An Ingredient in Toothpaste Makes Orange Juice Taste Vile. Here's How.

It’s the sort of slip-up you usually make only once. In the rush of a weekday morning, you brush your teeth, wander into the kitchen, and take a generous gulp of orange juice (OJ). Suddenly: ugh.

Why does a mouth that feels clean and mint-fresh end up tasting so unpleasant the moment it meets orange juice?

In brief, toothpaste includes a detergent that breaks down fat. Because your taste buds and taste receptor cells rely in part on fatty structures, brushing can temporarily interfere with how they work.

Before you swear off brushing to “protect” your taste buds, it’s worth knowing the effect is short-lived-typically just a few minutes. And brushing with toothpaste remains essential for oral health.

So what’s actually going on, and how do the taste receptors spread across your tongue normally do their job?

I’m a psychologist and have spent more than 40 years investigating the science of how people experience taste and flavour. Here’s the biology behind that notorious toothpaste-and-OJ moment.

A bittersweet symphony: sweet and bitter taste receptors

From an evolutionary point of view, your brain is set up to crave sweet sugars-important fuel for body and mind-while strongly avoiding bitter substances that may signal toxins. That’s why sweet and bitter receptors matter so much for survival.

Every cell in your body is enclosed by an outer layer called the cell membrane, built largely from fats known as lipids. In sweet and bitter taste receptor cells, those membranes also include a specialised molecule called a G protein-coupled receptor (GPCR).

Some GPCRs are tuned to pick up sweet tastes. They largely ignore compounds that aren’t sweet and respond to sugars your body can use. Other GPCRs detect bitter tastes, keying in to the many naturally occurring compounds that can be poisonous-effectively serving as an internal alarm.

Salty chips and sour sweets: ions, cations and signals

Your experience of saltiness and sourness works a bit differently. These tastes arise when positively charged ions-known as cations-move through tiny openings in the membranes of salty and sour receptor cells.

For saltiness, the relevant cation is positively charged sodium from sodium chloride (ordinary table salt).

For acidic, or sour, tastes, the key cation is a positively charged hydrogen ion. Different acids contain different chemical compounds, but what they share is the hydrogen cation.

When you eat crisps, sodium cations from the salt pass through specific openings in the salty receptor’s membrane, generating the sensation of saltiness.

In much the same way, hydrogen cations in a favourite sour sweet pass through dedicated channels in the sour receptor’s membrane, sending a “sour” message to the brain.

Toothpaste, sodium lauryl sulfate and orange juice (OJ)

Many people enjoy orange juice with breakfast because it’s naturally high in sugar. It also contains citric acid, which provides hydrogen cations. The result is a pleasing blend: sweet with a gentle sour edge.

But brush your teeth first, and that same orange juice can taste dreadful. What changed?

It isn’t simply that mint and sweetness don’t get along. Toothpaste commonly contains sodium lauryl sulfate, a detergent that helps remove dental plaque from teeth. Plaque is the sticky film of germs that contributes to tooth decay and can cause bad breath.

If you’ve ever washed up, you’ll have seen what detergent does in a bowl of greasy water: it breaks up oily fat so it lifts away easily and rinses clean.

A similar disruption happens in your mouth. The detergent in toothpaste can interfere with the lipids in the cell membranes of your taste receptors. By disturbing that lipid layer, brushing temporarily alters how taste signals are processed-especially the balance between sourness and bitterness.

An extra factor worth considering is timing: orange juice is acidic, and enamel can be slightly softened for a short period after acidic foods and drinks. Many dentists advise waiting a little while after acidic items before brushing, and likewise allowing a brief gap after brushing before drinking orange juice can reduce the chance of an unpleasant taste experience.

It’s also why some people prefer an SLS-free toothpaste (without sodium lauryl sulfate). These formulations can be gentler for some mouths and may lessen the dramatic “OJ tastes awful” effect-though plaque control and individual sensitivity vary.

Testing it out: a study with sodium lauryl sulfate

In 1980, I ran a study with two colleagues in chemistry to examine how the tongue responds to sweet, bitter, salty and sour tastes after exposure to sodium lauryl sulfate, the detergent used in many toothpastes.

We carried out an experiment with seven student volunteers at Yale. They sampled very strong solutions of sweet sucrose, sour citric acid, salt, and bitter quinine-both before and after holding a 0.05% sodium lauryl sulfate solution in their mouths for one minute.

You can try a home version of this: taste something sweet (such as sugar), a pinch of table salt, orange juice and tonic water before brushing, then repeat after brushing, and note what changes.

What we observed was that the perceived intensity of sucrose, salt and quinine dropped slightly. The most striking shift, however, was that an additional bitter quality appeared in the sour taste of citric acid.

That’s why, instead of tasting sweet with a pleasant tang, orange juice can taste bitter after you’ve brushed your teeth.

Linda Bartoshuk, Research Professor of Psychology, George Washington University

This article is republished from The Conversation under a Creative Commons licence. You can read the original article there.