1. If a Raw Onion Has Sugar, Why Doesn't It Taste Sweet?
Japan's official food composition table lists 100 grams of raw onion as 90.1 g water and 8.4 g carbohydrate.[1] Not all of that carbohydrate is sugar; 1.5 g is dietary fibre, the stringy part that we do not digest. Still, a vegetable that is mostly water plus a good share of carbohydrate suggests the onion contains sugars, even if its sharp taste makes them hard to notice.
What a raw onion has plenty of is something else. When you cut it, cells break open and a plant enzyme meets sulphur-containing compounds that were stored separately. They react into the sharp, tear-making chemicals. In a study of more than 100 onion samples sold in the UK, a team from the University of Liverpool and partners measured one marker of this sharpness (pyruvate) and also asked taste panels to rate the onions. Strength went up as the marker went up. The panels could not pick out a sweet flavour in onions, except in those with very low sharpness.[2]
So the sugar is there, but a strong, sulphurous bite may be covering it. One way to picture it is a small light hidden behind a bright one: the sweetness could be there while the sharp taste is stronger. This is our guess; the panel result fits it but does not prove it.
2. How Does Losing Water Make the Onion Taste Sweeter?
When onions go into a hot pan, water evaporates, and a raw onion has a lot of it to lose. Cooks and food-science courses describe browning as starting only once the water has gone: one university chemistry-of-food class that fried onions and checked them every five minutes put it as the food browning "just as the water evaporates".[4]
This matters for sweetness through simple arithmetic. Imagine 100 g of onion loses 60 g of water. Forty grams are left, still holding the same 8.4 g of carbohydrate. That is about 21 percent, roughly two and a half times the 8.4 percent we started with. This is an imagined case; real onions lose different amounts depending on heat and time, and only part of that carbohydrate is sugar. But it shows the idea: the sugar does not have to increase for each bite to hold more of it.
3. What Makes the Colour and Smell of Fried Onions?
Once the water is mostly gone, the temperature in the pan can climb and the onions turn gold, then brown. Two kinds of change are usually named. Caramelization is sugar browning when it is heated by itself. The Maillard reaction is a different set of reactions, in which sugars react with amino acids, the building blocks of protein. Both are forms of browning without enzymes, and both make new flavour and colour.[3] Onions contain sugars, and probably amino acids too, so both kinds of change may be involved; the sources we read do not say how much comes from each.
Different sugars start to caramelize at different temperatures. For example, one reference table gives fructose at about 105 °C and galactose at about 160 °C.[3] Those are figures for the sugars on their own, so they are a rough guide, not a rule for what happens inside a slice of onion.
Browning also makes new smells, and some of them are what we call sweet, even though they are scents rather than sugar on the tongue. That is another reason a browned onion seems sweeter than a boiled one.
4. Does Cooking Actually Make More Sugar? What the Sources Do Not Say
This is the part people often state with confidence, so it is worth being careful. You may read that heat breaks the long sugar chains in onions (fructans) into small sweet sugars, or that the sharp chemicals turn into sugar. In the sources we could read, neither claim comes with a measurement from onions being fried. The Liverpool study looked only at fresh, uncooked onions.[2] The sources on browning describe what happens to sugars as they are heated, but they do not count how much sugar a frying onion has at each minute.[3]
That leaves three plausible explanations, and one open question. If the sharp taste weakens, sugar may be easier to taste; we did not find this tested in fried onions. Water leaves, so each bite holds more sugar. Heat changes sugars into new coloured and scented compounds. Whether the amount of sugar itself rises as well, and by how much, we could not confirm. The weight of each effect probably depends on the kind of onion, the heat and the time, but the sources we read do not split them into numbers.
5. How Can You Watch Water Leave an Onion at Home?
With an adult, slice an onion and weigh it on a kitchen scale. Cook it gently in a pan with a little oil, and every five minutes lift out a spoonful onto a plate. Look at the colour and size, smell it, and, once it has cooled, taste a very small piece. Weigh the pan's contents at the start and end if your scale allows. A big drop in weight before the colour changes would fit the idea that water goes first. Take care with hot pans and knives, and never taste food while it is hot.
To go further, look at the sources below. The Liverpool paper is about why people find some onions mild and others strong. A useful next question is whether anyone has measured sugar in onions as they cook.