Sourdough and fructose cooperation ?

Hi

I'm making a sweet bread called "tsoureki" for some years now with yeast (compressed type). Also I'm using fructose, instead of sugar (health issues). It's sweeter than sugar and I've adjusted my recipes accordingly.

Last month I've tried some with sourdough, and the taste was great, but I'm facing a problem with the fermentation. The "yeast" recipe had a 28% fructose, with 8.4% yeast and rises like a mountain within 1 - 2 hours. For the "sourdough" version I've tried first to put a 10% sugar on a mix of 50% starter (100% hydration) with acceptable results. In my last attempt i raise the fructose to 22% and the dough made 8 hours to rise to a bake-able size.

My question is, how much sugar a sourdough can handle ?? Does anyone ever tried to use so much sugar in a recipe ?

If I use more water this can help the sourdough, or not ?

This is the Bakers % list for the sourdough version

100% Bread Flour (Robin Hood 14% protein)

50% starter (100% hydration)

12% milk evaporated 2% fat

10% butter pure (no water)

25% whole eggs 

22% fructose (sugar)

1,4% salt

1,2% mahlepi

0,2% vanilla

0,2% mastic

And this is the "yeast" version

100% Bread Flour (Robin Hood 14% protein)

8.4% fresh yeast (compressed)

15% warm water

10% milk evaporated 2% fat

10% butter pure (no water)

25% whole eggs 

28% fructose (sugar)

1,4% salt

1,2% mahlepi

0,2% vanilla

0,2% mastic

Thanks for any help
Dimitris