Science Questions

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

Ben -   It’s a great effect and it’s a lovely, lovely question because I actually had – I had to look this up and as soon as I read the question, I thought, “That’s brilliant!  Why didn’t I look this up before?”  But it has all to do with the substance in toothpaste called sodium laureth sulfate.  There’s a few similar chemicals that do the same thing.  It’s a surfactant which means that it lowers the surface tension of a liquid.

Kat -   Those are classic cleaners, aren’t they?

Ben -   Yes.  You’ll find it in detergents, you’ll find it in all sorts of different things that rely on breaking surface tension, and it’s 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.  There’s 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 it’ll taste bitter and it will be painfully cold.

Chris -   I met someone a little while back who’s at the Oxford University.  He’s a chemist and he showed me a wonderful trick with glucose because glucose comes in two 'handednesses'.  There’s right handed glucose and left handed.  What that means is, it’s a bit like if I had a glucose molecule and I put it in front of a mirror, you’ll have a molecule in one configuration in your hand, you’d 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 it’s sweet?

Kat -   Cherries.

Ben -   I’d assume it would be sweet because it’s the same atoms, isn’t it?  Built into their molecule, just kind of reflected.

Kat -   No, if it can’t be recognized.

Chris -   No.  It tastes like salt, which is salty.  It was disgusting.  It’s just a salty sort of (luhhrr) flavor.  It wasn’t very nice at all because it’s the wrong shape to fit into the taste receptors on your tongue, just like Kat says.

Ben -   Wow!

July 2009