Why does eating an orange taste bitter after brushing your teeth?

If you eat an orange or drink a glass of orange juice right after brushing your teeth in the morning, you may find that the orange juice seems to become bitter and more sour. Why is this?

The problem is not the orange juice, but the toothpaste used to brush your teeth that temporarily changes our taste buds. To understand this, we need to talk about the taste receptors on the tongue and the ingredients in the toothpaste.

Taste buds are really like flower buds

First, let’s talk about the taste receptors on the tongue.

In the past, there was a saying that different parts of the tongue are responsible for sensing different tastes, the tip of the tongue is responsible for sweetness, and the root of the tongue is responsible for bitterness. Some people even drew a taste map of the tongue. But in fact, this statement is incorrect.

There are many taste buds all over our tongue. The name of taste buds is very descriptive. Each taste bud has taste receptors responsible for sensing the five tastes of sour, sweet, bitter, salty and umami. These receptors are hidden inside the taste buds like independent petals.

Taste buds, image source: Wikipedia

When food comes into contact with taste buds, each receptor starts to work and recognizes the taste it is responsible for. Then the various taste receptors gather the information they sense together, and the brain restores the taste of the food.

When we usually drink orange juice, the sugar in the orange juice will activate the sweet receptors, and the bitter and sour things in it will also be detected by the corresponding receptors. However, because the sugar content in orange juice is relatively high, the sourness and bitterness are "masked", so the brain's overall perception of orange juice is sweet.

Sodium lauryl sulfate makes orange juice bitter

In toothpaste, there is a substance called sodium laureth sulfate (abbreviated as SLS or SLES), which is sometimes written as sodium lauryl polyoxyethylene ether sulfate.

It is a very common surfactant with certain cleaning and foaming effects. It is relatively cheap and is widely used in toothpaste, shampoo and soap.

In addition to cleaning and foaming, this substance can also inhibit the ability of the sweet taste receptors in the taste buds, making us less sensitive to sweetness, which is why orange juice is no longer sweet. However, SLS does not "tamper" with the sour taste receptors. The reason why we feel that orange juice has become sour is because the sweetness is suppressed and the sour taste is highlighted.

So why does orange juice become bitter?

This is because the surfactant SLS will destroy the phospholipids in the mouth.

Phospholipids would originally bind to the bitter receptors on the taste buds, making them less "sensitive". But when the phospholipids are destroyed, the bitter receptors can fully bind to the bitter molecules in the orange juice. In addition, without the "masking" of sweetness, the bitter taste becomes more obvious.

But fortunately, this situation will not last too long and can basically be recovered within 1 hour.

Other things that can "change" your taste

In fact, it’s not just SLS in toothpaste that can change your taste.

For example, the previously popular West African "miracle fruit", this small fruit looks like a smaller version of a cherry tomato, and it can also "cheat" our taste buds.

Miracle fruit, Image source: Wikipedia

This fruit itself has no special taste, not even sweet. But after eating this fruit, if you eat something sour, even lemon or vinegar, it will not taste sour, but sweet.

This is because the miracle fruit contains a "miracle fruit protein". After combining with the taste buds, it can activate the sweetness receptors when the pH value is lowered (that is, in an acidic environment), allowing us to feel the sweetness when eating lemons or drinking vinegar directly.

This sweetness overwhelms the signals from the sour taste receptors, giving us the illusion that the lemon is sweet.

Of course, the effect of miracle fruit is the same as that of SLS in toothpaste, which can only last for a short time, usually within half an hour. If you want to show your friends how to eat lemons, be sure not to eat miracle fruit too early.

By the way, there are a few toothpastes on the market that do not contain SLS surfactants. If you are interested, you can also try using such toothpaste. After brushing your teeth with it, drink orange juice, and the orange juice will not become sour and bitter.

References:

[1] https://www.scientificamerican.com/video/why-toothpaste-makes-orange-juice-t2013-07-02/

[2] https://health.howstuffworks.com/mental-health/human-nature/perception/orange-juice-toothpaste.htm

[3] ALLISON ANNMA, Chambers D H. Effects of residual toothpaste flavor on flavor profiles of common foods and beverages[J]. Journal of Sensory Studies, 2005, 20(2): 167-186.

Author: Tian Dawei, popular science creator

Reviewer: Zhang Yu, researcher/PhD, Chinese Center for Disease Control and Prevention, national health science expert

The article is produced by Science Popularization China-Creation Cultivation Program. Please indicate the source when reprinting.