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Similar questions have been asked previously, although none have specifically answered the question of whether true caramelisation really happens when 'caramelising' onions (sucrose & glucose caramelises at 160°C while fructose caramelises at 110°C). Slow cooking onions will induce the Maillard reaction, with its caramel-like flavours, evaporate off their water to concentrate the natural sugars and also break down the pungent trisuphides into milder sulphides and disulphides. Together, this is more than enough to make cooked onions taste sweeter. I have yet to find any authoritative source, however, to show that true caramelisation actually happens, or that starch pyrolysis (the break down of starches into sugars) really does take place when 'caramelising' onions. Does this exist or is caramelisation a misnomer?

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If I understand your question correctly, you're asking:

  1. Does "true" caramelization happen when browning onions?
  2. Does starch pyrolysis take place when browning onions?

Ad 1) I think it's indisputable that onions contain sugar and that sugar can be caramelized. 100g of raw onion contain about 90g of water, 9g of carbohydrates, 4 of which are sugar, and about 1 g protein per USDA. Cooking out the water allows the batch to reach temperatures higher than 100 °C, allowing for a slow caramelization process, as fructose can begin caramelizing at 105 °C.

Ad 2) The other 5 g of carbohydrates are mostly fructan, a polysaccharide, more specifically a polymer chain of fructose molecules. Fructane plays the role of starch in alliums such as onions and garlic. This study says - in the abstract, at least - that starches are almost nonexistent in most parts of the onion. I conclude that starch pyrolysis is therefore not really a factor in caramelizing onions. More so, if the fructane chains are broken down into fructose, more sugar is available for caramelization.

Additionally, I recommend this article from Daniel Gritzer at Serious Eats, although the pure caramel science is a bit hidden there.

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  • A sugar solution has to be extremely sweet to raise the boiling temperature significantly, even to 105 C (at which point caramelization would still take days). It just seems unlikely that the water content of caramelized onions is so low.
    – Sneftel
    Commented Jul 4, 2023 at 17:51
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    @Sneftel you can make them successfully overnight in a slow cooker on low, with the water they give off barely simmering - and present - so you don't even have to get to 100°C.
    – Chris H
    Commented Jul 4, 2023 at 19:00
  • But the fructane breaking down could explain the increased sweetness
    – Chris H
    Commented Jul 4, 2023 at 19:01
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    Exactly -- the temperature requirement for caramelization just doesn't seem to be achievable. Perhaps for crispy fried onions, but not for soft ones.
    – Sneftel
    Commented Jul 5, 2023 at 13:29
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    @LightBender No, my point is that if it's only the surface that "caramelizes" that supports your idea that it's actual caramelization in high-temperature areas,
    – Sneftel
    Commented Jul 6, 2023 at 12:28

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