Why Does Orange Juice Taste Bitter After Brushing Your Teeth?
― A foaming agent temporarily switches off your tongue's "sweetness"
The juice hasn't changed. Your tongue has. The ingredient that makes toothpaste foam well stays on the surface of your tongue for a while. It is thought to make sweetness harder to sense, and so it lets the bitterness and sourness that were normally hidden come to the front.
It's morning, and you brush your teeth in a hurry. Your mouth feels fresh and minty.
Then you take a sip of orange juice from the fridge. At once, a bitter, sour, medicine-like taste spreads across your tongue. Yesterday it was so delicious.
Milk or water doesn't bother you much. So why does only orange juice change its taste so much?
There are just two reasons
The foaming agent in toothpaste stays on your tongue and temporarily weakens the machinery that detects sweetness. Almost none of the juice's sweetness gets through.
The surface of your tongue has an oil-like film that softens bitterness. The foaming agent is thought to strip this film away, the way detergent lifts oil.
Orange juice contains sweetness, sourness and a little bitterness all at once. Normally the sweetness wraps around the other two. When that sweetness disappears and the bitterness grows, the same drink tastes like something completely different.
The culprit is toothpaste's foaming agent
Many toothpastes contain an ingredient called sodium lauryl sulfate. On ingredient lists it is often written as "sodium lauryl sulfate" or "SLS", and its job is to make foam.
One end of this molecule is attracted to water, and the other end is attracted to oil. It belongs to the same family as detergents and shampoos, and the whole family is called "surfactants". They are good at gathering and sticking at the boundary between water and oil.
The surface of the cells that sense taste on your tongue is also made of an oil-like film. When this molecule sticks there, the machinery that picks up sweetness is thought to stop working properly.
The film that guarded against bitterness gets stripped away
The higher bitterness on the right of Figure 1 comes from the second reason. On the surface of your tongue lie oil-like molecules called phospholipids. They are thought to cover the parts that sense bitterness and soften it.
The foaming agent dissolves this film like a detergent and removes it. Then the small amount of bitter compounds that oranges naturally contain reach the bare tongue directly. The sourness of the juice also feels sharper, because sweetness no longer wraps around it.
Water and milk don't bother you because their taste doesn't rely on a mix of sweetness and sourness in the first place. The more a drink uses sweetness to hide bitterness and sourness, the more obvious the change.
Chew the leaves of gymnema, a climbing plant that grows in India and elsewhere, and sugar is said to taste like "plain sand" for a while. A compound in the leaves blocks the machinery that senses sweetness. The action of toothpaste's foaming agent has often been studied by comparing it with this leaf compound.
Does rinsing bring the taste back right away?
When you rinse, the foaming agent left in your mouth is diluted by the water. Use the slider in Figure 2 to change how much water you rinse with. After two rinses, the concentration drops to somewhere between one-tenth of a hundredth and less than one hundredth of where it started.
In practice, though, the taste often does not come back right away after rinsing. Part of the foaming agent is not dissolved in water at all but stuck to the surface of your tongue. What has stuck on does not come off easily when diluted, so you have to wait for your saliva to wash it away bit by bit.
The surest fix is to swap the order. Drink the juice first and brush afterwards, and the problem never arises. Choosing a toothpaste without sodium lauryl sulfate on the ingredient list is also said to make the change smaller.
Summary
Orange juice tastes bitter right after brushing not because of the juice, but because your tongue has temporarily changed. The foaming agent makes sweetness harder to sense and strips off the film that softened bitterness. The bitterness and sourness that sweetness used to hide now arrive bare.
Taste isn't decided by the drink alone.
Change the surface that receives it, and the same juice becomes a different drink.
We also cover what your tongue picks up in Where on your tongue do you feel the fizz of carbonated drinks? and Why does mint feel cool when it isn't cold? For why food loses its taste when your nose is blocked, see Why does food lose its taste when you have a cold?
- Before brushing, take a sip of orange juice. Note how sweet and sour it tastes.
- Brush with your usual toothpaste, rinse lightly, then take another sip. Compare which tastes weakened and which got stronger.
- After about 30 minutes, drink it again. See how far the taste has come back.
Look at the ingredient list on the toothpaste box too. Compare a toothpaste with "sodium lauryl sulfate" and one without on different days, and you may notice the difference. How strongly you notice it varies from person to person.
For those who want more ― terms, formulas and links to textbooksWe state clearly what level each part is, from middle school science to university courses
- Middle schoolCovered in middle school science
- High schoolCovered in high school chemistry and biology
- High school+Advanced high school material, or treated as a textbook sidebar
- UniversityNot covered in high school; university specialist courses (surface chemistry, sensory physiology)
- ResearchNot taught even at university as settled fact; topics researchers are still investigating
Middle schoolTerms: this phenomenon has names
- Surfactant: a substance with a water-loving part and an oil-loving part in one molecule. The foaming agents in detergent, shampoo and toothpaste belong to this family.
- Sodium lauryl sulfate: a surfactant often used in toothpaste. Ingredient lists write it as "sodium lauryl sulfate" or "SLS".
- Taste modification: when one substance strengthens or weakens how another taste is sensed. The compound in gymnema leaves is of the same kind.
Middle schoolHigh schoolChecking with a formula: how far does rinsing dilute the foaming agent?
