Emily Liman of the University of Southern California in Los Angeles says protons released by sour-flavor acids enter directly into the cell -- without any of the sodium-involving steps her team expected.
"This mechanism is very appropriate for the taste system because we can eat something that has a lot of protons and not much sodium or other ions, and the taste system will still be able to detect sour," Liman says in a statement.
The study, published in the Proceedings of the National Academy of Sciences, describes how people perceive sour flavors.
"In the past, it's been difficult to address this question because the taste buds on the tongue are heterogeneous. Among the 50 or so cells in each taste bud there are cells responding to each of the five tastes, Liman says.
To measure "sour" only, Liman and colleagues first bred mice with sour "marked" with a yellow florescent protein.
"Once we've understood the nature of the molecules that sense sour, we can start thinking about how they might be modified and how that might change the way things taste," Liman says.