First, I should point out that I am not a Sourdough fan, I have always been someone who prefers to add yeast and make quick breads for family and friends at the drop of a hat (such as a phonecall from a friend dropping around for a visit) and to this end I have spent some time trying to find the best yeast activation nutrient. I have spent quite a few years trying to find things that work, I think I have worked through dextrose powder, to the various malts, molasses and ever other conceivable combination of each including adding some ascorbic acid to the mix.
That was until I found Rapadura sugar, an evaporated cane juice that retains all the natural goodness, minerals and vitamins. It is a beautiful golden colour with a fantastic complex flavour and is now my default sugar for baking. Every time I use rapadura to activate yeast it goes ballistic and in most cases I get a nice foamy mixture in minutes, and my proof times to double are often in as little as 1/2 hour which means I can often go from mix to serve in as little as two hours.
Here is a video example of how effective rapadura is, this is 7g of the sugar with 7g of yeast and a little tepid water. Its a 2sec per frame time lapse, from adding the water to the end is approximately 15 minutes. To the point of being ready to use, it was fully active in under two minutes (shortly after you see the yeast bloom across the surface of the water).
So there you go, if you can find yourself some Rapadura give it a go, you will be pleasantly surprised I think.