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What’s the Chemical Difference Between Hot- and Cold-Brew Coffee?

Scientists put on their barista aprons and found out.

Popular Mechanics

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Cold brew coffee in a clear cup

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What’s in your cold brew coffee, and how does it differ from a traditional hot brew?

For a presentation poster, scientists brewed many batches to identify differences in the chemistry between cold and hot brew coffee. They confirmed ongoing study results that show hot brewed coffee has more antioxidants (and acidity) than cold brew. They also took their research a step further and examined these levels in different temperatures of roasts, from light to dark.

For their research, the scientists from Philadelphia’s Thomas Jefferson University had to develop the most standard possible brew. Coffee aficionados already face this question every day, and they agree that a burr grinder works best to grind beans into a uniform consistency. And many use water heaters to reach a consistent recommended temperature just below boiling.

Both of these mechanics are imperfect. That’s on top of varying levels of roast even within narrow ranges like “light” and “medium.” Coffee is one part science and one part fault-tolerant art form.

So to try to homogenize their research as much as possible, the researchers did everything they could to control the parameters. “They developed a procedure for when the water should be added to the ground coffee, how to pour the water and for how long, how to shake the solution, how to press the brewed coffee and how to analyze it. They set time limits for each step, with margins of just a few seconds,” the American Chemical Society (ACS) said in a statement.

What the scientists found will interest coffee lovers. Overall, pH of hot and cold brews at the same “roastiness” are similar, which goes against popular wisdom that cold brew coffee is less acidic than hot. (Other studies back up this finding.) The researchers observed that pH gets higher—more basic—as the level of roasting gets higher, so the darkest roasts have the lowest acidity.

Brewing hot coffee also results in a product with much higher antioxidant content. For lighter roast coffees, the difference is smaller, but for dark roast coffees, hot brewing extracts far more antioxidants than cold brewing. Hot brewing also has higher amounts of specific kinds of acids and dissolved solids, despite its overall similar pH.

The researchers don’t speculate about this, but it could be this higher level of some acids that results in the perception that hot brew is more acidic than cold brew.

The researchers presented their poster as part of the ACS’s spring national meeting. The meeting itself was canceled because of COVID-19 (coronavirus), but participants were invited to share their research presentations online. Even so, the website compiling that research is a bit of a ghost town.

Caroline Delbert is a writer, avid reader, and contributing editor at Pop Mech. She's also an enthusiast of just about everything. Her favorite topics include nuclear energy, cosmology, math of everyday things, and the philosophy of it all.

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This post originally appeared on Popular Mechanics and was published April 6, 2020. This article is republished here with permission.

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