tl;dr version: why does S/Bayanus ferment sucrose to an apparent attenuation of more than 95% while S/Cerevisiae ferments it to an apparent attenuation of about 65-70%?
I've been experimenting with sugar wash fermentations, using white table sugar (sucrose) and S/Cerevisiae yeast strains marketed as distilling yeast. Attenuation proceeded from an OG of 1,100-1.110 to an SG of around 1.030 (apparent attenuation about 65-70%).
This matches what Beersmith predicts, e.g. using 5kg dextrose in an 18 litre batch size with, say, US-05.
When I replace S/Cerevisia strains in Beersmith with Red Star Champagne yeast (S/Bayanus) my projected FG drops from 1.032-1.033 to about 1.003. Based on experience I'm happy to accept that (although I haven't tried it yet) but what I don't understand is why champagne yeast (S/Bayanus) ferments sucrose so much better. What Beersmith predicts confirms what I have heard from other home distillers who use champagne yeasts for sugar fermentations, so I'm glad to accept it as a fact but I don't understand why this is.
I prehydrated the yeast, pitched generously and provided ample levels of yeast nutrients and DO. The yeasts I have tested have an alcohol tolerance of 15% V/V or more, while my fermentation produces no more than about 10%, so it's not the yeast's alcohol tolerance that is the limiting factor here.
According to Beersmith, replacing the sucrose with dextrose (compensating for the fact that dextrose is a monohydrate) makes no difference. Not that I expected any, since yeast enzymatically breaks down sucrose into two monosaccharides and then ferments the monosaccharides.
From what I previously understood, I believed that pretty much all yeast strains should ferment sucrose entirely. But I was obviously wrong: based on what I see in Beersmith and on my hydrometer this is not true, and it looks like S/Bayanus ferments sucrose much further than S/Cerevisiae.
Why is this and how does it work?