Yeast Alcohol Tolerance

[tradowsk]

A question I've had but never really found a good answer for: what happens to yeast when they reach their alcohol tolerance? Do they start dying off? Or do they just go dormant?

For example, say I had a wine where the yeast reached their tolerance and stopped fermenting even though there was some sugar left. If I added some water to dilute the alcohol, would it start fermenting again? What if I waited a few weeks/months and then added the water? Would the yeast still be alive/active sitting for so long at their tolerance?

Or, what if I added simple syrup or honey instead of water? That would still dilute the abv, so would it kick off more fermentation?

 

[BernardSmith]

Hi tradowsk. Not a biologist or a microbiologist but two or three quick thoughts.
1. Yeast cells that are thousands of years old have been reactivated after millennia of dormancy in several experiments in the middle east in attempts to replicate beers that were consumed by the ancient Egyptians (and others). Here is one reference. Researchers Brew Beer with 5,000-Year-Old Yeasts | Archaeology | Sci-News.com. So bottom line, yeast is a living organism but because of its relative simplicity it can survive in conditions that defy our expectations for more complex life forms.
2. When you add water to alcohol you dilute the alcoholic content and so any yeast cell that was unable to continue to ferment at the higher concentration is far more likely to begin to ferment in the lower concentration.
3. When you add syrup or honey you are not in fact working to lower the overall concentration of alcohol. Certainly, the immediate impact MIGHT be to lower the ABV but if there is enough fermentables in the addition that ABV will tend to rise back to where it was ... That said, yeast have limits in dealing with the density of the substrate they are being asked to ferment. Too much sugar in solution (too high an SG) means that they are unable to transport sugars through their cell walls, though, clearly when there is already alcohol in solution with water the SG of that solution is going to be less dense than if there is only water (which is why you can ferment to an SG lower than 1.000).

 

[user 36973]

The yeast does physically change in response to alcohol somewhere around the 9-10% level. See here for some more info.