One of the mistakes I learned growing up was brushing my teeth right before eating breakfast, sitting down, and with a minty mouth, going for the OJ. One sip is all it took to realize something had gone horribly wrong. What are they putting in the toothpaste to make OJ lose its wonderful flavor?
When you brush your teeth, you may notice that the toothpaste starts to foam up in your mouth. This is what its designed to do, and the chemical responsible for the suds is called Sodium Lauryl Sulfate (SLS). While the foaming may not be an issue for most, it does have an unwanted, temporary side effect on your taste buds by decreasing the sensitivity of the 'sweetness' receptors on your tongue. It also destroys phospholipids that usually act to decrease the sensitivity of the bitterness receptors.
This combination tips the once-balanced flavor scale towards the sour/bitter receptors. The result is that when you take a sip of orange juice, less sweetness (and more bitterness) is detected by your tongue, making the taste more bitter overall. Eating and drinking other items with a high acidity will yield the same result.
You will find SLS in almost all toothpastes, with exceptions being the natural brands like Tom's.
Sodium Lauryl Sulfate is also used in detergents, soaps and shampoos to produce the same foaming effect, but I don't recommend putting those in your mouth.