee (both hot and cold) using a Toddy full immersion brewer until the coffee reached equilibrium,[2] and we then diluted them to precisely 2% total dissolved solids (TDS)—this way, differences in the strength of the brew wouldn’t affect the comparison. We then measured the brews with a high-resolution colorimeter to get quantitative, non-subjective data about the color of the brews.

**Figure 1.** The six vials display coffees from three different origins, roasted to three different roast degrees. They were all brewed to equilibrium using full immersion and then diluted to 2% TDS. The third vial (from left) was a roasted to a “dark” roast degree and then brewed at 92°C. The sixth vial was roasted to the same fixed roast degree and then brewed at 4°C. You can read the full research at Yeager et al., 2022.

Our systematic quantitative measurements[3] (led by graduate student Sara Yeager)[4] strongly corroborated our initial qualitative findings (see figure 1). The roast level strongly affected the color, with darker roasts yielding blacker brews, but the brew temperature also had a significant impact on the color, with cold brew indeed more reddish than the corresponding chilled hot brew. This color effect is potentially quite important because color can affect perceived sensory characteristics, and color is typically the first assessment of quality a consumer makes. Many consumers moreover associate reddish colors with sweetness, suggesting that cold brew coffee is advantageous if perceived sweetness is a desired sensory quality for the brew.

Acidity and Flavor

Aside from color, we also measured a host of chemical and physical properties for each brew. A key chemical finding is that the chilled hot brews invariably had a lower pH (i.e., were more acidic) than the cold brew coffee (see figure 2). The roast level has a much more pronounced effect on the pH, with light roast averaging about 4.9, medium roast about 5.1, and dark roast about 5.5. But at fixed roast level, and when we carefully controlled the TDS to be precisely 2%, the cold brews were about 0.1 to 0.2 pH units higher than the chilled hot brews. These results support the notion that cold brews are indeed less acidic than hot brews, at least when they are all brewed fully to equilibrium.

Of course, we didn’t go to all the effort of brewing 27 different types of coffee under careful conditions without bothering to taste them. Postdoctoral researcher Mackenzie Batali assembled a panel of tasters who underwent extensive training and calibration with appropriate sensory references. The calibrated panel then blind tasted all 27 types, in triplicate, at 2% TDS and 4°C to assess each brew for a variety of different sensory attributes (e.g., bitter, berry, citrus, smoky). Each of the 10 trained panelists assessed the intensity of more than two dozen sensory attributes in each of the 81 samples, yielding a huge amount of data.[5]

The key sensory finding is that, even when averaged over wildly different coffee origins and roast levels, the cold brew coffees were statistically significantly more floral and less bitter, less sour, and less rubbery than the chilled hot brews (see figure 3). Again, we took care to make sure all the brews were served at exactly the same strength and the same serving temperature, so the results really implicate the key role of the brew temperature in affecting key sensory attributes. Floral is typically considered a desirable attribute, while sour, bitter, and especially rubbery are typically considered negative attributes,[6] suggesting that in general, cold brew is better than chilled hot brew for yielding desirable flavor profiles.

Brew Time and Extraction Dynamics

A key aspect of these two studies is that everything was measured after brewing the coffee to equilibrium (taking more than 24 hours in the case of 4°C brew temperature) and then diluting it to a specific strength. We wondered whether we would see different results if we used different brew times. Accordingly, we started up a new series of experiments to investigate the dynamics of cold brew. Specifically, we wanted to answer the questions, How does the flavor profile of cold brew develop versus brew time? and How does it compare to chilled hot brew?

We first examined the dynamics of extraction in full immersion coffee for different brew temperatures. In other words, how does the TDS vary with brew time? Led by Dr. Jessie Liang (then a grad student), we set up an experiment similar to the previous ones, testing three origins, three roast levels, and three brew temps (4°C, 22°C, or 92°C), and every few minutes during the brew, we used a pipette to withdraw a small sample to measure the TDS at that point in time.[7] What we observed is that, regardless of coffee origin, roast level, or brew temperature, the TDS almost always followed a similar trend with a shape that we termed the “universal brew curve”[8] (see figure 4). Upon first adding the water to the coffee grounds, as soon as we could take a sample, we invariably measured a significant non-zero TDS. In other words, a large quantity of soluble material very rapidly dissolves into the liquid upon contact, yielding what we termed the “rinse concentration.” After the rapid initial rinse, the TDS increases more gradually at a rate that depends very sensitively on the brew temperature. Eventually the TDS reaches the “equilibrium concentration,” where it stops increasing with time. Importantly, full immersion hot brews also follow this universal brew curve, with a characteristic time of about 20 to 30 minutes to reach equilibrium, in contrast to room temperature, which takes about five hours, and fridge temperature, which takes 10 or more hours, to reach equilibrium.

