Yeast are tiny one celled fungi. There are both benevolent and obnoxious strains. One can find yeast commercially for brewing, as a nutritional supplement, and for baking. In bread making, yeast digests sugars and creates carbon dioxide. The structure of the dough traps the gas, giving yeast bread its loft and softness. Too little yeast will result in dense, heavy loaves, while too much may overwhelm the structure of the dough and cause it to either collapse in on itself or create large pockets or bubbles. I am sure we have all come across commercial loaves of bread with gaping holes.
There are several options when buying dry yeast:
Active dry yeast is good for longer storage, but is less tolerant to thermal-shock. In other words, you need to be absolutely sure the warm water or milk used in the recipe is not over 122°F or 50°C. This form should also be added to a liquid before mixing into the dough, as the live cells are typically encapsulated and protected by dead cells.
Instant dry yeast is more perishable than active, but contains a higher percentage of live cells. It may be added directly to the dough. However, a small amount should always be tested to ensure it is viable.
Rapid rise yeast is a form of instant yeast developed to reduce rising time. It is believed to produce less flavorful product, but it is useful in some bread machine recipes.
When reading recipes remember that 1 packet of yeast is equivalent to 2 1/4 tsp dry yeast.
In my experience, our local grocery store only carries yeast in the tiny packets or jars. Neither option is particularly frugal. Stores such as Sam’s Club or some restaurant supply stores will sell yeast in two pound packages for approximately the cost of 3 small packets. Dry yeast should last at least six months in the freezer. If you divide it before storage, it may last over a year, but that shelf life is not guaranteed. It is always important to test the yeast to be sure it is still viable. If you are just starting out on your bread baking journey, don’t make bulk yeast your very first purchase. Wait until you’ve had several successes before committing to the large bag, as no one needs a constant reminder of a failed project each time they open the freezer. Allow frozen yeast to come to room temperature before using.
The other option when purchasing yeast is to buy it fresh. It may be found in cakes and is sold under the names: fresh yeast, cake yeast, baker’s compressed yeast, or wet yest. This is often stocked at health food or restaurant supply stores and is highly perishable.
When substituting in recipes calling for dry yeast remember one small cake of fresh or compressed yeast is 0.6 oz and is equivalent to 2 1/4 teaspoons dry yeast.
To test yeast for viability add a pinch to a weak solution of warm sugar water. The yeast should make tiny bubbles that produce a creamy foam.
Yeast growth is encouraged by the sugar in the dough and is kept in check by the addition of salt. If your home is cool or drafty, create a warm place for rising by heating your oven to 200°F and then turning it off. Placing a shallow dish of water on the lower rack will help keep the humidity optimal. Additionally the dough may be covered by a damp towel.
Some recipes call for bread to rise once, and others are allowed a second. Recipes calling for a second rise typically have a stronger yeast flavor.
Finally remember that nutritional or brewer’s yeast is not the same as yeast for baking or brewing, it is dead and will not produce the gas necessary for rising.