Mike Tommasi wrote:

> Sugar content remains unchanged in absolute terms (it does not
> evaporate), it adds to whatever sugar was already there, and throughout
> the various processes (dilution with stock or other liquids, reduction,
> etc.) the concentration will change accordingly.

What's likely to happen to some of the sugar is the Maillard reaction,
the process that's responsible for the browning of bread and other tasty
events. It's also responsible for the brown color of caramelized sugar.
However, a sweet wine has enough sugar that I suspect only a fraction
will be consumed in the Maillard reaction.

Mark Lipton