These extraction dynamics have a huge implication for comparisons of hot brew versus cold brew: we need to make sure that the TDS is the same in each brew measured before attempting to make comparisons. For example, a five-minute hot brew might have a TDS of 1%, while a 24-hour cold brew could have a TDS of 4%. Because extraction is so influential, the hot brew would naturally have a much lower acidity simply because it has less dissolved solids in it. One could also do a very long hot brew and compare it to a very short cold brew and obtain the opposite result. Before making judgments about the acidity of cold brew compared to hot brew, first ask, “Is the TDS the same in both beverages?”

**Figure 3.** Effect of brew temperature on specific sensory attributes, as measured by a trained and calibrated panel of 10 tasters on 100-point scales, of full immersion coffee brewed to equilibrium and then diluted precisely to 2% TDS. Data averaged over three different coffee origins and three different roast levels. Adapted from Batali et al. 2022.

These results are still in peer review, so here we provide a sneak preview based on Dr. Liang’s dissertation.[9] A snapshot of all the sensory attributes versus time is shown in figure 5, with several trends readily apparent. First, the rinse concentration almost invariably had the most different sensory characteristics compared to the coffee sampled during other phases, regardless of brew temperature or roast level. For many attributes (e.g., bitter and sour), the intensity was lower in the rinse concentration, as expected, because the TDS is lowest at that time point. Intriguingly, the very important sensory attribute of sweetness had higher intensities in the initial rinse concentration. This result accords with a number of studies that have indicated that perceived sweetness in black coffee is negatively correlated with TDS.[10] The results in figure 5 show that the same trend with sweetness also occurs for cold brews, so if the only concern is maximizing perceived sweetness, then the appropriate brewing procedure is to add water and immediately remove the coffee grounds to get that initial rinse brew.

Of course, that procedure won’t necessarily yield a balanced brew, so longer brew times are necessary to extract more solids and develop the brew. A perhaps surprising aspect of the sensory data in figure 5, however, is how many of the sensory attributes vary little with brew time after the initial rinse. Notable exceptions are bitter, burnt, and rubber, which continued to evolve even well after the initial rinse (especially for dark roast). This observation is consistent with the idea that the molecules responsible for bitter, burnt, and rubber sensory attributes generally tend to be less readily soluble than others, meaning they require more time to extract. The most important observation is that the cold brews systematically had less bitter, burnt, and rubber intensity than the chilled hot brews, regardless of time point, corroborating the main results from our prior study (see figure 3).

Conclusions

Taken together, these experiments strongly support the notion that cold brew differs significantly from chilled hot brew: how it looks, how acidic it is, and, most importantly, how it tastes. In comparison to chilled hot brew, cold brew consistently exhibited attributes typically considered more favorable.

What does this mean for coffee industry professionals? Should coffee industry professionals avoid using chilled hot brew? Doubtless many will continue to be attracted to the speed of chilled hot brew, which inarguably has the advantage of yielding a chilled coffee beverage within a few minutes rather than hours. But the key take-home message here is best summarized as “Good things come to those who wait.” And importantly, the wait times might not be as long as some expect. Our dynamic data suggests that cold brews are done within about 5 to 6 hours for room temperature and about 10 to 12 hours for fridge temperature, where “done” means that the TDS is effectively at its final equilibrium value and the sensory attributes cease to change appreciably.

So, if you’re accustomed to chilling hot brew, you might consider experimenting with some true cold brews—you might be pleasantly surprised. And if you’re already accustomed to performing 24-hour or longer cold brews, you might consider experimenting with shorter brew times—you might also be pleasantly surprised. In either case, we hope the research summarized here helps you in the quest to brew the most delicious coffee.

