Chemistry of Cooking

What is Yeast?

We have been eating raised, or leavened, breads for 6,000 years, but it wasn’t until the investigations of Louis Pasteur 150 years ago that we began to understand the process of how bread actually rises.  The key is the gas-producing metabolism of a particular class of fungus, the yeasts.  The word “yeast” comes from a root word that meant “to seethe, boil, bubble over,” the image in which fermentation seems to be a kind of cooking and generating a transformation from within.

The yeasts are a group of microscopic single-celled fungi, and a relative of the mushrooms.  There are over 160 species of yeast, each used for different purposes.  Yeasts are very small living organisms whose food is mostly sugar and whose waste products, when grown anaerobically, are mostly carbon dioxide (CO₂) and alcohol.  The overall equation for the conversion that takes place in yeast cells is:

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

The yeasts used in baking produce little alcohol.  One common yeast used in almost all baking is Saccharonmyces cerevisiae, which, in an oxygen-rich environment, converts sugar (mostly glucose) into CO₂ and water.

Forms of Baker’s Yeast

Commercial yeast is sold to home and restaurant cooks in three common forms, each with a different genetic strain of S. cerevisiae with different traits. These forms are as follows:

Cake or compressed yeast is a moist block of fresh yeast cells, direct from the fermentation vat.  Its cells are alive, and produce more leavening gas than the other forms.  Cake yeast is perishable, must be refrigerated, and has a brief shelf life, about one to two weeks.  As soon as sugar is added to the block of yeast, it will almost immediately begin fermenting the sugar and will produce some water so turning the whole block into a bubbling liquid. This mixture can be added to any recipe requiring yeast.

Active dried yeast, which was introduced in the 1920’s, has been removed from the fermentation tank and dried into granules with a protective coating of the yeast.  The cells are dormant and can be stored at room temperature for months.  The cells can be reactivated by soaking them in warm water, 105-110°F/41-43°C and then add some sugar and leave them until a vigorous fermentation has started before mixing it into the dough.  At cooler temperatures, the yeast cells recover poorly and the results will be less than optimal.

Instant dry yeast, an innovation of the 1970’s, is dried more quickly than active dry yeast, in the form of small porous rods that take up water more rapidly than granules. Instant yeast does not need to be prehydrated before mixing with other dough ingredients, and produce carbon dioxide more vigorously than active dry yeast.¹⁰