Cold-brew coffee is so hot right now, and not just with hipster consumers. Scientists at Thomas Jefferson University in Philadelphia have been taking a deeper look at the underlying chemistry to better understand how the cold-brew method alters coffee's chemical characteristics, with an eye toward pinpointing the best way to cold brew the perfect cup. They had originally planned to present their results last month at the American Chemical Society meeting in Philadelphia, but the COVID-19 pandemic shut that down. So instead, they presented the information in a virtual poster session.

Rather than pouring boiling or near-boiling water over coffee grounds and steeping for a few minutes, the cold-brew method involves mixing coffee grounds with room-temperature water and letting the mixture steep for anywhere from several hours to two days. Then it is strained through a sieve to filter out all the sludge-like solids, followed by filtering. This can be done at home in a Mason jar, or you can get fancy and use a French press or more elaborate Toddy system. It's not necessarily served cold (although it can be)—just brewed cold.

Co-author Niny Rao tasted her first cup of cold-brew coffee a few years ago while attending an ACS conference in San Diego. "I was like, 'Oh, my god, this is great!'" Rao told Ars. "It's not metallic. It's non acidic. It has a little bit of sweetness to it, and it's very full and flavorful."

Ever the scientist, Rao did some Googling when she got home and tried to make her own cold brew. That first experiment did not go well. And thus a research subfield was spawned. Rao teamed up with her TJU colleague Megan Fuller, and the two went down the experimental rabbit hole to unlock the chemical secrets of a good cold brew.

"It turned out that there is a lot of research on coffee but not much research on cold-brew coffee," said Rao. That's partly because the biggest coffee-brewing countries (Italy, Turkey, Brazil, Colombia, for example) are all devoted to hot-brew coffee, like espresso. The cold-brew trend is mostly centered in North America.

"There are a lot of studies on espresso," said Rao. "We thought it would be a good idea to put some information out there for consumers and enthusiasts like me who want to make their own cold-brew coffee."

In one paper, published in 2018, Rao and Fuller measured levels of acidity and antioxidants in batches of cold- and hot-brew coffee. But those experiments only used lightly roasted coffee beans. The degree of roasting (temperature) makes a significant difference when it comes to hot-brew coffee. Might the same be true for cold-brew coffee? To find out, the pair teamed up with one of their undergraduate students, Meghan Grim, to explore the extraction yields of light-, medium-, and dark-roast coffee beans during the cold-brew process.

"A lot of chemicals are more soluble at high temperatures than at lower temperatures," said Rao. "So when you're brewing hot-brew coffee, when you pour water even just below the boiling point over coffee grounds, you are extracting all kinds of compounds. Some of them may not be that desirable. For the cold brew, you are selectively extracting compounds that are only soluble at lower temperatures." That may include key flavor compounds.

They used the cold-brew recipe from The New York Times for their experiments, with a water-to-coffee ratio of 10:1 for both cold- and hot-brew batches. (Hot brew normally has a water-to-coffee ratio of 20:1, but the team wanted to control variables as much as possible.) [corrected]

Reproducibility is always a challenge, of course: even professional baristas recognize how hard it is to make a cup of cold brew that's consistent every time. Rao noted that—despite performing experiments with the same beans, using the same machine, on the same settings—there were still differences between the early batches she and her colleagues made. But eventually they settled on a strict, standardized procedure, carefully controlling when water was added to the coffee grounds, how long to shake (or stir) the solution, and how best to press the cold-brew coffee.

For the lighter roasts, Rao et al. found that caffeine content and antioxidant levels were roughly the same in both the hot- and cold-brew batches. But there were significant differences between the two methods when medium- and dark-roast coffee beans were used. Specifically, the hot-brew method extracts more antioxidants from the grind; the darker the bean, the greater the difference. Both hot- and cold-brew batches become less acidic the darker the roast. An academic paper on the results is forthcoming.

"My advice to consumers has always been to drink what they like," said Rao. "But if you want to craft a coffee beverage with antioxidants or acidity in mind, you may want to pay attention to roast. If you want a low-acid drink, you may want to use a darker roast. But remember that the gap between the antioxidant content of hot- and cold-brew coffee is much larger for a darker roast."

Next, Rao and her colleagues plan to extend their research to exploring how the cold-brew versus hot-brew processes and roasting temperatures affect the flavor compounds in raw coffee beans, called furans. "I was hoping to finish that project by now, but, well, the pandemic put a ding on that plan," she said. "The [university lab] building is completely shut down." As for home experiments, cold-brew requires significantly more coffee than the hot-brew method, and her household is rationing the precious coffee supply just like everybody else.

Thanks to all that in-depth research, Rao now makes a mean cup of cold-brew coffee and can offer a few useful tips. First, "Even differential temperature makes a difference," she said, recommending that home brewers leave their coffee on the counter for 10 hours or so rather than putting it in the refrigerator. If you do go the refrigerator route, she advises letting it steep even longer.

Second, source your coffee wisely, since different coffee beans from different regions can have very different flavor profiles. Also, pay attention to the roast, which is a major factor in coffee chemistry in general. "When you apply heat to beans, you start to see color changes—the smell is different," said Rao. She uses a blend of beans for her home cold brews: half medium roast, half dark roast.

Finally, Rao suggests triple-filtering your cold-brew coffee. "Extraction is a continuous process," she said. If there are unwanted particles in your coffee grinder, that can impart an undesirable moldy or musky flavor to the final cup. Instead of using a regular sieve or filter paper, she stacks two filters. "It takes a while for the coffee to filter, but I end up with better coffee," she said.

But seriously, though: 10 hours to make a cup of coffee? "You can set it up after you get home [in the evening] and the next morning you just filter, and then you have coffee," said Rao. For those of us who are perhaps less patient and/or less likely to plan ahead that efficiently, it's probably best to stick with the classic hot-brew method.

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