Why does salt make watermelon taste sweeter?
― Your tongue doesn't measure flavour. It compares.
Salt is salty, yet sprinkle it on watermelon and the fruit tastes sweeter. Odd, but the watermelon has not gained any sugar. What changed is how your tongue and brain perceive it. And only a tiny pinch works. Add too much and the sweetness fades away.
On a summer veranda, your grandmother is sprinkling salt over a slice of watermelon. "It tastes sweeter this way," she says. You take a bite, half doubting her.
She's right. It is sweeter than before. And you hardly taste the salt at all.
Then you get carried away and pile on the salt. Now it's just a salty, watery fruit. It's the same salt, so why does the amount flip the result?
There are only two reasons it tastes sweeter
Your tongue and brain don't measure the strength of a taste in absolute terms. When a little of a different taste is mixed in, the main taste stands out more clearly. This is called "contrast."
Once there is enough salt to taste clearly salty, saltiness and sweetness start to cancel each other out. Sweetness is boosted right around the edge of noticing the salt, or not.
So salt on watermelon is not really seasoning. It is more like a small signal that nudges the sensitivity of your sweet-sensing system. Let's go through it step by step.
Your tongue tastes by comparison
The surface of your tongue holds several thousand tiny taste buds. Cells inside them detect sweet, salty, sour, bitter and umami, each through its own route.
The signals travel along nerves to the brain, which combines them and decides, "this food tastes like this." The brain does not look at one taste alone. It sets the strength of each taste in balance with the others.
It is a bit like how a grey patch looks lighter on black paper than on white paper. A faint saltiness acts as the "background," and the sweetness, the "lead," stands out. In taste science this is called the contrast effect. A pinch of salt in sweet red-bean soup works the same way.
There is more. Mouse studies have reported that the sweet-sensing cells of the tongue have an entry point that takes in glucose together with sodium, a component of salt. So salt may also boost the sweet signal directly. Researchers are still investigating this.
Too much salt makes the sweetness disappear
As you add more salt, there comes a point where "salty" becomes clearly noticeable. Cooking textbooks put the rough concentration at which you notice saltiness at about 0.2%.
Beyond that point, saltiness and sweetness weaken each other. This is called the suppression effect. It is why watermelon covered in salt doesn't taste sweet.
In Figure 1, move the slider to change the amount of salt on one slice. A "pinch" held between two fingers (about 0.5 grams) makes the whole slice about 0.2% salt, right at that rough threshold. To reach the same concentration as body fluids (0.9%), you would need more than four times as much salt.
In reality, the salt sits only on the surface of the watermelon. Your first bite is much saltier than the average for the whole slice. If even a "pinch" feels like too much, the trick is to sprinkle more lightly.
Much of watermelon's sweetness comes from fructose, the sugar common in fruit. When fructose is cold, more of its molecules are said to take the sweeter shape. Chilled watermelon with just a little salt: this old way of eating it has two reasons behind it.
Salted caramel, and the pinch of salt in sweet red-bean paste, also use the contrast effect. In every case, the salt is far less than the sugar.
Summary
Watermelon tastes sweeter with salt not because it has more sugar, but because your tongue and brain judge each taste against the others. A faint saltiness becomes the background, and the sweetness rises into view. But once the salt passes the point where you clearly notice it, sweetness and saltiness cancel each other out.
Salt doesn't add sweetness. It makes sweetness stand out.
That's why only a pinch you barely notice works.
For how cold dulls sweetness, read Why does melted ice cream taste so sickly sweet? For how smell shapes taste, read Why does food lose its taste when you have a cold? And for why "spiciness" isn't a taste at all, read Why doesn't water put out the heat of chili peppers?
- Pour 100 mL of water into each of two cups, and dissolve one teaspoon of sugar (about 3 grams) in each.
- Add a tiny amount of salt, about one ear-pick scoop, to just one cup and stir well. Sip from each in turn and compare the sweetness.
- Next, add about half a teaspoon of salt to the salted cup and compare again. How did the sweetness change?
Ask someone to close their eyes and taste the cups without telling them which has salt. That makes it harder for expectations to sway the result. Take just one sip at a time.
For those who want to know more ― terms, formulas and links to textbooksWe mark the level of each topic, from middle-school science to university courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school biology and chemistry
- High school+Advanced high-school material, or a textbook sidebar
- UniversityUniversity-level courses not taught in high school (sensory physiology, food science)
- ResearchNot even taught as settled fact at university; researchers are still working on it
Middle schoolTerms: this phenomenon has names
- Taste bud: a tiny organ on the tongue and elsewhere that detects taste. Taste-sensing cells are clustered inside.
- Contrast effect: when a little of a different taste is added, the main taste seems stronger. Watermelon with salt and red-bean soup with salt are classic examples.
- Suppression effect: when two tastes are both clearly present, they weaken each other.
- Threshold: the lowest concentration at which you can notice a taste.
Middle schoolHigh schoolChecking with a formula: how salty is the salted watermelon?
