Currently, scientists believe that flavors are the result of interactions between taste molecules and receptors on your tongue. Molecules of a certain shape will interact with receptors that are shaped to accept them. But these receptors can be manipulated, which is probably the best explanation for why orange juice and toothpaste taste gross together.
The likeliest culprit for the offensive reaction is the foaming agent found in almost all toothpastes. Sodium lauryl sulfate (SLS) is a surfactant -- a sudser -- added to toothpaste. It creates the froth that toothpaste becomes after you begin brushing by lowering the surface tension of the saliva in your mouth and allowing bubbles to form. While it aids in spreading the toothpaste throughout your mouth, it also creates the impression of cleanliness; a mouthful of foam just feels cleaner.
But SLS has other properties, too. For one, it suppresses your sweet receptors, so it has a dampening effect on the generally sweet taste of orange juice. In addition, SLS destroys phospholipids. These fatty compounds act as inhibitors on your bitter receptors. So by inhibiting sweet receptors and destroying phospholipids, SLS dulls the sweetness and promotes the bitter taste in orange juice.
This is not the only explanation of why orange juice and toothpaste form a bad flavor, but it's the most widely accepted one. Another explanation, posed by a researcher at the United States Department of Energy, suggests that the horrible taste is the result of interaction between the stannous fluoride in toothpaste and the acetic acid in orange juice.
While research into the science of taste is as intense as minty toothpaste, investigations into the orange juice/toothpaste interaction are actually fairly sparse. The authors of one study, published in the Journal of Sensory Studies in 2005, concluded that it takes at least an hour for the effects of minty toothpaste on the taste of orange juice to dissipate [source: Allison, Chambers].
But this study examined the effects of only "strongly mentholated toothpaste." So is the same bad taste created with toothpastes that don't contain a minty flavor? Any toothpaste with SLS will create the bad taste, says University of California - Davis sensory scientist Dr. Hildegarde Heymann And don't forget, SLS is present in just about every brand of toothpaste. Dr. Heymann should know, she's worked as a flavor scientist in the toothpaste industry. Of course, you don't need a Ph.D. to figure this out. The simple misstep of taking a sip of O.J. after brushing your teeth is experiment enough.
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