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I use onions to add a sweeter taste to some dishes, for example in tomato sauce for pasta. I cook them in oil, with some salt, before adding the other ingredients. It seems to me that when I turn the temperature too hot, the onions don't become sweet at all, but rather retain some of their spiciness. They also look less "glassy". Why is this?

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Wikipedia: "Caramelization is a complex, poorly understood process that produces hundreds of chemical products, and includes the following types of reaction: <eight bullet points>". Uh oh. +1, though! –  Jefromi Oct 19 '10 at 13:27
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@Jefromi: This isn't caramelization. Caramelization involves browning while the question evidently refers to short pan-frying or sautéing (which is what gives them the translucent appearance described in the question). –  Aaronut Oct 19 '10 at 13:39
    
@Aaronut: Hm, my bad. I thought this was referring to cooking long and slow, so that they brown (but not dark) evenly all the way through, instead of browning rapidly just at the surface on high heat - and get very very sweet. I grew up with my mom calling that "caramelized onions" - is that not right? –  Jefromi Oct 19 '10 at 14:22
    
@Jefromi: If they're fully-browned then they are indeed caramelized. Caramelization doesn't actually require a low temperature though, it's just easy to burn them first at high heat. In either case, this question appears to be about the taste of regular, non-caramelized onions cooked at low heat vs. high heat. –  Aaronut Oct 19 '10 at 14:28
    
I'm keen to know the answer to this because I want to avoid sweet onions (the opposite of what you want, but the answer will work for us both). –  Francis Davey Jan 27 '13 at 21:23
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Well, you're definitely right. Onions cooked at a high temperature act differently than ones cooked at a low temperature, per "the bible" aka On Food and Cooking. However, it doesn't go in to much of an explanation as to why. Probably the most relevant aspects of what is in there are:

When onions and their relatives are heated, the various sulfur compounds react with each other and with other substances to produce a range of characteristic flavor molecules. The cooking method, temperature and medium strongly affect the flavor balance. Baking, drying, and microwaving tend to generate trisulfides, the characteristic notes of overcooked cabbage. Cooking at high temperatures in fat produces more volatiles and a stronger flavor than do other techniques. (p.311)

Since a low temperature produces less volatiles, I assume that the natural sugars of the onion shine through instead.

It's probably also important to understand where the oniony flavor comes from. The spice, as you describe it, is the onions natural defense mechanism. However, it doesn't exist defacto in the onion, but is rather the result of a chemical reaction. The onion stores very reactive sulfur in the cell fluid, and a seperate trigger mechanism in a storage vacuole. When you cut / crush / cook/ peel the onion, you break the vacuole and the enzyme and sulfur cause a chemical reaction giving off the spice. (p272 & 310)

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I need to sit down and read that book cover to cover some day. –  hobodave Oct 19 '10 at 18:44
    
Then maybe I could answer EVERY question on the site. muahahaha –  hobodave Oct 19 '10 at 18:45
    
@hobodave, I tried to do that, but it's like going back to high school biology class. It is some serious, technical science reading. It was a bit much for me to "read". Instead, I reference it whenever I'm interested in a particular thing. I'm excited about his new book, which is supposed to largely be a response to people telling him OFAC is awesome, but it's not very approachable. –  yossarian Oct 19 '10 at 19:20
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And by no means do you need to INCREASE the number of questions you answer. Leave something for the rest of us. Jeez. –  yossarian Oct 19 '10 at 19:21
    
User TXCraig1 gave the following explanation on The Naked Scientists forum: "Onions contain a lot of fructans (fructans are fructose polymers - as opposed to starch which is a glucose polymer). Cooking them hydrolizes the polymers, breaking them down into fructose and fructooligosaccharides." thenakedscientists.com/forum/index.php?topic=9846.0 –  Juha Palomäki Oct 21 '13 at 5:01
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