Sucrose fermentation using S/Cerevisiae vs. S/Bayanus

Question

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%?

Details:

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?

Answer 1

While I'm not entirely sure what you're proposing is true (see below), here are some points to consider which potentially support your claim:

• Saccharomyces bayanus has an active fructose uptake system, while S. cerevisiae relies on 'facilitated diffusion' to uptake fructose. Meaning? S. bayanus actually expends cellular energy to bring fructose into the cell from outside. While the net energy gained from metabolizing each molecule of fructose is therefore lower (by ~25%) than it would be if no energy had been expended, it could be more efficient overall. Facilitated diffusion, on the other hand, relies only on the concentration gradient of fructose between the outside and inside of the cell (a higher concentration outside basically forces fructose in through the cellular membrane). However, this 'driving force' becomes less and less as the concentration outside the cell decreases. This suggests that S. cerevisiae may be less efficient at sucrose utilization (given that sucrose is just glucose and fructose bound together).
• S. bayanus also has the ability to grow in a vitamin-free medium, whereas S. cerevisiae does not. This could be an important differentiation in the case of a sugar wash, where vitamins/nutrients should be limited to what you provide.

Honestly, though, I'm just a bit skeptical, for a few reasons:

• I personally couldn't find any evidence online suggesting that strains of S. bayanus were more able to ferment sucrose-based sugar wash. If you had any to share, I think that would be helpful in establishing whether or not it's actually true.
• I'm pretty sure BeerSmith isn't working complex physiological considerations into its attenuation calculations (that is to say, I think attenuation is considered only as the percentage of drop in gravity from OG, and doesn't involve calculating complicated interactions between yeast species and sugar source). This seems born out by the fact that you see no difference in the program's predictions when you replace sucrose with dextrose (both being 100% fermentable). My guess is that the yeast you switched to simply has a higher attenuation value associated with it (again, just a guess, I'm not very familiar with BeerSmith).
• I've never done a sugar wash before, but your finishing gravity of 1.030 seems very high for what other people online report (see: here, here, eg.). It seems like sugar washes benefit from long fermentation and a very close eye on things like pH and nutrient availability. From my own experience, I've always found it to hold true that (for all intents and purposes) both sucrose and dextrose are fully fermentable by all commonly used yeast strains (at least within that strain's own limitations). This could suggest that your fermentations with S. cerevisiae are not reaching their full attenuation potential.
• Another thing to watch out for is that (at least according to Wikipedia) many strains marketed as S. bayanus are found to actually be S. cerevisiae.
• You should always try to remember that comparing different yeast strains (and in this case species) is never going to be a 1-to-1 comparison. Strains and species have highly specific requirements for nutrients and environmental conditions (e.g. the idea I mentioned above that S. bayanus can grow in a medium free of vitamins) and just because you're treating the fermentations the same way (yeast nutrient, DO, etc.) doesn't necessarily mean you can assume the outcome represents the theoretically capabilities of the particular yeast. Even just different strains of S. cerevisiae will respond very differently to environmental factors such as pH, nitrogen availability, ethanol levels, temperature etc., and I think this especially holds true in a very nutrient-limited situation like a sugar wash. Just for example, consider that, under nitrogen starvation, S. cerevisiae is known to irreversibly lose function of its sugar transporter molecules (basically the cell will break down and use its own internal proteins/enzymes as a nitrogen source). This indicates that, though you may be feeding with a decent amount of yeast nutrient, if the levels and (possibly more importantly) the timing of this addition isn't right, it can lead to a non-reversible change in yeast physiology which could affect fermentation performance.

I suppose the real point here, though, is that yeast and fermentation are an incredibly complicated thing. I think it's advisable, then, to be cautious about the assumptions you make about one species, especially if they're based on 1. the predictions of a computer program and 2. your experience with a different yeast.

_{*Info not linked was taken from the book Brewing Yeast and Fermentation}

Answer 2

One thing I have since learned is that the pH of a sugar wash can crash quite severely (down to 3 or even 2.5). S/Bayanus tends to handle a lower pH better than S/Cerevisiae does. The former feels fine at a pH of 2.8-4 while the latter prefers 4-4.5. I'll experiment with buffering the pH of a wash and see how the two compare then.