The quantity we care about is "the salt concentration within one slice of watermelon." We compare it with the rough threshold for noticing saltiness, then work out how many times larger the watermelon's sugar is than the salt.
| In symbols | c ＝ ( m ÷ M ) × 100 |
| In words | Salt concentration [%] ＝ weight of salt ÷ weight of watermelon × 100 |
| Where it comes from | It is simply the definition of "mass percent concentration" from middle school. The strength of a taste depends on what fraction of the liquid touching the tongue is the taste substance, that is, on concentration. |
| Symbol | Meaning and unit |
| c | Salt concentration (%) |
| m | Weight of salt added (g) |
| M | Weight of one slice of watermelon (g) |
| Weight of one slice of watermelon (without rind, example) | 250 g |
| A "pinch" of salt (the amount held between two fingers) | said to be about 0.5 g |
| Sugar content of watermelon | said to be about 10% |
| Rough concentration at which saltiness is noticed | said to be about 0.2% |
| Concentration of body fluids (physiological saline) | 0.9% |
| Weight of salt ÷ weight of watermelon | 0.5 ÷ 250 ＝ 0.002 |
| Convert to percent (c) | 0.002 × 100 ＝ 0.2 |
| Weight of sugar in one slice (g) | 250 × 0.1 ＝ 25 |
| How many times the salt is the sugar? | 25 ÷ 0.5 ＝ 50 |
| How many times the current salt concentration is the body-fluid concentration? | 0.9 ÷ 0.2 ＝ 4.5 |
A "pinch" of salt gives a concentration of 0.2%, right at the edge of being noticed. The salt weighs only 1/50 of the sugar, and it is still only 1/4.5 of the body-fluid concentration. This "noticed or not" amount is what brings out the sweetness.
High schoolHigh school+Where are taste signals "compared"?
High schoolWhen a taste-sensing cell receives a taste substance, it produces an electrical signal and sends it to the brain through the taste nerves. Sweet, umami and bitter are thought to be detected by receptors (proteins) on the cell surface, and saltiness by a channel that lets sodium pass through.
High school+The contrast and suppression effects do not necessarily happen only on the tongue. They are also thought to occur at the stage where the brain combines signals from separate routes into one "impression of taste." Food science calls these influences between tastes "taste interactions," and sorts them into contrast, suppression and synergy (where umami substances strengthen each other).
UniversityThe sweet receptor, and another sweetness sensor
The main sweet receptor is a pair of two proteins, T1R2 and T1R3. Separately, it has been reported that SGLT1 (sodium-glucose cotransporter), the transporter that absorbs glucose in the gut, is also present in sweet-sensing cells. Because this transporter carries glucose together with sodium, it would make sense that salt strengthens the response to glucose. In mice, adding salt is reported to have increased the nerve response to glucose.
📖 Derivations and further reading: Taste (Wikipedia, in Japanese) ／ Sodium-glucose cotransporter (Wikipedia, in Japanese)
ResearchWhat is still not clear
- Does the same transporter work on the human tongue? The way salt strengthens the glucose response was studied in mice. How much it matters in people is still being checked.
- Where in the brain does the contrast effect happen? It is unclear how much comes from the tongue's cells and how much from the stage where the brain combines signals.
- Individual differences in the ideal amount of salt. The concentration at which saltiness is noticed is said to vary with person, age and everyday diet, and the amount that brings out sweetness best is thought to differ from person to person.
In other words, this article too is "an explanation as far as we know today." A way of eating learned from long experience is now being explained at the molecular level.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: "concentration of solutions," "sense organs" | Salt concentration calculation, taste buds on the tongue |
| High school | Biology: "reception of stimuli and nerves" | How taste signals reach the brain through nerves |
| High school+ | Home economics / cooking science: "taste interactions" | Contrast and suppression effects |
| University | Sensory physiology, food science | Sweet receptor and sodium-glucose cotransporter |
| Research | Neuroscience of taste | Role of the transporter in humans, where contrast happens |
| ― | Links to daily life | A pinch of salt on watermelon or red-bean soup, salted caramel |
- Wikipedia, "Taste" (味覚, in Japanese)
- Wikipedia, "Taste bud" (味蕾, in Japanese)
- Yasumatsu K, et al. "Sodium-glucose cotransporter 1 as a sugar taste sensor in mouse tongue." Acta Physiologica (2020).
- Takashi Yamamoto (山本隆), Bimi no kozo: naze "oishii" no ka (美味の構造 ― なぜ「おいしい」のか, "The Structure of Deliciousness: Why Does Food Taste Good?"), Kodansha Sensho Metier (講談社選書メチエ)
- Ministry of Education, Culture, Sports, Science and Technology (文部科学省), Standard Tables of Food Composition in Japan (日本食品標準成分表), values for watermelon
*This article is a popular-science explanation. The figures given are rough guides to help you understand the mechanism. How salty things taste varies from person to person. If you have been told to limit your salt intake, follow your doctor's instructions.