Science Questions

Why does brushing your teeth alter the flavor of substances afterwards?

Sun, 5th Jul 2009

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Question

Joseph Han asked:

Why does brushing your teeth alter the flavor of substances afterwards?

Answer

Ben -   Itís a great effect and itís a lovely, lovely question because I actually had Ė I had to look this up and as soon as I read the question, I thought, ďThatís brilliant!  Why didnít I look this up before?Ē  But it has all to do with the substance in toothpaste called sodium laureth sulfate.  Thereís a few similar chemicals that do the same thing.  Itís a surfactant which means that it lowers the surface tension of a liquid.

Kat -   Those are classic cleaners, arenít they?

Ben -   Yes.  Youíll find it in detergents, youíll find it in all sorts of different things that rely on breaking surface tension, and itís in the toothpaste, to make sure that you get a good foam from the toothpaste while you clean your teeth and look a bit rabid.

Kat -   Speak for yourself...

Ben -   But they also interact with that taste buds in two key ways:  They inhibit the taste buds that perceive sweetness, so whatever you eat afterwards will taste less sweet and then they break up fatty molecules called phospholipids and these phospholipids live on the surface of our tongue, and they inhibit the receptors for bitterness.  So, not only do we get the effect through the knocking down of the sweetness, but actually boosting the bitterness that you get as well.  So, that means anything you eat will taste less sweet and much more bitter which is why orange juice in particular, which we know is normally very sweet, is really quite foul.  Thereís also menthol in there, and that has a temperature effect which fools your sensory nerves into being more sensitive to cold.  So, fresh orange juice, fresh from the fridge may sound great and refreshing, will be good with your breakfast, but itíll taste bitter and it will be painfully cold.

Chris -   I met someone a little while back whoís at the Oxford University.  Heís a chemist and he showed me a wonderful trick with glucose because glucose comes in two 'handednesses'.  Thereís right handed glucose and left handed.  What that means is, itís a bit like if I had a glucose molecule and I put it in front of a mirror, youíll have a molecule in one configuration in your hand, youíd have the molecule with its mirror image in the reflection.  And nature is just the same.  There are both forms of the sugar in nature.  It just so happens that the human body uses the D-form, the right handed form.  He brought with him some left handed glucose and I tasted it and guess what it tasted like?

Ben -   I have no idea.

Chris -   Do you think itís sweet?

Kat -   Cherries.

Ben -   Iíd assume it would be sweet because itís the same atoms, isnít it?  Built into their molecule, just kind of reflected.

Kat -   No, if it canít be recognized.

Chris -   No.  It tastes like salt, which is salty.  It was disgusting.  Itís just a salty sort of (luhhrr) flavor.  It wasnít very nice at all because itís the wrong shape to fit into the taste receptors on your tongue, just like Kat says.

Ben -   Wow!

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Joseph_Han asked the Naked Scientists: Hello All, The podcasts are a Godsend and have successfully staved of brain death during many hours of monotonous work. So cheers for that. I've a question as well. Why, after one brushes his or her teeth, does everything taste so strange? Is it anything to do with the diatoms in toothpaste? The bacteria in your mouth? I'm sure I could find an answer somewhere, but I'd much rather ask you lot. Take care, and many thanks! Joseph Han from Virginia in the US. What do you think? Joseph_Han, Sat, 4th Jul 2009

I would compare it to eating a mint and then trying to eat an orange or an apple, naturally they will taste strange. Chemistry4me, Sat, 4th Jul 2009

Mouthwash, toothpaste, gum, etc. all have menthol in them. Menthol has the effect of triggering sensors in your mouth that usually detect the temperature of something, and the effect is that your mouth feels colder than it actually is.

Madidus_Scientia, Sun, 5th Jul 2009

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