Cold-brew vs hot-brew coffee: what's the difference?

Cold-brew may be one of the hottest trends in coffee-making, but not much is known about how this process alters the drink’s chemical characteristics. Scientists have found that the content of potentially health-promoting antioxidants in coffee brewed without heat can significantly differ from coffee prepared with the same beans in the traditional way, particularly for dark roasts. The researchers are presenting their results through the American Chemical Society (ACS) SciMeetings online platform.

“This study can inform coffee enthusiasts about how they may want to craft their own coffee at home, based on science and on analytical chemistry,” said Meghan Grim, an undergraduate student who worked on the project.

In the cold-brew process, ground coffee is mixed with room-temperature or colder water and steeped for as long as two days, sometimes in a refrigerator. It can be consumed cold or hot. In hot-brewing, ground coffee is mixed with boiling or near-boiling water and steeped at most for a few minutes. In either case, the coffee grounds are sometimes pressed and the beverage is then filtered to remove the grounds.

“After I first tried cold-brew coffee, I wanted to make it at home, but it didn’t turn out too great,” said Niny Z Rao, PhD, the project’s principal investigator. Rao then looked into the scientific literature to learn what she had done wrong. “It turns out there weren’t any studies on cold-brew coffee at the time, so we decided to do some,” Rao said.

The researchers had previously assessed acidity and antioxidants in hot- and cold-brew coffee from lightly roasted beans. They also analysed the effect of medium and dark roasting, and of medium and coarse grinding, on a few coffee attributes. Researchers are now roasting Columbian beans at five different temperatures — from light roasting at 174°C to dark roasting at 209°C — and studying the impact on acidity, antioxidants and other natural chemicals in hot- and cold-brew coffee.

With the lighter roasts, researchers found that the content of caffeine and antioxidants is pretty similar in hot and cold brews. However, there are significant differences with the darker roasts.

“Hot-brewing extracts more antioxidants from the grind than cold-brew, and this difference increases with the degree of roasting,” Rao said.

Through this, it can be deduced that hot-brewing dark roasts produces a potentially healthier drink. Hot-brew also contains more of certain kinds of acids, and total dissolved acids. The pH is similar for hot and cold brews at equivalent roast, though both become less acidic as roasting temperature increases.

“My advice to consumers has always been to drink what they like. 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,” Rao said.

One of the biggest challenges researchers faced in these projects was reproducibility.

“We have performed experiments in which we’re using the same beans, the same machine, the same settings, and it comes out not quite the same as the previous batch. The same goes for brewing. To create a cup that’s consistent every time is really difficult. I have a lot of respect for the baristas who can do that,” Rao said.

To approach this level of reproducibility, researchers standardised their processes and 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 analyse it. Researchers also set time limits for each step, with margins of a few seconds. This produced more reproducible results.

Researchers have now begun comparing the impact of the two brewing processes and degree of roast on furans, flavour compounds found in raw coffee beans that are also generated through roasting. Such compounds are important in cold-brew coffee, which does not have the same distinct aroma as hot-brew, as it lacks the steam to carry volatile organic compounds to the nose.

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