Yeast and how it work

 

 

Yeast and How it Works

 

Did you ever wonder why flour tastes like sawdust but a French or Italian?

Bread made with that same flour and little else has a pleasant, sweet taste?

It’s the yeast.

Yeast is the magic ingredient of the baking world. While the rest of our ingredients are inanimate, yeast is alive and bakers have learned to cultivate yeast as a living thing in their bread and pastry dough. In this chapter, we will explore the different types of yeast and learn to cultivate yeast in different ways to create different products.

Our grandparents used--and many commercial bakers still use—fresh yeast rather than the dry yeast that we buy in the store. Fresh yeast performs marvelously well but is fragile, must be kept refrigerated, and used right away—hardly the conditions of today’s carefree baking.

The yeast that we buy is granular. Each little grain is a manufactured ball of starch or dextrose containing many yeast cells. When these grains are dissolved, the yeast is released into the dough.

The yeast on the grocers’ shelves typically comes in two forms: either instant active dry yeast or active dry yeast. The difference is in how the yeast cells hydrate or absorb water. Instant active dry yeast does not have to be hydrated in water prior to mixing as active dry yeast does.

Active yeast is mixed in water, the particles are dissolved, and the yeast is allowed to grow until the mixture becomes foamy. Then it is added to the flour. The cells of instant dry yeast are porous to absorb water and can be put directly in the flour without waiting for the yeast to hydrate.

And yes, yeast is alive. It is neither plant nor animal but a fungus. We add it to the flour in its dormant state and create a growing culture with moisture and the proper temperature. Under the right conditions, the yeast multiplies rapidly and a loaf of bread, when it is ready to go into the oven, may contain millions of live yeast cells.

So how do we nurture these little creatures? Like most other living organisms, they require three conditions for growth: moisture, food, and a hospitable environment. In such an environment, yeast will grow rapidly. Yeast feeds on sugar or converts the starch in the flour to sugar for food. (Without the capability to convert starch to sugar for food, yeast would not thrive in sugar free breads such as French bread.) As the yeast cells feed, they expel carbon dioxide and alcohol (ethanol). The carbon dioxide gas rises through the bread dough and is captured by the gluten structure in the dough to form gas pockets. The alcohol is evaporated in baking. 

 

The alcohol and other excretions impart a “yeasty” flavor to the dough. Master bread bakers manipulate the ratio of these two byproducts, carbon dioxide and alcohol—usually with temperature and acidity--to control the rise time and the flavors in the breads. In this chapter, you will learn to do the same. Bread wouldn't be bread without yeast and yeast can't work without sugars. Since yeast is alive, it needs food for fuel, in this case, simple sugars. But flour is mostly starch and table sugar (sucrose) is too complex for the yeast to digest before the sugar is broken down in the biological and chemical actions of the fermentation process. Amylase and invertase, enzymes present in the flour or created by the yeast, break down the starch molecules into sugars. While some of these simple sugar molecules become food for the yeast; others create the sweet flavor we find in fine bread—even a French bread where there is no sugar added.

Generally, a long, slow fermentation makes for bread with better flavor, texture, and moisture retention. Many fine types of bread call for “retarding” or slowing down the growth of the yeast with refrigeration. If dough is refrigerated, the yeast grows more slowly As long as the dough is above 40 degrees, fermentation still takes place but at a slower rate. As the temperature of the dough approaches 40 degrees, the yeast growth slows and stops. When the dough is warmed and the growth of the yeast takes off, there is plenty of sugar present for the yeast and an excess of sugar to sweeten the bread.

Creating the Right Environment for Yeast As a baker, you need to know how to create the right environment for the yeast to work in. Let’s look at the factors that you need to understand and control to create wonderful breads:

·         Moisture: You must create a moist environment to dissolve the yeast granules, to hydrate the yeast cells, and to create the right environment for growth. Most of the time, you will want your bread dough as moist as you can handle without being sticky. Bread dough that is too dry will take a long time to rise because the yeast will not multiply as rapidly and because the dry dough is stronger and more difficult to lift. Especially with a bread machine, it is important to measure the amount of water carefully with an accurate measuring cup.

·         Acidity: Yeast prefers a slightly acidic environment. Our grandmothers discovered they could create that with a tablespoon or two of lemon juice. We recommend that you do the same with a dough conditioner. (A dough conditioner has other conditioning roles.) Try several conditioners until you find the one that works best for you.

·         Saltiness: Salt impedes the growth of yeast and slows down the rise. Measure salt carefully. An extra half teaspoon of salt will significantly increase the time it takes the dough to rise. Conversely, you can speed up yeast growth with sugar.