26.4 Yeast

What yeast is and what it eats

Yeasts are fungi made of a single cell, and they are found almost everywhere in nature. They can feed on many things, including alcohols, acids and amino acids, but they prefer sugar. They can use simple sugars (glucose, fructose, galactose, mannose), the double sugars maltose and sucrose, and even ethanol.

The yeasts used in food are facultative: they can breathe oxygen, ferment, or do both at once. Yeasts are always present in the air, and they need food and warmth to grow.

Alcohol and gas

Yeast empties its NADH carriers by a different route from the lactic acid bacteria. It turns pyruvate into ethanol in two steps.

• An enzyme cuts a piece off pyruvate. That piece leaves as carbon dioxide gas, and what remains is acetaldehyde.
• A second enzyme adds electrons from NADH to acetaldehyde, which becomes ethanol. The emptied carrier goes back to glycolysis.

Every glucose a yeast ferments therefore gives off both alcohol and gas. Saccharomyces and other fermenting yeasts work this way. The kitchen uses both products. The gas is what makes bread dough rise. The alcohol, made on purpose, gives wine and beer. Made by accident, the same process makes fruit and vegetables ferment and rot. Food with unwanted yeast in it often turns slimy.

Why wine stops at about 16%

With oxygen or without?

Pasteur thought that yeast makes alcohol only when it runs out of oxygen, as a kind of starvation. That idea has since been shown to be wrong.

What actually happens is called the Crabtree effect. Breathing oxygen gives a yeast more energy from each sugar than fermenting does. Even so, when Saccharomyces finds itself in a lot of sugar, it starts making alcohol at once, while still breathing oxygen at the same time. It does this in beer wort (the sugary liquid that becomes beer) and in bread dough, however much air they hold. One idea is that the yeast makes alcohol to kill rivals that compete for the same sugar.

In short, Saccharomyces cerevisiae ferments when sugar is high, even with plenty of oxygen; breathes oxygen when sugar is low and oxygen is plentiful; and ferments when there is no oxygen. It can stand alcohol that most of its rivals cannot.