What is the maximum number of carbons in sugar that can be fermented by yeast?

Yeast is capable of fermenting sugars primarily through the action of enzymes that break down carbohydrates into simpler sugars, which are then converted into alcohol and carbon dioxide during fermentation. The sugars that yeast can ferment include monosaccharides and disaccharides, with glucose and fructose being among the most readily utilized.

The most common fermentable sugars that yeast can process generally contain up to six carbon atoms, which is characteristic of hexoses like glucose and fructose, as well as disaccharides such as sucrose and maltose. These six-carbon sugars provide the necessary energy and carbon skeleton for yeast during the fermentation process.

While yeast can metabolize sugars with fewer than six carbon atoms—such as the five-carbon sugar ribose—the capability to ferment sugars beyond six carbons becomes less efficient, as yeast does not effectively ferment complex carbohydrates and larger sugars. Specifically, sugars with seven or more carbons, like heptose, are not fermentable by typical brewing yeasts.

Thus, the ability of yeast to ferment sugars is maximally efficient with six-carbon structures, confirming that the maximum number of carbons in sugar that can be fermented is six.