You may think it might be the common mint flavour of toothpaste clashing with other flavours, but in the case of orange juice and many other things, this isn't actually what's going on. The culprit here is thought to be two compounds almost universally added to toothpastes: sodium lauryl sulfate and sodium lauryl ether sulfate, which are anionic surfactants, meaning they lower the surface tension of water.
Why is that desirable in toothpaste? Because it works as something of a detergent, and makes the toothpaste foam up to help it spread around inside your mouth easier. Besides any cleaning effect, this has the by-product of making you feel like the toothpaste is doing something, which toothpaste manufacturers have found to be a great way to get people to buy more of their toothpaste.
Mint is added to toothpaste for this same reason, as it leaves your mouth feeling cool, clean, and fresh, particularly if it's well distributed throughout your mouth.
As Tracy Sinclair, one-time brand manager at Oral-B stated in the book The Power of Habit, by Charles Duhigg:
Consumers need some kind of signal that a product is working. We can make toothpaste taste like anything — blueberries, green tea — and as long as it has a cool tingle, people feel like their mouth is clean. The tingling doesn't make the toothpaste work any better. It just convinces people it's doing the job.
(Interestingly enough, besides any real cleaning effect, sodium lauryl sulfate is added to shampoo for similar marketing reasons, as people perceive that foaming shampoo works better than non-foaming, whether a particular brand's foaming shampoo actually cleans better than some other non-foaming shampoo or not.)
Back to your taste-buds. The sodium lauryl sulfate interacts with your sweet taste receptors, making them less sensitive, and thus dulling the sweet flavour. In addition to that, it also destroys phospholipids in your mouth, which are compounds that have the same type of effect sodium lauryl sulfate has on sweet taste buds, except the phospholipids dampen your bitter taste buds.
The net effect is that your sweet taste buds are dampened while your bitter taste buds become more sensitive. So when you drink something like orange juice, which normally has an overpowering sweet taste that masks an underlying bitter taste, it is going to taste drastically different – in this case extremely bitter.
So if your morning routine includes brushing your teeth before eating, you can simply find toothpaste that is free of sodium lauryl sulfate, and sodium lauryl ether sulfate and the food you eat directly after shouldn't taste awful, unless you're bad at cooking, of course.
--Sodium lauryl sulfate has been shown to act as a shark repellent. There is also evidence that it is effective as a microbicide when spread on your skin, particularly effective in helping to prevent infection from viruses like Herpes simplex and HIV, which are non-enveloped viruses.
Image by wwarby under Creative Commons license.
Daven Hiskey writes for the wildly popular interesting fact website TodayIFoundOut.com. To subscribe to Today I Found Out's "Daily Knowledge" newsletter,click here or like them on Facebook here. You can also check 'em out on YouTube here.