On Tue, 23 Mar 2021 15:33:42 -0700 (PDT), RichD
> wrote:

>Some recipes call for caramelizing onions, apples, whatever.
>Apparently, after heating for a while, a sudden change of state
>occurs; caramelization
>
>Any chemists here, who can describe the chemistry of this process?
>Somehow, it seems the sugars concentrate.

It is called the "Maillard reaction".
Maillard browning is a chemical reaction that usually occurs between
amino acids (the building blocks of protein) and those carbohydrates
known as reducing sugars – although the reaction has been known to
occur between reducing sugars and whole proteins.
Janet US