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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?

hawkeye
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    _All_ food contains sugar, it's just not all the sweet stuff that you're thing about. Check out [the Maillard Reaction](http://en.wikipedia.org/wiki/Maillard_reaction) – Mike G Apr 17 '15 at 13:52
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    @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 Apr 17 '15 at 15:06
  • @rumtscho when I say "sugar" I'm not referring to glucose/fructose/things that are perceived to humans as sweet. I am not a chemist, but it's my understanding that _all_ digestible calories are a form of sugar. In the case of vegetable oil, it would be glycerol. Feel free to prove me wrong :) (no, seriously - if I'm wrong please tell me) – Mike G Apr 17 '15 at 15:15
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    @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 Apr 17 '15 at 15:20
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    (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 Apr 17 '15 at 15:22
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    @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 :() – Mike G Apr 17 '15 at 15:27
  • @mikeTheLiar this is indeed getting too long for comments, and unlike the digestion part, "Where does the sugar for maillard come from in bread" is ontopic. It's best to write it up as a separate question. I'll answer it later, both to give others the opportunity to get some rep and because I spent a bit too much time doing SE stuff today. – rumtscho Apr 17 '15 at 15:36
  • @mikeTheLiar You might also consider asking on biology.stackexchange.com for some of that; the issue of what your body actually does with all those macronutrients isn't really on topic here. – Cascabel Apr 17 '15 at 17:00
  • There are some great answers to your question already, but also some wonky ones. I'm new to cooking.se, but I think this sort of question would be more accurately and informatively answered at chemistry.stackexchange.com. Next time you have a similar question, consider posting there instead! – Curt F. Apr 18 '15 at 23:19

4 Answers4

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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.

TFD
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rumtscho
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    Do you mean 4% instead of 40%? – Michael McGriff Apr 17 '15 at 13:42
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    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 Apr 17 '15 at 13:44
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    Ah I see now, just seeing 4.24 / 100 = 40% confused me. Thanks for your explanation. – Michael McGriff Apr 17 '15 at 13:48
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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).

Luis Cota
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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.

scott pratt
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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.

Escoce
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    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 Apr 17 '15 at 17:42