It’s harvest time in the Northern Hemisphere, and it will soon be asserted, far and wide, that one of the reasons alcohol levels are climbing faster than Bush’s poll ratings are dropping is that currently available commercial yeast strains yield more alcohol than their fungal ancestors did. Alas, this claim is widely wrong—at least within the reality-based winemaking paradigm.
The notion that modern (post-modern?) yeasts deliver more alcoholic bang for the Brix is another urban wine legend, something that could only be true if the laws of chemistry-as-we-know-it were repealed. We know two basic things about yeasts: they are the only single-celled fungi; and what they do is convert sugar to alcohol, until they die off. More specifically, the yeastie beasties take one sugar molecule and turn it into two ethanol molecules, two carbon dioxide molecules, some heat and very small amounts of some other by-products. There’s only so much carbon in the sugar to redistribute into the new substances; all the yeast nutrient in the world can’t talk a yeast cell into making two and a half ethanol molecules, or three.