I started to respond to this last evening but we lost power with the extremely strong winds.
I don't personally sweeten many wines so I just heat water and sugar, but the citric acid is reported to hel conversion. See the following:
<H2>Making Invert Sugar</H2>
Inverted sugar syrup can be easily made by adding roughly one gram of
citric acid or
ascorbic acid, per kilogram of sugar.
Cream of tartar (one gram per kilogram) or fresh
lemon juice (10 milliliters per kilogram) may also be used (1 tsp lemon juice per 1 pound sugar).
The mixture is boiled for 20 minutes, and will convert enough of the sucrose to effectively prevent crystallization, without giving a noticeably sour taste. Invert sugar syrup may also be produced without the use of acids or enzymes by thermal means alone: two parts granulated sucrose and one part water simmered for five to seven minutes will convert a modest portion to invert sugar.
All invert syrups are created by hydrolysing sucrose to glucose (dextrose) and fructose by heating a sucrose solution, then relying either on time alone, or time and the
catalysis reaction of an acid or enzymes to speed the reaction. Commercially prepared acid catalyzed solutions are neutralised when the desired level of inversion is reached.
All constituent sugars (sucrose, glucose and fructose) support fermentation, so invert sugar solutions may be fermented as readily as sucrose solutions.