WILLIAM RISTENPART is a professor of chemical engineering at the University of California, Davis, and the founding Director of the UC Davis Coffee Center. The opinions expressed herein are entirely his own.

References

[1] A more detailed account of this study is published in Jiexin Liang, Mackenzie E. Batali, Catherine Routt, William D. Ristenpart, and Jean- Xavier Guinard, “Sensory Analysis of the Flavor Profile of Full Immersion Hot, Room Temperature, and Cold Brewed Coffee Over Time,” Sci Rep 14 (2024): 19298, https://doi.org/10.1038/s41598-024-69867-6.

[2] As part of our research into cold brew, we performed a

detailed investigation of the concept of equilibrium in full immersion brewing. See 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.

[3] The beverage color results are published in Sara E. Yeager, Mackenzie E. Batali, Lik Xian Lim, Jiexin Liang, Juliet Han, Ashley N. Thompson, Jean-Xavier Guinard, and William D. Ristenpart, “Roast Level and Brew Temperature Significantly Affect the Color of Brewed Coffee,” Journal of Food Science 87, no. 4 (2022): 1837, https://doi.org/10.1111/1750-3841.16089.

[4] As part of her dissertation, Sara also wrote and published a tour-de-force meta-analysis of acid concentrations present in coffee. See Sara E. Yeager, Mackenzie E. Batali, Jean- Xavier Guinard, and William D. Ristenpart, “Acids in Coffee: A Review of Sensory Measurements and Meta-Analysis of Chemical Composition,” Critical Reviews in Food Science and Nutrition 63, no, 8 (2023): 1010, https://doi.org/10.1080/10408398.2021.1957767.

[5] Mackenzie E. Batali, Lik Xian Lim, Jiexin Liang, Sara E. Yeager, Ashley N. Thompson, Juliet Han, William D. Ristenpart, and Jean-Xavier Guinard, “Sensory Analysis of Full Immersion Coffee: Cold Brew Is More Floral, and Less Bitter, Sour, and Rubbery Than Hot Brew,” Foods 11, no. 16 (2022): 2440, https://doi.org/10.3390/foods11162440.

[6] See, for example, the “hedonic penalty/lift” data in figure 4 in Jean-Xavier Guinard, Scott Frost, Mackenzie Batali, Andrew Cotter, Lik X. Lim, and William D. Ristenpart, “A New Coffee Brewing Control Chart Relating Sensory Properties and Consumer Liking to Brew Strength, Extraction Yield, and Brew Ratio,” Journal of Food Science 88, no. 5 (2023): 2168, https://doi.org/10.1111/1750-3841.16531.

[7] Dr. Liang’s dissertation for her PhD in food science and technology is titled “The Extraction Dynamics and Sensory Quality of Full Immersion Brewed Coffee” (University of California, Davis, 2023), https://escholarship.org/uc/item/8cd8n7j8.

[8] An exception is when we tested extremely high mass ratios of coffee grounds to water. Coffee grounds typically float upwards to the top of the brewer because they initially contain a lot of trapped gas, but as that gas escapes and water infiltrates into the grounds, they eventually sink. Sometimes at high mass ratios the coffee grounds undergo ”sudden collapse” events, where most of the grounds suddenly sink and stir up the fluid, causing a big jump in the TDS. These sedimentation physics are explored in detail in Jiexin Liang, Hudson G. Brown, Megan M. Dunkin, and William D. Ristenpart, “Concentration Gradient Inversion via Sedimentation of Coffee Grounds in Full Immersion Brewing,” Journal of Food Engineering 357 (2023): 111619, https://doi.org/10.1016/j.jfoodeng.2023.111619.

[9] Liang, “Extraction Dynamics and Sensory Quality.”

[10] See, for example, Mackenzie Batali, Carlito Lebrilla, Jean-Xavier Guinard, and William D. Ristenpart, “Less Strong, More Sweet,” 25, November 28, 2019, https://sca.coffee/sca-news/25-magazine/issue-11/less-strong-more-sweet.

We hope you are as excited as we are about the release of 25, Issue 22. This issue of 25 is made possible with the contributions of specialty coffee businesses who support the activities of the Specialty Coffee Association through its underwriting and sponsorship programs. Learn more about our underwriters here.

Worth the Wait: How Cold Brew Differs from Chilled Hot Brew | 25, Issue 22