From the American Chemical Society:
It’s happened to many of us: Half asleep in the morning, you finish brushing your teeth and reach for your daily glass of orange juice. After taking a big swig, you spit it back out, making a face like you’ve just chewed on a lemon. Turns out, a specific chemical in your toothpaste is responsible for that nasty taste. This week, Reactions explains why toothpaste and orange juice don’t mix.