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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- Allison, A.M.A. and Chambers, D.H. "Effects of residual toothpaste flavor on flavor profiles of common foods and beverages." (Abstract) Journal of Sensory Studies. 2005. http://www.cababstractsplus.org/google/abstract.asp?AcNo=20053091920
- Francis, Prof. Greg. "Lecture 36: Why toothpaste ruins your orange juice." Perdue University. http://www2.psych.purdue.edu/~gfrancis/Classes/PSY310/L36.pdf
- O'Hare, Nick. Why does orange juice taste so awful after you brush your teeth?" The Guardian. September 27, 2007. http://entertainment.timesonline.co.uk/tol/arts_and_entertainment /books/article2538282.ece
- Petrusso, Annette. "Toothpaste." How Products Are Made. 1995. http://www.enotes.com/how-products-encyclopedia/toothpaste
- "Bitter taste in your mouth?" University of Utah. February 21, 2003. http://unews.utah.edu/p/?r=031406-55
- "Orange juice and toothpaste." U.S. Department of Energy. http://www.newton.dep.anl.gov/askasci/chem00/chem00216.htm
- "Why does orange juice taste odd after toothbrushing?" Science Made Simple. http://www.sciencemadesimple.co.uk/page50g.html
- "Why does orange juice taste so bad after brushing your teeth?" Australian Broadcasting Corporation. June 22, 2006. http://www.abc.net.au/goldcoast/stories/s1669605.htm
- "Why does orange of grapefruit juice taste so nasty after I brush my teeth?" University of Wisconsin-Madison. November 20, 2007. http://www.news.wisc.edu/14477