Of the sugar you've just added. Onion caramelization is a complex process, a prime example of the Maillard reaction. It involves several chains of chemical transformations, while the added sugar is only a step away from caramel.
> An earlier version of this recipe suggested using sugar and baking soda for even faster results.
https://www.seriouseats.com/quick-caramelized-onions-recipe
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> help things along with a little bit of sugar. In science speak, caramelization is the oxidation of sugars. Here, those sugars are the natural sugars in the onions, but the addition of a small amount of white sugar adds an extra caramelization boost.