I have been experimenting with sugar wash fermentations. I've tried sucrose (cane sugar) as well as dextrose (corn sugar) and observed stalled fermentations in both, in spite of sufficient yeast nutrients, DO and yeast alcohol tolerance. I'm suspecting that the pH of the wash may be the culprit. As other already have noted (but something I had yet to learn when I started) the pH of a fermenting sugar wash can crash to 3 or even 2.5 or so. Unsurprisingly, my yeast (a S. Cerevisae strain) appears to struggle with this low pH. A resumption of fermentation after raising the pH with calcium carbonate appears to confirm that.
As I understand it, this acidification is the result of organic acid production by the yeast during fermentation (which also plays a role in ester formation). In order to prevent too much acidification and ester production (the latter being undesirable in a wash intended to produce a neutral spirit) distillers recommend to buffer the wash, e.g. with calcium carbonate. I am trying that at the moment, but no-one seems to agree on how much calcium carbonate to use.
That said, organic acid production is a normal thing for yeast to do. However, this is far more of an issue in sugar wash fermentations than in beer fermentations: while in fermenting wort the yeast tends to drop the pH to 4-4.5 or so, it rarely drops to 3 or below. This suggests that wort has some sort of buffering capacity.
What causes this? What wort component(s) have this buffering effect?