Why do yeast make alcohol even when there is plenty of air?

Yeast make alcohol in the presence of air because, when sugar is plentiful, sugar itself switches them toward fermentation. It sounds backwards, since yeast can breathe, and breathing with oxygen earns far more energy from sugar: 32 ATP, the cell's energy currency, per glucose, against 2 to 5 for fermentation. But in a sugary liquid, S. cerevisiae, the species of baker's and brewer's yeast, ferments anyway. This is called the Crabtree effect, after the English biochemist Herbert Grace Crabtree.

How does it work? As glucose concentration rises, glycolysis, the first stage of sugar breakdown, speeds up, and it makes a good amount of ATP by itself. That cuts the need for the slower oxygen-based route, and so oxygen consumption goes down. In the lab, S. cerevisiae made no ethanol in air below about 150 mg/L of glucose, but above that it did.

It may also be a weapon. The effect is believed to have evolved as a way to beat competitors, since ethanol is antiseptic, about when the first fruits fell from the trees. Ripe fruit full of sugar is a place where yeast live, and a yeast that quickly turns sugar into a poison for its rivals gets the fruit for itself.

Strangely, the effect was found through cancer research: Crabtree discovered it while studying tumor cells, whose metabolism, called the Warburg effect, also favours glycolysis over oxidative phosphorylation.