Yeast's Crucial Roles in Breadbaking

[Isa]

Yeast's Crucial Roles in Breadbaking
It acts as a leavener, dough developer, and flavor builder

by Shirley O. Corriher

Yeast is the driving force behind fermentation, the magical process that allows a dense mass of dough to become a well-risen loaf of bread. And yet yeast is nothing more than a single-celled fungus. How does it do it?

Yeast works by consuming sugar and excreting carbon dioxide and alcohol as byproducts. In bread making, yeast has three major roles. Most of us are familiar with yeast's leavening ability. But you may not be aware that fermentation helps to strengthen and develop gluten in dough and also contributes to incredible flavors in bread.

Yeast makes dough rise
The essentials of any bread dough are flour, water, and of course yeast. As soon as these ingredients are stirred together, enzymes in the yeast and the flour cause large starch molecules to break down into simple sugars. The yeast metabolizes these simple sugars and exudes a liquid that releases carbon dioxide and ethyl alcohol into existing air bubbles in the dough.

If the dough has a strong and elastic gluten network, the carbon dioxide is held within the bubble and will begin to inflate it, just like someone blowing up bubblegum. As more and more tiny air cells fill with carbon dioxide, the dough rises and we're on the way to leavened bread.

Yeast strengthens bread dough
When you stir together flour and water, two proteins in the flour -- glutenin and gliadin -- grab water and each other to form a bubblegum-like, elastic mass of molecules that we call gluten. In bread making, we want to develop as much gluten as we can because it strengthens the dough and holds in gases that will make the bread rise.

Once flour and water are mixed together, any further working of the dough encourages more gluten to form. Manipulating the dough in any way allows more proteins and water to find each other and link together. If you've ever made homemade pasta, you know that each time you roll the dough through the machine, the dough becomes more elastic; in other words, more gluten is developed. And with puff pastry dough, every time you fold, turn, and roll the dough, it becomes more elastic.

Yeast, like kneading, helps develop the gluten network. With every burst of carbon dioxide that the yeast releases into an air bubble, protein and water molecules move about and have another chance to connect and form more gluten. In this way, a dough's rising is an almost molecule-by-molecule kneading. Next time you punch down bread dough after its first rise, notice how smooth and strong the gluten has become, in part from the rise.

At this stage, most bakers stretch and tuck the dough into a round to give it a smooth, tight top that will trap the gases produced by fermentation. Then they let this very springy dough stand for 10 to 15 minutes. This lets the gluten bonds relax a little and makes the final shaping of the dough easier. This rounding and resting step isn't included in many home baking recipes, but it's a good thing to do.

Fermentation generates flavor in bread
As Harold McGee, the author of On Food & Cooking, has pointed out, big molecules in proteins, starches, and fats don't have much flavor, but when they break down into their building blocks -- proteins into amino acids, starches into sugars, or fats into free fatty acids -- they all have marvelous flavors. Fermentation, whether it's acting on fruit juices to make wine or on flour to make bread, does exactly that -- it breaks down large molecules into smaller, flavorful ones.

At the beginning of fermentation, enzymes in the yeast start breaking down starch into more flavorful sugars. The yeast uses these sugars, as well as sugars already present in the dough, and produces not only carbon dioxide and alcohol but also a host of flavorful byproducts such as organic acids and amino acids. A multitude of enzymes encourages all kinds of reactions that break big chains of molecules into smaller ones -- amylose and maltose into glucose, proteins into amino acids.

As fermentation proceeds, the dough becomes more acidic. This is due in part to rising levels of carbon dioxide, but there are also more flavorful organic acids like acetic acid (vinegar) and lactic acid being formed from the alcohol in the dough. (This is similar to what happens to a bottle of wine that has been left uncorked for a while: the alcohol combines with oxygen to make vinegar.) The acidity of the dough causes more molecules to break down. The dough becomes a veritable ferment of reactions. Eventually, the amount of alcohol formed starts to inhibit the yeast's activity.

Yeast has help in producing flavorful compounds. Bacteria are important flavor builders as well. There are bacteria in the dough from the beginning, but as long as the yeast is very active, it consumes sugars as quickly as they're produced, leaving no food for the bacteria, which also like sugar. But when bakers chill a dough and slow down its rise, the cold dramatically reduces yeast activity. The bacteria, on the other hand, function well even in cold temperatures, so they now have an opportunity to thrive, producing many more marvelously flavorful acids.

