I love the taste of lightly fried sliced onion. I've heard this referred to as 'caramelizing' the onion.

Is there sugar in the layers of an onion that is changed to caramel, or is this just a phrase?

My question is: If you 'caramelize' an onion, do they contain sugar?

  • 6
    All food contains sugar, it's just not all the sweet stuff that you're thing about. Check out the Maillard Reaction Commented Apr 17, 2015 at 13:52
  • 8
    @mikeTheLiar It's not true that "all food contains sugar". A bottle of oil contains no sugar whatsoever. There are many foods which contain sugars, but by no means all of them.
    – rumtscho
    Commented Apr 17, 2015 at 15:06
  • 14
    @mikeTheLiar then you understood something wrong. The human body can get energy from proteins, fats and carbohydrates (I've seen conflicting opinions on ethanol, so I'll leave it out). Only the carbohydrates are split down to sugars, the others aren't. The energy from fat comes when the body splits it into glycerol and fatty acids, but 1) the glycerol itself is not split further, and 2) the glycerol is an alcohol, not a sugar. Also, even if something is split to sugar in the stomach (like starch), it is incorrect to say that it "contains sugar". If you cook up a mixture of starch (cont.)
    – rumtscho
    Commented Apr 17, 2015 at 15:20
  • 5
    (cont.) it does not act like a mixture of sugar and water, because it contains no sugar at all. For the cook, it is completely irrelevant that the body can split it into sugar later. Just like starch does not contain feces, even though it can be turned into them by digestion, it does not contain sugar :) So, to sum it up, there is sugar (both sucrose and other types) in more food than people think of, but by far not in all food.
    – rumtscho
    Commented Apr 17, 2015 at 15:22
  • 1
    @rumtscho "If you cook up a mixture of starch it does not act like a mixture of sugar and water, because it contains no sugar at all." - but isn't the sugar in the starch was causes various crusts/breads/etc to brown when cooked (I mean the chemical reaction)? (I'd really love to take this into chat 'cause this sounds fascinating but I'm at work :() Commented Apr 17, 2015 at 15:27

4 Answers 4


Yes, onions contain sugar, just like most fruit and vegetables. It is not simply a common phrase, it is true caramelization.

They have 4.24 g of sugar per 100 g in total (wet weight). For dry weight 40% is sugar. See the USDA nutrient database for more details.

  • 1
    Do you mean 4% instead of 40%? Commented Apr 17, 2015 at 13:42
  • 12
    No, I meant 40% of the dry matter. The 4 grams are per 100 grams of whole onion, which is mostly water. It has 1.1 g of protein, 0.1 g of fat, 4.24 g of sugars and almost 5 grams of nonsugar carbohydrates. The sugar is 40% of these nutrients.
    – rumtscho
    Commented Apr 17, 2015 at 13:44
  • 5
    Ah I see now, just seeing 4.24 / 100 = 40% confused me. Thanks for your explanation. Commented Apr 17, 2015 at 13:48

Onion does contain sugars when raw, but they are pretty much indigestible and tasteless. Cellulose (vegetable fiber), for example, is a complex carbohydrate which only ruminants can digest with the aid of bacteria in their stomach.

With caramelization, complex sugars in onion split into simpler ones, which are the ones we can taste, by the action of heat. Therefore, fried onion tastes sweeter, and so does tomato, etc.

When sugar cane crops are ready to be harvested, dry leaves are burnt in situ in order to increase the yield of sucrose by the same effect: a fraction of the existing complex sugars are turned into sucrose (saccharose).

  • Good answer, cota: mean cook?
    – Chef_Code
    Commented Apr 18, 2015 at 6:59
  • I beg your pardon?
    – Luis Cota
    Commented Apr 18, 2015 at 19:10
  • I also suppose that frying destroys some of the compounds which give the onion its pungent taste.
    – IMil
    Commented Feb 14, 2018 at 8:43

The answer is: An onion contains sugar whether you "caramelize" it or not. "Caramelizing" an onion (or anything else) helps to emphasize the natural sweetness of the onion. You're basically cooking off the water in the onion and concentrating the sugars.


Onions are very sugary. People eat so much sugar these days however, that they don't notice. When someone does something like the atkin's diet or similar, they begin to taste how sweet all those veg really are.

  • 8
    This has been flagged a couple times. I think it is an answer (it says right there, they're sugary) but it's just not the most helpful one. The sugary foods part comes across a bit soapboxy (there are a lot of other reasons we don't taste onions as sweet), and "very" could use elaboration/specificity, like rumtscho showed.
    – Cascabel
    Commented Apr 17, 2015 at 17:42

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