In a handy video, the American Chemical Society has explained why orange juice (and food in general) tastes like soap after you brush your teeth. The basic gist is that a detergent called sodium lauryl sulfate, found within your toothpaste, blocks the sweet receptors on your tongue and ramps up your bitterness receptors. The ACS has a more detailed explanation below:
Next, hopefully they’ll tackle the question of why toothpaste makes water taste extra cold. (…Or is that just me?)
Other gustatory questions you can chew on: