The beans aren’t really to blame. In fact, a number of airlines now get their grounds from popular brand name roasters like Starbucks (Delta) or Dunkin’ Donuts (Jet Blue). The real culprit: the water used in airplane brew. Planes have tanks filled with "potable water," a.k.a. water used for washing your hands in the bathroom and brewing coffee and tea, and it’s not the cleanest—a 2012 EPA report found that 12 percent of commercial airplane water tested positive for coliform bacteria, which usually indicates other gut-rocking bugs like E.coli are present. And according to one American Airlines flight attendant we spoke with, “those tanks are probably only cleaned out every six months to a year.” Yikes.
The brewing process is next to blame—at 35,000 feet, water boils at a much lower temperature, which messes up the extraction process so that only some of the coffee solids get dissolved, says Andrew Hetzel, a coffee quality and trade consultant. And if the flight attendant flips on the warming pad too soon, the first few drops of brew burn and taint the whole batch, said one Air Canada flight attendant.
Finally, your own taste buds weaken the flavor of in-flight joe. Cabin pressure, dry air, and altitude reduce your taste perception by about 30 percent (hello, heavily salted dinners), which means that already crummy coffee never stood a chance. (British Airways is on top of this: They’ve partnered with Twinings tea to create a special full-bodied blend for flying—a bloody great idea.) For the record, a few of the flight attendants we spoke with simply believe the majority of the population has forgotten what boring, black coffee tastes like: “People have become so accustomed to sweetness, they’re shocked when our coffee doesn’t taste like a caramel macchiato,” said one Southwest flight attendant.
This story originally appeared on Travelandleisure.com.