I tell my kids to brush their teeth in the evening. I’ve got two motives really. The first is that I want them to take care of their teeth (duh!), but the second is that I know that anything they like to eat in the evening after a proper dinner isn’t good for them and toothpaste will make it take like crap. I know this from experience, but what’s causing all that fun in their mouths?
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 in the video linked below.