From Fine Cooking #43, pp. 80-81

[mudbug]

More on yeast...

Yeast: profiles in leavening

Know your yeast:
The more you know about yeast, the more you can appreciate the joys of working with it. Many bakers are unconditionally loyal to a particular format or brand. In professional circles, consistency is very important so yeast is chosen carefully. At home, we need a yeast which suits most of our baking needs. But where to start?

Fresh yeast:
Your grandmother probably used fresh yeast. Purists adore it. Doughs made with it are supple and bouncy and the yeast fragrance is subtle. Fresh yeast, usually the choice of professional bakeries, is best for doughs which will not undergo excessive handling. The strains used to make fresh yeast are different from those used to make dry yeast. Dry strains are selected for their stability under stress (drying, rehydration, poor handling). Dry yeast contains 5% to 8% moisture compared to 70% to 72% in fresh. After re-hydration (adding water to proof) or mixing with other ingredients, there is a "lag phase" which the dry yeast requires in order to become active again. Fresh yeast, of course, has no lag phase.

Fresh yeast does have its drawbacks. It is far less stable a product than dry. Fresh yeast in compressed form or in "cream" form is delivered every other day to the commercial baker - sometimes by the tanker truck load. Home bakers must rely on a professional baker to obtain fresh yeast or purchase compressed yeast in the dairy case (whenever it's available). Since fresh yeast does not require proffing, it's difficult to tell if it's truly fresh. Fresh yeast keeps for 10 to 20 days. If you buy a one pound block (which is what I do, although some bakeries will sell you half a pound) you may wish to freeze it. To do this, wrap it well in waxed paper then in plastic wrap and seal. It is important to keep the yeast from drying. Allow the yeast to defrost gradually in the refrigerator the night before you are planning to use it. The longer the yeast is frozen, the more it will lose potency. When in doubt, discard.

Active dry yeast:
Most cookbooks still call for "active dry yeast". Bakers who honed their skills with this yeast, know what to expect from it. "Active dry" is being replaced by "instant yeast". When "active dry" is called for, you may substitute "instant" if you like (see the substitution guide). As with fresh yeast, active dry is a live culture - with one notable exception. Under most circumstances, it must be proofed or reconstituted with water and a bit of sugar before use. Once it is exposed to air and moisture, it starts to lose its potency. A container of active dry yeast should be well sealed and refrigerated or frozen. Always take note of the expiration date.

Instant yeast - a.k.a. "Fast Rising" or "Bread Machine Yeast":
Instant yeast is very active and very tolerant. It offers the baker a wide margin for error or experimentation. It activates rapidly in warm water and can be added to other ingredients in its dry state. It's a good keeper - 3 to 4 months in the freezer. Instant yeast is a good choice for rich coffee cakes and sweet breads which you may wish to freeze and for doughs which will see a slow rise in the refrigerator.

Instant yeast's qualities become liabilities when you use too much of it. How do you know if this is the case? A premature rise and an overt yeasty, "beery" odor. Problems also arise when you allow a dough to proof too long. Make sure you do not allow doughs to rise beyond double their original size - 60% to 70% is fine. You can always opt for more fermentation in the final rise. Over-fermented doughs reduce the shelf life of the final product. The solution is very simple: less is more. Decrease the yeast portion by 15% to 25% increments. You can use too little yeast, but you won't compromise taste and structure if you let it rise (albeit) slowly. This may not work, however, with rich or heavy breads (i.e. rye) which need good fermentation power and mixtures which contain perishables. No two bakers, or kitchens, or ovens, are alike. Don't be afraid to do some tweaking.

Substitution guide:
1 tablespoon active dry = 2 1/2 teaspoons instant = 3/4 ounce fresh yeast.

From: Baker Boulanger: La Technique

[amber]

hi my name is amber and i am a student at the university of connecticut. i am doing an article on bread for a journalism class. i am trying to get any tips, recipes, history of, uses of, etc. of bread. i was hoping there was someone in the connecticut area that could help me and might want to do a phone interview. please email me. thanks. amber

[Isa]

Fascinating Cchiu!


I'm always amaze by how much litterature yeast has inspired. There is always something else to learn.

[mudbug]

Yummmmmmmm!! Isa!

[kokopuffs]

I have used SAF Red Instant Yeast for about a year, now. It seems equally active as when I first got it a year ago.