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Imagine I liquefy 10 onions with a powerful blender and I subsequently reduce the liquid. What would happen? Would I end up with a caramelized onion paste? Or something else?

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    Just did this with one onion, a food processor and a rice cooker on quick cook. You have to add a little water to steam up the mashed onion, and some Olive oil to get a decent fry before the cooker shuts off. After 3 or 4 fry cycles, you get a nice, tasty brown caramelized puree. Probably could use a fry pan instead of a rice cooker, but I like the way the cooker turns off before burning anything. On a stove, you may not need the initial water, but you will need an oil. One of those 20krpm ninja mixers might give you a different product. – Wayfaring Stranger Jul 16 at 3:40
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    Why don't you try it and tell us? – Rob Jul 16 at 10:55
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    @WayfaringStranger How was it different from traditional caramelized onions? – Behacad Jul 16 at 12:08
  • @Behacad Just smaller bits. You'd really have to chop fast to break down the cell structure. I expect cooking that would give you something that still looks about the same. – Wayfaring Stranger Jul 16 at 14:47
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    @WayfaringStranger I added a few details to my own answer, but it tasted good. I'm not an expert on caramelized onions, but I do eat them regularly and I thought the taste was the same. – Behacad Jul 17 at 15:09
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You would first boil the onions and then most likely burn them.

1st: boiling. Caramelization needs higher temperatures than 100 Celsius but as far as there is water in the mixture, temperature won't rise. Therefore you'll get boiled onions.

2nd: burning. Once the water is gone temperature rises very fast and since caramelization takes time you get to burn them first.

Chopping vs blending:

Chopped onions keep the fluids inside the cells and release them gradually. This makes the temparature rise over 100 °C and prevents it to go too high. This way is much easier to set the stove to the right temperature.

Water and oil:

Water and oil don't like each other very much so trying to add oil to a watery mixture is a bad bad idea. Chooped is the only way if you use an oiled pan.

Notes:

  • Caramelization happens around 120-165 Celsius, below the browning is too slow and above will be fast burnt
  • If you want carmelized onion paste, blend afterwards.
  • To speed up the process and get a paste at the same time you can add baking soda to the onions. Maillard reaction (browning) is much much faster in alcaline environment, which you'll get by adding the baking soda. Also cells will degrade and you'll end up with a paste. Worth a try just to see the results.

Guide: Here is a guide to carmelized onions from serious eats where they address the how and the why.

  • Someone seemingly tried this and the onions did not burn. Look at the comment by @WayfaringStranger – Behacad Jul 16 at 12:09
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    Yes, but he had a cooker. It means its' closed and does the cooking by itself (i.e. keeps the temperature in check). In a pan at first oil and water will sparkle in all directions and when water is gone oil will heat up fast and burn the onions. And also the oil will burn if you don't put a lot of it (i.e. like when you deep-fry). But I won't insist. I guess you'll try and tell us what happened – Francesco Zambolin Jul 16 at 12:22
  • But how is this true of the blended product and not finely chopped onions that you're caramelizing? Once the water is gone the onion can burn, but if you do it well they turn beautiful and sweet and brown. – Behacad Jul 16 at 13:16
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    As I said in my answer, by blending you destroy the cells and fluids come out. Now those fluids will prevent the temperature to rise above 100°C. But if the fluids remains in the cells they do not take part in evaporation and temperature can rise. Cooking destroys gradually the cells and fluid come out preventing the temperature to rise too high. By the time the onions are cooked they are also caramelized. You need time for browning to take place – Francesco Zambolin Jul 16 at 13:26
  • Ah I see. Hopefully I can try and we can see if you're correct. – Behacad Jul 16 at 14:58
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I attempted this as an experiment and the result was very similar to traditional caramelized onion:

https://imgur.com/f0R56HL

I'm not expert so it is hard for me to comment on the changes in flavour, but it seemed surprisingly stereotypical of caramelized onion to me. The texture is softer, perhaps like a putty.

One benefit of this technique is that this process took approximately 35 minutes, which is shorter than other methods I've tried to caramelize onions.

I pureed two onions in a high-powered blender until it was liquified into a thick white liquid. I cooked it over high heat until it started to stick a bit and progressively reduced the heat until it was done. I added some oil towards the end.

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    Nice, can you write the step you took? Consistance of blended onions(fine chopped or puree), how much oil and when you added, stove heat (low, medium, high) etc.. Would be useful. thx – Francesco Zambolin Jul 18 at 12:12
  • I get away with the water plus oil because I use a closed rice cooker. At first, the water steams the 5 inches of chopped onions I have in the lot and gets them releasing their own water. After it boils down a couple inches, I refill the pot with more chopped onion, and cook (boil then fry) until I have nice caramelized onions for onion soup. The temp sensor prevents burning, but sometimes I have to cycle a few times to get things brown enough. I find it a lot easier than messing with pans or ovens. – Wayfaring Stranger Jul 18 at 17:40

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