You know how it is. You got up late and you’re in a rush. You swig a glass of orange juice after you’ve brushed your teeth. Instead of a sweet fruit flavour, the juice tastes like a foul concoction. At its worst, the rest of your breakfast can taste revolting too. Something’s going on.

What do I need? Orange juice. Three types of toothpaste: 1. ordinary mint; 2. mint without foaming agents (this is available at wholefood shops – check the label for the absence of sodium lauryl sulphate, which is a foaming agent used in many toothpastes); 3. nonmint (again, one that doesn’t contain SLS). If you can’t get hold of the SLS-free varieties you can try the experiment using ordinary mint toothpaste.

What do I do? Take a swig of orange juice and note how sweet it tastes. Brush your teeth with nonmint toothpaste, then take another swig of juice and see how it tastes. Repeat with toothpaste No 2 and toothpaste No 1.

What will I taste? After brushing with non-mint, non-foaming toothpaste, you might expect the juice to taste foul but, surprisingly, it tastes just like normal sweet juice. Brushing with nonfoaming mint toothpaste makes the juice taste weird, but not too bad. Normal mint toothpaste transforms the juice into a bitter, medicine-like flavour.

What’s going on? You’ve just experienced the orange-juice effect, something that’s been known about since the 1970s. Foaming agents, usually the SLS, are added to toothpaste to help disperse the paste around your mouth and for easy rinsing, but they have a nasty after-effect. They interfere with the taste buds on your tongue, suppressing the ability to taste sweetness and salt, and enhancing any bitter flavours. The powerful mint flavour that most of us want from our toothpaste adds to the problem. While it doesn’t interfere with our tastebuds in the same way as the foaming agents, the mint flavour overpowers the taste of anything that is consumed afterwards. The rule – and it makes sense for the health of your teeth too – is brush your teeth after meals, not before.

PS: Foaming agents aren’t a vital ingredient, but, as with soap, if we don’t see any bubbles we think it isn’t working. The same goes for mint. If a toothpaste doesn’t knock you out with its mint flavour, we assume it’s not effective. In Japan, most toothpastes are fruit-flavoured, and in the West many children’s toothpastes are fruity too, not minty.

Explore Books

• Book Extracts

• Book Reviews

• Books Group

• Audio Books

Times Recommends

• Superfreakonomics by Steven D Levitt and Stephen J Dubner
• The Times picks the 50 best paperbacks of 2009
• Vincent van Gogh’s letters show the soul of the artist