We estimate the concentration of foaming agent left in the mouth after brushing, and how much rinsing dilutes it. The amount of toothpaste and its ingredient share differ between products and people, so these figures are rough guides.
| In symbols | C ＝ C₀ × ( v ÷ ( v ＋ V ) )ⁿ |
| In words | Concentration after rinsing ＝ starting concentration × (liquid left in the mouth ÷ liquid left in the mouth plus water), multiplied once for each rinse |
| Where it comes from | Adding water does not change the amount of dissolved substance (conservation of matter). The amount stays the same while the liquid grows, so the concentration falls by the same factor the liquid grew. Each spit-out repeats this. |
| Symbol | Meaning and unit |
| C | Concentration after rinsing (mg per mL) |
| C₀ | Concentration when brushing ends (mg per mL) |
| v | Volume of liquid left in the mouth after spitting out (mL) |
| V | Volume of water in one rinse (mL) |
| n | Number of rinses |
| Toothpaste used each time | About 1 g (1000 mg) |
| Share that is foaming agent | About 1–2% depending on the product (1.5% here) |
| Liquid mixed with the foam while brushing | Assume 5 mL |
| Liquid left in the mouth after spitting out | About 1 mL |
| Water per rinse | Assume 10 mL |
| Concentration where foam particles start to form | About 0.008 mol per litre (1 mol is 288 g) |
| Amount of foaming agent (mg) | 1000 × 0.015 ＝ 15 |
| Concentration when brushing ends, C₀ (mg per mL) | 15 ÷ 5 ＝ 3 |
| Concentration where foam particles form (mg per mL) | 0.008 × 288 ≒ 2.3 |
| Fraction left after one rinse | 1 ÷ 11 ≒ 0.091 |
| Concentration after one rinse | 3 × 0.091 ≒ 0.27 |
| Concentration after two rinses | 0.27 × 0.091 ≒ 0.025 |
| How many times smaller than at the start | 3 ÷ 0.025 ＝ 120 |
The concentration when brushing ends is slightly above the level at which foam particles (micelles) form. This is why your mouth fills with foam. After two rinses, the amount dissolved in water falls to about one 120th. The taste still does not return right away because the part stuck to the surface of your tongue is not included in this formula.
High schoolHigh school+Molecular shape and how taste is received
High schoolSodium lauryl sulfate is a molecule with a long chain of 12 carbon atoms, ending in an electrically charged sulfate group. The chain side is attracted to oil, and the sulfate side is attracted to water. Cell membranes are made of phospholipids arranged in two layers. A surfactant pushes its chain side into the membrane, disturbing the arrangement or pulling membrane molecules away.
High school+Sweetness and bitterness are sensed when proteins called receptors in the membranes of taste cells catch molecules. Sweetness is thought to use one combination of receptor, while bitterness uses a few dozen types. When the membrane is disturbed, sweetness detection is thought to weaken, and bitter molecules reach their receptors more easily.
UniversitySurface chemistry and taste-modifying substances
The concentration at which surfactants begin to gather into particles in water is called the critical micelle concentration. For sodium lauryl sulfate in pure water it is about 8 millimoles per litre. Below this, molecules adsorb to the surface one by one; above it, they dissolve as micelles. In taste research, DeSimone, Bartoshuk and colleagues in 1980 compared this ingredient with gymnemic acid from both physical-chemical and sensory angles. Two effects are discussed: suppression of sweetness, and removal of the phospholipids that held bitterness in check.
📖 Derivation of the formula, and further reading: Critical micelle concentration (Wikipedia, in Japanese) ／ Sodium lauryl sulfate (Wikipedia, in Japanese)
ResearchWhat is still not clear
- The fine mechanism of weakened sweetness Whether it binds to the receptor itself or acts through disturbance of the membrane is not settled.
- Why people differ Some people hardly notice anything, while others taste strong bitterness. Individual differences in bitter receptors may play a part.
- Time to recover It changes with the amount of saliva and the way you brush, and few figures clearly state how many minutes it takes.
In other words, this article too is "an explanation within what is known today". The more familiar the phenomenon, the more likely its details are still under study.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: concentration of solutions, sense organs | The dilution calculation for rinsing; tasting with the tongue |
| High school | Chemistry: surfactants / Biology: cell membranes | Molecular shape and the phospholipid film |
| High school+ | Biology: receptors and receiving information | Sweet and bitter receptors |
| University | Surface chemistry, sensory physiology | Critical micelle concentration, taste-modifying substances |
| Research | Molecular biology of taste | How sweetness weakens, individual differences |
| ― | Links to daily life | Drink juice before brushing; choose by ingredient list |
- DeSimone, J. A., Heck, G. L., & Bartoshuk, L. M. (1980). Surface active taste modifiers: a comparison of the physical and psychophysical properties of gymnemic acid and sodium lauryl sulfate. Chemical Senses, 5, 317–330.
- Wikipedia, "Sodium lauryl sulfate" (ラウリル硫酸ナトリウム; in Japanese)
- Wikipedia, "Critical micelle concentration" (臨界ミセル濃度; in Japanese)
- Wikipedia, "Taste" (味覚; in Japanese)
- Takashi Yamamoto, Bimi no kozo: naze "oishii" no ka (山本隆『美味の構造 ― なぜ「おいしい」のか』), Kodansha Select Mook (講談社選書メチエ)
※This article is a science explainer for general readers. The figures given are rough guides to help you understand the mechanism. How taste is perceived varies from person to person. If you have concerns about your teeth or mouth, please consult a dentist.