Understanding Yeast

Yeast - any of various small, single-celled fungi of the phylum Ascomycota that reproduce by fission or budding, the daughter cells often remaining attached, and that are capable of fermenting carbohydrates into alcohol and carbon dioxide.

Yeast a leavening agent

Yeast is a living plant that digests sugar to form carbon dioxide and alcohol. The alcohol evaporates during baking. Yeast needs moisture, food in the form of starch and is sensitive to temperature:

80 degrees - Ideal
138 degrees - kills yeast

Functions of Yeast in the Breadmaking Process

There are three main functions of yeast in dough. They are leavening, dough maturation and development, and flavor development. These are achieved through the following steps (excerpted from Technology of Breadmaking edited by Stanley P. Cauvain and Linda S. Young).

All of the processes that have evolved for the manufacture of bread have a single, common aim. That aim is namely to convert wheat flour into an aerated and palatable food. In achieving this there are a number of largely common events that occur. Those specific to yeast are as follow:

• The mixing of flour (mainly wheat) and water, together with yeast and salt, and other specified ingredients in appropriate ratios.

• The development of a gluten structure (hydrated proteins) in the dough through the application of energy during mixing, often referred to as "kneading".

• The incorporation of air bubbles, as well as fermentation gases, within the dough during mixing.

• The continued 'development' of the gluten structure created as a result of kneading, in order to modify the rheological properties of the dough and to improve its ability to expand when gas pressures increase because of the generation of carbon dioxide gas in the fermenting dough. This stage of dough development may also be referred to as 'ripening' or 'maturing' of the dough.

• The creation or modification of particular flavor compounds in the dough.

• The fermentation and expansion of the shaped dough pieces during 'proof'.

• Further expansion of the dough pieces and fixation of the final bread structure during baking.

Additionally leavening effects the volume, crust, texture, taste, wholesomeness and shelf life of leavened breads.

Compressed Yeast-

Keeps two weeks in refrigerator

Active Dry Yeast-

Keeps six months in airtight container, activated by warm water twice its weight

Instant Dry Yeast-

Doesn't need to be activated

Yeast Equivalents:

Other Leaving Agents:

Baking Powder
Acid reacting salts and biocarbonate of soda need heat and water to react and form carbon dioxide.

Single: Require only Moisture

Double: Require heat for complete reaction

Baking Soda
Chemical sodium Bicarbonate

Needs moisture and acid

Mechanical

Creaming-

Beating Fat and Sugar to Incorporate air

Foaming-

Beating eggs with or without sugar to incorporate air