Saccharomyces cerevisiae, baker's or brewer's yeast, are fungi which naturally occur, well, all over the place. Because yeasts are everywhere, it's possible to leave a batter (or grape juice) out and cultivate a new colony of yeast to grow in your food, but this is probably not advisable for most people - especially since yeast (specifically the desirable strain) is commonly available in grocery stores. Although many other strains are generally regarded as safe (S. bayanus and S. pastorianus used extensively in commercial beer and wine making), in cooking and baking, the word yeast refers to S. cerevisiae.
To allow yeast to feast on just wheat flour and water is time consuming, sometimes taking several days to produce enough flavor and volume of small bubbles for delicious, tender bread. Many recipes aid the growth of the yeast by providing a little extra fuel in the form of cane sugar (be careful, an environment too saturated with sugars can shut down yeast activity resulting in a dense loaf) and making sure the temperature is just right to promote yeast activity (around 95°F [35°C]).
The amount of yeast to use, the length of time to allow the yeast to grow, and the balance of other ingredients that may promote or inhibit yeast activity are all unpredictable variables when creating a recipe from scratch. It takes a lot of trial and error to produce a recipe with accurate rise times for a particular amount of yeast (doubling yeast in a recipe won't allow you to halve the rise time) so it's best to start off by sticking with the amounts and times printed in a recipe before experimenting.
Commercial yeast production starts with a small group of healthy yeast organisms that is carefully grown by providing them with nutrients (supplied to them in a slurry called wort). As they multiply via budding (splitting themselves into new yeast cells), the yeast is transferred from test tubes to flasks to tanks. The tanks (called fermentation tanks) start off small and contain a specially formulated wort (usually a mixture of molasses, minerals, and vitamins) enabling the yeast to reproduce quickly and grow (and to be transferred to ever larger fermentation tanks). Fleischmann's has some multi-story tanks that have a capacity of over 60,000 gallons (225,000 L)! When the producer decides it's more cost effective to sell the yeast than to keep multiplying them, they wash and separate the yeast from the wort and other debris and proceed to prepare them for the different types of yeast products.
There are three main types of yeast available to the home cook: fresh, active dry, and instant.
Fresh yeast are live yeast cells mixed with carbohydrates (commonly corn starch) that has been compressed into small square cakes, wrapped, and refrigerated. The yeast is kept cold so it doesn't grow before being incorporated into a recipe and is only viable for about one to two weeks. After that, the yeast runs out of nutrients and dies. Fresh yeast is the most active (that is, gas producing) of the three types of yeast commonly available. According to , a 0.6 ounce (17 g) cake of fresh yeast is interchangeable in a recipe to one packet (1/4 ounce or 7 g) of dry yeast. A 2 ounce cake is equivalent to three 1/4-ounce packets of dry yeast.
|Close up of fresh yeast (all close up pictures at the same magnification)|
Active Dry Yeast
Introduced in the 1940's, active dry yeast was a major innovation in how people would use yeast and bake breads. To make active dry yeast, live yeast cultures are dried after being removed from the fermentation tanks. A protective layer of yeast debris is allowed to coat the coarse clumps of yeast forming the tiny granules. Active dry yeast is simply dehydrated, dormant yeast cells clumped into grains that await reactivation. To revive the yeast, the grains must be soaked/dissolved in warm water (about 110°F or 43°C is considered optimal) prior to mixing with the dough or batter. Active dry yeast changed the world of baking because it was a shelf stable product that had consistent performance when used. Families on the move and cooks who didn't have constant access to a refrigerator could still use yeast once active dry yeast was made available. (Fleishmann's introduced their active dry product shortly after America entered World War II with the intent of providing yeast to soldiers.)}?>
|Close up of active dry yeast|
Instant yeast is the name cookbooks give to the third kind of commonly available yeast - but it's almost never sold under that name. Fleishmann's calls their instant yeast product "RapidRise" while Red Star Yeast uses the label "Quick-Rise". With the popularity of bread machines rising, yeast companies are also selling instant yeast as bread machine yeast. In any case, all of these different names mean the same thing - instant yeast. Instant yeast isn't really instant, it's about 50% faster in terms of rise time. To keep things simple, you just use the same amount of yeast as you would active dry, but you don't have to wait as long to get the same rise (which is why recipes typically say something like "allow to rest until volume has doubled, about 1 hour" because we really don't know how long it's going to take because we don't know if you're using the same yeast as we are). Instant yeast is made in a similar manner to active dry, but the drying process has been altered somewhat. According to Red Star, they use a lower heat to produce more porous granules while Harold McGee's On Food and Cooking claims it's a fast drying process. Whatever the process, the end result is that each yeast granule has more surface area and activates faster than active dry. In fact, they activate so quickly, you don't have to soak them in water first - the moisture of the dough or batter will be enough to get the yeast moving again.
When looking at the ingredients of instant yeast, it usually contains sorbitan monostearate and ascorbic acid (an antioxidant used in packaged foods as a preservative; a form of ascorbic acid is commonly known as Vitamin C) as well as yeast. My theory is that to make it a little more "instant", sorbitan monostearate is added as a wetting agent to speed up the absorption of water.
|Close up of instant yeast. Granules are smaller than active dry enabling them to moisten faster|