Q: Why does fresh orange juice taste so awful after you have just brushed your teeth?
A: The unpleasant taste is produced by the chemical reaction between the citric acid in the orange juice and the alkaline ingredients in the toothpaste. These alkaline ingredients (notably sodium monofluorophosphate) are designed to neutralise the acids produced by bacteria which lead to tooth decay. The same type of reaction will occur if you eat or drink anything else acidic (try salt and vinegar flavour crisps).
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