Yeast, a single cell living organism, has been around man for thousands of years. Ancient Egyptians used it to leaven their bread and make alcoholic beverages. It wasn't until the 1860's that yeast was discovered, via microscope, to be a living organism. Baker’s yeast came into being at the turn of the 20th century.
There are several categories of yeast:
- Baker’s yeast
- Nutritional yeast
- Brewer’s yeast
Since cooking is the theme, we will restrict our discussion to baker's yeast.
How it works
Yeast works by consuming sugar and excreting carbon dioxide and alcohol.
Carbon dioxide, being a gas, causes bread to rise. Saccharomyces cerevisiae
multiplies in about 1 1/4 hours at 86 degrees, hence waiting an hour or so
for dough to double in size.
Types of Yeast
Baker's yeast is used in home and commercial baking. It is derived from a brewer's yeast called Saccharomyces cerevisiae. There are several strains of yeasts to choose from:
- Active Dry Yeast
- Instant Dry Yeast (quick rise)
- Instant Dry Yeast for Bread Machine
- Compressed Yeast
Each yeast is processed slightly different and has its place in baking.
(CY), aka wet yeast or fresh yeast, is generally sold to consumers in block sizes of 2 oz or 8 oz. It has a high moisture content (about 70%) and has a shelf life of 8 weeks since packaging. It must be kept 35 to 38 deg F to maintain its freshness and activity. It produces the most carbon dioxide of all the yeasts.
Active Dry Yeast
(ADY) has most of the moisture removed from it (about 8% moisture
content). The yeast is exposed to high heat which kills off many of the
cells. This puts the yeast in a semi dormant state which increases the
shelf life to about a year. It comes in 3 pack strips or a 4 oz jar. ADY
needs hydration, called proofing (putting in 110 F water for 10 minutes)
to work. Proofing sloughs off the dead cells so the live ones can work.
ADY requires two risings of the dough.
Fast Rise Yeast
(IDY) is an amalgamation of two strains of yeast to create a superior
yeast. IDY is a smaller particle size than ADY; hence faster activation and quicker rise. IDY will reduce the rise time of dough by up to 50%. Less heat is
used in the process. Because every cell is living, IDY does not need
hydration to work (the yeast can be mixed directly with recipe
ingredients without proofing). Only requires one rising of the dough.
Some say the taste is cleaner than active dry yeast because there is no
dead yeast cells.
IDY should not be used in refrigerated doughs.
Bread Machine Yeast
(BMY) is a quick rise yeast with ascorbic acid added as a dough
conditioner (helps dough stretch better). The granules are very fine so
they mix well in a bread machine.
Most of the time one yeast can be substituted for another.
Use 2 parts compressed yeast for 1 part ADY.
Use 3 parts compressed yeast for 1 part IDY or BMY.
Use 1 1/4 parts ADY for 1 part IDY or BMY.
Use 3/4 parts IDY or BMY for 1 part ADY.
Yeast proportions to use are based on the amount of flour in a recipe.
- Use 3/4 tsp ADY for every cup (4 oz) of flour
- Use 1/2 tsp IDY or BMY for every cup of flour
The water used with yeast can have an adverse affect on it, especially hard water. We suggest using bottled water with yeast recipes.
Another problem can be the dryness in the air, especially in the winter when the humidity is low. Yeast can dry out.
High altitutes can also have an affect on recipes with yeast. Flour is susceptible to drying out.