If you like your coffee on the colder side, you may have noticed that there’s a distinct flavor difference between iced coffee and cold-brew coffee. That difference is real, and knowing why it exists requires a fascinating lesson in chemistry.
WHAT’S THE DIFFERENCE?
Although they might look the same, iced and cold-brew coffee are made in very different ways. Iced coffee involves brewing coffee hot, then either cooling it down slowly or pouring it over ice to chill it more quickly. Cold brew, as the name implies, involves brewing the coffee in cold water. As UCLA’s Science and Food blog puts it, “Hot brew quickly produces fragrant java with bite and acidity, whereas cold brew rewards patience with condensed coffee that is smooth and sweet.” Coffee takes longer to brew cold than hot because heat speeds up chemical reactions—a principle that’s at play in the flavor differences, as well.
A number of chemical reactions take place during the brewing process. The coffee grounds release gases, some into the air, but others into the brew itself. Oils, acids, and other aromatic elements, known as coffee solubles, are extracted from the grounds during the brewing process, too. All of this happens much faster when you use hot water to brew coffee than when you use cold water. With hot brew, that means that you get that room-filling aroma and full-bodied flavor of hot coffee—but it also means that the brew degrades and oxidizes much faster. When oils oxidize, they make coffee taste sour; when acids degrade, they make coffee taste bitter—and the hotter the brew, the more bitter the result.
Cold-brew coffee oxidizes and degrades, too, but it takes much longer. If you keep cold brew cold, you can all but guarantee that there won’t be any bitterness. But cold brew coffee isn’t just less bitter—it tastes quite different, thanks to the way certain solubles more readily dissolve in cold water than others. According to UCLA, “The compounds that don’t dissolve are the ones often attributed to unfavorable flavors: these stay in the grounds that are subsequently tossed away. Consequently, cold brews take on a much sweeter, floral profile.”
In the end, though, the style of cold coffee that’s “better” just comes down to personal preference. Fans of cold-brew like it for its smooth sweetness; those who prefer their coffee hot or iced like it for its bold flavors, stronger aroma, and even, yes, its bitterness.
References: (1) https://scienceandfooducla.wordpress.com/2014/08/19/coffee-brewing-chemistry-hot-brew-and-cold-brew/ (2) Emma Bladyka, “Coffee brewing: wetting, hydrolysis and extraction revisited”, Specialty Coffee Association Of America, pp. 1-7..