A glass of juice is a great addition to breakfast: it's healthful, and it tastes good. But, what if you've already brushed your teeth? Then your breakfast juice can taste bitter. Why does brushing your teeth ruin your morning juice? That's the question for this Moment of Science.
Brushing your teeth alone is not responsible for making juice taste bitter, unless you brush your teeth with foamy toothpaste. One ingredient in particular, sodium lauryl sulfate, is responsible for causing that after-brushing taste. Sodium lauryl sulfate is in all sorts of detergents. If you check your cupboards, it's very likely that you'll find it in some of your shampoos and soaps as well as in your toothpaste. So, in some sense, you're washing your mouth out with soap.
And sodium lauryl sulfate doesn't just alter the taste of juice: it can alter the taste of almost every food or drink you consume, it's just more noticeable in juice because it makes the acid in the juice taste bitter and lessens the sweetness of the sugar. This taste alteration can last up to two hours.
Sodium lauryl sulfate isn't really necessary for cleaning teeth, although many people like using foamy toothpaste. If you brush with baking soda, or with paste that does not contain sodium lauryl sulfate, you will avoid the after-brushing bad taste altogether.