Fermentation versus Respiration --- the Energy Generation Paradox

As an aspiring winemaker, I was surprised to learn that even in the presence of oxygen, the wine yeast, Saccharomyces cerevisiae prefers to use glucose to generate ATP via fermentation instead of respiration. This is a paradox because respiration leads to the generation of 680 kilocalories (kcal) of energy yielding between 36-38 molecules of ATP for each molecule of glucose whereas fermentation leads to only 56 kcal of energy and only a net 2 molecules of ATP.¹
If glucose levels are high, even if there is oxygen present, S. cerevisiae would rather ferment glucose than respire glucose. This is called the Crabtree effect or glucose inhibition. When glucose concentrations are high, it costs the yeasts more to synthesize the enzymes necessary for respiration than it does to just ferment glucose.² This is of benefit to the winemaker because the net result is the transformation of glucose to ethanol and carbon dioxide.

Approximately 94% of the glucose is converted into ethanol and carbon dioxide. The other byproduct of fermentation that we need to account for is the 56 kcal of energy that is produced by the fermentation of 1 mole of glucose (180 grams). Less than half of that energy (22 kcal) is stored as ATP for use by the yeast in other biochemical reactions, the rest (34 kcal) is lost as heat.³ The implication here is that for every reduction in Brix of 1^o there is a concommitant rise in temperature of 2.3 ^oF.

Since yeasts die at temperatures above 100 ^oF, it is important to take into account the generation of heat during fermentation.
1. VID252 Wine Production UC Davis Online Course
2. The Crabtree Effect: A Regulatory System in Yeast
3. VID251 Introduction to Winemaking UC Davis Online Course