Handling and using yeast

Here are a few tips to keep in mind when handling yeast.

• Active dry can be used at 50% of the weight of fresh yeast.  Instant dry can be used at 40% of the weight of fresh yeast.
• Yeast responds to warm water only. 
• It is extremely sensitive to cold temperatures, too much air, and too much heat.  
• Yeast dies within a couple of hours. Though most of the yeast is dead by the time the bread is placed in the oven to bake, there is still enough active yeast to produce carbon dioxide during the early stages of baking. Bread continues to rise in the early stages of baking, but as it progresses, the high temperature kills off the remainder yeast. Most bread deflates slightly toward the end of baking.
• A few things inhibit the yeast's ability to ferment. Salt, shortening, and animal fats can affect the rising action. Recipes that use yeast as the leavening agent, usually contain small amounts of salt and have some sugar for balance.
• Inactivated or dry yeast may at times be dead before use. To check for vitality, add a package to warm water and watch for gas production or expansion. Only after that, combine yeast with flour. Yeast will not activate if the water is too hot, too cold, or if a liquid other than water is used (i.e. milk).

Remember that yeast is a living organism and that the rising action is a result of its metabolic activity.

Baker's yeast

There are several types of Baker's yeast:

Cake Yeast, also known as wet, fresh, or compressed yeast, is characterized by a high moisture content. Cake yeast is processed one step further than cream yeast and is taken directly from the fermentation vat. the yeast cells are alive and produce more leaving gas than other forms. It is very perishable and needs to be refrigerated. Shelf life is short, between 2 and 8 weeks.
Active Dry Yeast is yeast is processed one step further than Cake Yeast. It is taken from the fermentation tank and dried into granules with a protective coating of yeast debris. Due to the low moisture content, the yeast is in a semi-dormant state and is therefore more stable than cake yeast.  At the time of usage, the baker needs to reactivate the yeast by soaking them in warm water, 105-110 degrees F/41-43 degrees C, before adding mixing into the dough. The shelf life of an unopened package extends from a few months up to 2 years.

Instant Yeast, also known as "fast-rising" or "fast-acting" yeast, is a "dry" yeast that can shorten the rising time in traditional baking by as much as 50%. This yeast is dried quickly and into porous rods instead of granules. This rod formation allows them to take up water more rapidly. Instant yeast does not need to be prehydrated and usually produces more carbon dioxide gas than active dry yeast The shelf life of an unopened package extends up to 2 years.

Yeast metabolism

Baker's and brewer's yeast is Saccharomyces Cerevisiae, also referred to as the "sugar-eating fungus."

Yeasts feast on glucose and fructose from sugar, and on maltose from the broken-down starch granules in the flour.
Yeast metabolize sugars for energy and produce ethyl alcohol and carbon dioxide following this equation:

C6H12O6  ----->  2(CH3CH2OH) + 2(CO2) + ATP

In making beer and wine, the carbon dioxide escapes the liquid and concentrates the alcohol. In making bread, carbon dioxide and alcohol become trapped in the dough. The flexibility of the dough accomodates the expanding gas by inflating or "rising." The ethyl alcohol, along with other by-products of fermentation, give yeast-leavened breads their typical aroma. At the time of baking, the heat expells both carbon dioxide and alcohol from the dough, leaving a flavorful network of empty air pockets.
A small amount of added table sugar increases yeast activity, whereas a large amount decreases it. Too much sugar dehydrates the yeast. To compensate, bakers of sweet breads add more yeast than ordinary, and allow longer times for the bread to rise.

Yeast are also sensitive to salt, and are greatly affected by temperature. Cells grow and produce gas most rapidly at around 95 degrees F/35 degrees C.

History of yeast

The history of yeast dates further back than written language. Egyptian hieroglyphics, dating back 5,000 years, depic the Ancient using yeast  for alcohol fermentation as well as a leavening agent. It is believed that early fermentation occurred naturally through contamination of fruit and flour. The Ancient used yeast even though they did not understand it. The faculties of yeast were often attributed to magic.

Leaven, as it is described in the Bible, was a soft dough-like medium. A small portion was reserved as a starter dough for new bread. By saving "good" batches, man naturally selected the best yeast for his baking/fermentation needs. Yeast is therefore considered the first industrial microorganism.

Yeast was not identified as the living organism responsible for fermentation and leavening until after the invention of the microscope in the 1600s and the work of Louis Pasteur in the 1860s. Isolation of pure cultures gave way to commercial production and manufacturing starting at the turn of the 20th century.

Bread 101

What qualifies as bread? On the most basic level, bread is the result from cooking a mixture of milled grains and water. Here is a quick run down of the most common ingredients in bread:

• Flour - Wheat flour is most commonly used in raised bread because it contains two proteins, glutenin and gliadin, which form gluten when combined with water. As the baker kneads the dough, the gluten develops and becomes elastic. This elasticity allows the incorporation of carbon dioxide gas into the dough.
• Starch is a carbohydrate that makes up 70% of the flour by weight. Starch granules release sugars that the yeast feed on. Starch reinforces gluten and absorbs water during baking, helping the gluten contain the carbon dioxide.  
• Water is the most important liquid in bread. It dissolves and activates yeast and blends with the flour to create gluten.  
• Yeast is a live, single-cell fungus that begins feeding on the sugars in flour and releases the carbon dioxide that makes bread rise. Yeast also adds many of the flavors and aromas associated with bread.
• Baking powder and baking soda are chemical leavenings that participate in the reactions between acidic and alkaline compounds that produce the carbon dioxide necessary to inflate dough or batter. These chemical leavenings act much faster than yeast, and are best used in quick breads.
• Salt slows rising time, which allows the flavor of the dough to develop. Salt also adds structure to the dough by strengthening the gluten, which keeps the carbon dioxide bubbles from expanding too rapidly.
• Eggs add food value, color, and flavor. They also make the crumb fine and the crust tender. Eggs add richness and protein.
• Fat in the form of butter, margarine, shortening or oil add flavor and moisture to bread. Fat slows moisture loss and helps bread stay fresh longer.
• Other liquids such as milk, buttermilk, cream or juice may be added for flavor or to enhance texture. Only add warm liquids to dry ingredients. Too cool liquids slow or stop yeast action. Too hot liquids destroy the yeast and prevent bread from rising.
• Sweetners such as sugar, brown sugar, honey, molasses, jams, and dried fruits may be used to add flavor and color to the crust.