Image source: Please, brush your teeth (This site has an extensive collection of tooth brushing photos.)
Ever wonder why orange juice tastes awful right after you brush your teeth? The short answer is there’s something in toothpaste – sodium laurel sulfate – that interferes with your taste buds, especially the taste of sweetness. This heightens your sensitivity to sour and bitter tastes.
That’s the executive summary. Here’s a slightly longer explanation.
What’s in your toothpaste?
Probably more than you think. Toothpaste contains flourides, abrasives, detergents, thickeners, and water softeners. It also has sweeteners to hide the bad taste of all that other stuff. The ingredient that makes orange juice taste bad is a foaming detergent that cleans your teeth. As a detergent it helps dissolve food particles. The foaming action distributes the toothpaste in your mouth and carries away what the abrasives and detergents loosen up.
Foaming agents are called surfactants, and the one most commonly used in toothpaste is sodium laurel sulfate (SLS). Surfactants are added to shampoos, many soaps, and detergents. They carry away debris, yes, but they also signal to us that the product is working. If we didn’t see the bubbles, would we really believe it was doing anything? It’s perfectly OK to use toothpaste without a foaming agent, if you can find one. (Tom’s of Maine and Kiss My Face make SLS-free toothpaste.)
Contrary to Internet and email rumors, sodium laurel sulfate is not carcinogenic. But you don’t want to swallow it (it’s used in laxatives), leave it on your skin for long, or get it in your eyes.
But why does this affect taste?
A phospholipid with a head and two tails (Image source: AP Biology Wiki)
Substances that can be dissolved in fat, such as other fats, oils, wax, fat-soluble vitamins, are called lipids. Some types of lipids (fats) don’t like water. They’re called hydrophobic. Think of how, when you rinse a greasy frying pan under running water, not much happens. But when you apply a detergent, the grease breaks down and easily washes away.
Something similar happens in your mouth when you brush your teeth.
Every cell in the body, including the cells of the tongue, is surrounded by a membrane containing lipids. Not the water-hating fats like butter and grease, but a more complex structure called a phospholipid. Phospholipids have two tails on one end that hate water (hydrophobic), but the other end loves water (hydrophilic). In the membranes that surround cells, the phospholipids have lined themselves up so their heads are pointing in one direction and their tails in the other. Not only that, but two of these layers have gotten together with their tails pointing to the inside and their heads on the outside.
A phospholipid bilayer
The amazing thing about this structure, called a phospholipid bilayer, is that it can control what gets into a cell and what comes out. When you use toothpaste with a foaming detergent like sodium laurel sulphate you change the permeability of the cells on your tongue. You become less able to taste something sweet, and anything sour begins to taste bitter.
The orange juice effect can last half an hour. Eventually your own saliva dilutes the SLS, washes it away, and your ability to taste returns to normal.
A matter of taste
How do you taste?
What is a supertaster?
The genetics of supertasting
Are you a supertaster: Do you really want to know?
Are you a supertaster: Look at your tongue
Are you a supertaster: How does PROP Taste to you?
Are you a supertaster: DNA testing
Why do we love high-fat foods?
Do we taste fat?
The taste advantage
Grapefruit and the Pill
This is your brain on sugar — and sugar substitutes
The Pepsi challenge: How beliefs affect what you taste
Norman Swan, Super Tasters, ABC Radio National, June 9, 1997
For a detailed discussion of toothpaste ingredients, I recommend the Science Blog of Simon Quellen Field.