If you’ve ever left a hot cup of coffee to cool down, you’ll probably notice it doesn’t taste quite as good as your favorite cold brew coffee. Nor does the hot-brewed coffee you pour over ice to make a quick cold-brew substitute.
Hot- and cold-brewed coffee have obvious temperature and flavor differences, but the variances go deeper. Researchers are beginning to uncover some chemical differences between the two.
Cold-brewed coffee is a relatively new trend and hasn’t been studied as extensively as hot-brewed coffee, said Niny Rao, a chemistry professor at Thomas Jefferson University in Philadelphia who with colleague Megan Fuller has started researching cold brew. The research was inspired by Rao’s own struggles making cold brew at home.
“When I tried it, I realized it wasn’t working out,” Rao said. “So I convinced Dr. Fuller to help me in the lab to figure out exactly what was going on.”
The two have looked at how brewing methods and the ways coffee beans are roasted impact the caffeine, acidity and antioxidant properties of the resulting brew.
Coffee brewing is a science
“Coffee brewing is a very complex process,” Rao said. “Coffee itself is a very complex beverage.”
Peter Giuliano is chief research officer at the Specialty Coffee Association and executive director of the Coffee Science Foundation, which recently announced its own research project in partnership with cold-brew coffee company Toddy on cold brew’s chemical and sensory aspects.
“All the factors of coffee brewing — water temperature, grind size, brewing time, filter type, coffee type — have an effect on coffee flavor,” Giuliano said. “Just a little understanding of the science can help consumers radically improve the flavor of their home-brewed coffee.”
Hot brew has higher antioxidant levels
Coffee is rich in antioxidants, including polyphenol chlorogenic acid, or CGA, said Megan Meyer, director of science communications at the International Food Information Council. CGA has anti-diabetic, anti-cancer, anti-inflammatory and anti-obesity effects, and could help reduce the risk for some chronic diseases, research shows.
In terms of health benefits, research published in Scientific Reports in 2018 found hot-brewed coffee retains higher levels of antioxidants than cold brew.
The type of roast you choose also determines antioxidant levels. In her team’s latest research, which hasn’t been published yet, Rao said both hot- and cold-brewed coffee had similar antioxidant levels when lighter roasts were used. For dark roasts, hot brewing keeps more antioxidants intact than cold brew.
That doesn’t mean one brewing method is necessarily healthier than the other, Rao said. But if you’re worried about oxidative stress, the free radical-antioxidant level imbalance in the body, a hot-brewed dark roast may be best.
But heat breeds bitterness and acidity
Emily Rosenberg, director of education and training operations at Stumptown Coffee, previously explained to HuffPost that CGA also affects bitterness.
When the CGA in unroasted coffee is heated during the roasting process, it’s broken down into quinic acid and caffeic acid, which have an even more pronounced bitter, astringent flavor. And the longer coffee is applied to heat, the more gnarly the flavor gets as those acids develop. That means hot-brewed coffee tends to taste more bitter and acidic than cold brew, even if it’s made with the same beans.
Those lower levels of bitterness and acidity in cold-brewed coffee also make it less likely to cause heartburn or other digestive problems.
There’s more scientific basis to the claim that cold brew is less acidic. Rao’s research shows the pH levels of hot- and cold-brewed coffee are mostly comparable, ranging from 4.85 to 5.13. (A pH of 7 is neutral, with 0 being the most acidic.) But, hot-brewed coffee was found to have a higher concentration of acids.
Darker coffee roasts tend to have lower acidity levels than lighter roasts, Rao said.
“If you’re talking about drinking something that is lower in acidity, your best bet might be a cold-brewed, dark-roast coffee,” she said.
Caffeine levels are about the same
The Mayo Clinic clocks an 8-ounce cup of coffee as containing about 96 mg of caffeine, and 1 ounce of espresso has 64 mg (though the caffeine content can vary greatly between brands and varieties of coffee). For a moderate caffeine intake, the daily recommendation is 400 mg, or three to four cups, Meyer said.
Hot- and cold-brewing methods don’t specifically affect the amount of caffeine. The caffeine quotient is more likely determined by the coffee-to-water ratio you choose, Giuliano said. Less water produces a more caffeinated cup.
“Think of the difference between espresso versus drip coffee,” Giuliano said. “While they have similar levels of caffeine in their respective servings, when comparing total caffeine levels in the same volume, espresso has significantly more caffeine.”
Brewing time also impacts caffeine, acidity and antioxidant levels. A 2017 study published in Scientific Reports found that for cold-brewed coffee, most caffeine in medium- and dark-roast beans is extracted after 400 minutes.
Caffeine is comparable across roasts, Rao said.
“If you’re going to drink cold brew or hot brew for the caffeine, cold brew may have a higher caffeine level because of the grind-to-water ratio,” Rao said. “It’s not because one has more caffeine than the other.”
How science can help you make better coffee at home
Water-to-coffee ratio also influences coffee’s strength and ultimately its flavor, Giuliano said. He urged home coffee brewers to experiment with different amounts of coffee and water, and with different brewing methods, to find their ideal cup.
“I will say the No. 1 mistake people make is using too little coffee grounds when they are making coffee at home,” Giuliano said.
The “standard gold cup” recommendation for hot-brewed coffee is 1 ounce of coffee per 18 ounces of water, he said. The Coffee Science Foundation is still working on the ideal cold brew ratio, but The New York Times’ “Cold-Brewed Iced Coffee” recipe calls for one-third cup of ground coffee to 1.5 cups of cold water to brew a single cup. For a bigger batch, similar to Starbucks’ cold brew, The Kitchn uses 8 ounces of coffee to 8 cups of water.
If you want to consistently brew a great cup of coffee at home, Rao suggests using a scale to weigh coffee grounds, keeping a record of your preferred ratio and understanding the science of brewing methods.
“Knowing exactly what kind of compounds are in there, you can start kind of crafting this a little bit more intelligently and scientifically to make yourself something enjoyable,” Rao said.