Why does orange or grapefruit juice taste so nasty after I brush my teeth?

It’s all about phospholipids, says John Moore, director of the Institute for Chemical Education at UW–Madison.

Not a flavor guy himself, he poked around and learned that these oily, fatty materials affect receptors that sense the bitter flavor. Detergents in toothpaste, including sodium laurel sulfate, break up phospholipids that are normally on the tongue, Moore says. Although these detergents help with cleaning, he says the mechanical effect of brushing is responsible for much of the cleaning action.

Removing phospholipids changes the taste equation in your mouth. "Apparently some phospholipids inhibit receptors for the bitter taste," Moore says. "If you remove them, things that might taste a little bitter now taste very bitter."

After a search of the scientific literature, Moore found that much remains to be learned regarding these questions of taste. However, he suspects that the sweet flavor of grapefruit normally blocks the bitter taste, but in the absence of phospholipids, the bitter taste receptors are overwhelmed, causing that nasty taste in your mouth.

Moore suggests an experiment. Lightly brush your teeth with baking soda, an old-timey, detergent-free tooth powder that is too abrasive to use regularly. Eat some grapefruit. Let your palette clear, and repeat with standard toothpaste.

Notice the difference? Ecch! We did.