The subtle and smooth result of the flash-chilled method is why many coffee experts prefer it over cold brew. "Everyone who actually takes coffee seriously in what we call the 'Third Wave coffee movement' thinks [cold brew is] garbage," said Jeremy Tooker, the owner of Four Barrel Coffee in San Francisco, referencing the faction of coffee shops and roasteries that treat sourcing as sacrosanct, and evangelize the merits of single-origin beans. Featureless, harsh, "something of an abomination" are common descriptors you'll hear about the drink from baristas. Yuki Izumi, who runs the coffee program at Hi-Collar in New York City, said that cold brew is just "flat."
The "battery-acid" flavor is due to cold brewing's imprecise process that can take anywhere from overnight to an entire day, and the ratio of coffee to water can fluctuate without consequence. "You’re like, 'Five pounds-ish [of coffee grounds], to, whatever, five gallons of water, for I don't know, 12 hours,'" said Tooker. Forgot to strain the grounds from the water? Oops, but steeping for 24 hours is also fine. "All the rules go out the window. It’s obviously a compromise."
The origins of the process are also telling: Cold brew started as a graveyard for beans that were about to expire instead of letting them go to waste, according to Tooker. "The freshness at that point doesn't matter," he said. "[The cold brew] would taste the same [regardless of whether] you put in four-week old or five month old coffee in the system." The primary culprit here is the lengthy exposure to oxygen, which puts the steeping coffee at risk of ruin through a reaction called oxidation. (A sliced apple turns brown for the same reason.)
Flash-chilled coffee, on the other hand, tastes much fresher. "It brings out the character of the beans," said Izumi. When you flash-chill, the aromatics of the hot coffee don't have a chance to escape. The moment the scalding liquid hits the ice, it locks in particular olfactory notes and drastically reduces the chance of oxidation, which turns coffee rancid. Think of it like blanching vegetables. The entire process literally takes just two or three minutes -- a fraction of the time cold brewing takes -- and it's super-easy to do at home.