Saturday, February 02, 2013

Notes on yeast

Yeast is one of the most important baking ingredients. It contributes much to bread and pastry in contrast to the small amount used in most formulas. Through a process known as fermentation, yeast gives bread its light, airy structure and helps produce a pleasingly soft texture in pastries.

During fermentation, yeast feeds on sugars in the dough and changes them into carbon dioxide and ethyl alcohol. The yeast releases sugar from the flour through enzymatic action. It also feeds on sugars added by the baker in richer doughs.

Fermentation takes place when as carbon dioxide gas accumulates in the structure of the dough. Since these gasses can't escape, the dough rises as gasses are trapped in air pockets in the dough. This leavening action turns breads and pastries into the products that we all treasure.

Types of yeast

Three forms of yeast are available to the baker. While pre-ferments such as the Italian biga or Western sourdough starter provide leavening action through yeast or other microorganisms, my focus here is on commercially available yeast products for the baker. They are:

Fresh or compressed yeast is often used by professional bakers. Since it's a perishable product, fresh yeast must be stored  in the refrigerator as it has a relatively short shelf life. Red Star Yeast advises that one (2-ounce) cake will leaven about three pounds of flour.

The baker uses more fresh yeast by weight than any other form. To use, dissolve fresh yeast in 90- to 95-degree F. liquid (usually the water in the formula). The liquid must come from the total moisture in the recipe.

Active dry yeast is a dry granular form of yeast. Fleischmann's Yeast developed it during World War II for the U.S. armed forces. It's longer shelf life gave military bakers a yeast product that was stable at room temperature and one that tolerated transportation over long distances. Active dry yeast remains the standard for the Armed Forces Recipe Service. While active dry yeast can be stored at room temperature, refrigeration extends its shelf life.

To use, the yeast must be re-hydrated in warm water (100 to 110 degrees F.), a process called blooming or proofing. After five minutes, the yeast solution is added with the other wet ingredients in the formula. The baker uses about 50 percent less active dry yeast by weight than fresh.

Instant yeast is also a dry granular form of yeast. Unlike active dry yeast, instant yeast doesn't have to be dissolved in warm water before use. The baker mixes it directly in the dry ingredients. Instant yeast is formulated to quickly absorb water and give off more carbon dioxide gas. The baker uses about 50 percent less instant yeast by weight than active dry. Instant yeast is also called rapid-rise or quick-rise yeast.

Conversion between forms of yeast

Active dry yeast leavened my career as a ship's cook and baker in the U.S. Navy. After using active dry yeast for most of my civilian career as well, I've switched over to instant yeast. It's easier to use and doesn't require blooming in warm water with a bit of sugar. I like it because you mix it directly in the dry ingredients.

With three yeast products on the market, the baker must first determine the type of yeast used in the recipe. A well written bakery formula will state the type of yeast used. Many cookbooks will explain the type of yeast used its recipes.

Convertion from one form of yeast to another requires a bit of math for the baker. Since many recipes call for active dry yeast, let's begin there. These instructions are based on weights and not volume measurement. I will explain my rationale behind the use of weights in baking in a future article.

Fleischmann's and Red Star both package dry yeast (active dry and instant) in strips of three (1/4-ounce) envelopes for the home market. Each envelope is equivalent to 2-1/4 teaspoons. I purchase Red Star's SAF brand instant yeast in 1-pound packages for home and work.
To convert from active dry yeast to instant, multiply the weight of active dry yeast by .70. A recipe that calls for .75 ounce of active yeast, for example, you'd make the following calculation:

.75 oz active dry yeast x .70 = .525 oz instant yeast

To convert from fresh yeast to active dry, multiply the weight of fresh yeast by .50. A recipe that calls for .75 ounce of fresh yeast, for example, you'd make the following calculation:

.75 oz fresh yeast x .50 = .375 oz active dry yeast

To convert from fresh yeast to instant, multiply the weight of fresh yeast by .35. A recipe that calls for .75 ounce of fresh yeast, for example, you'd make the following calculation:

.75 oz fresh yeast x .35 = .253 oz instant yeast

Modern digital scales will accommodate decimals to the hundredths. To make the reverse calculation, divide target yeast by the conversion factor. For example, the recipe calls for instant yeast. Since you have active dry yeast, you need to convert from instant yeast to active dry yeast. Make the calculation as follows:

.50 oz instant yeast / .70 = .71 oz active dry yeast

The Artisan website contains an useful yeast conversion table. Conversion factors on the chart appear to be close to mine.

For the home baker, I don't see any reason why you can't round the amount of yeast used to the nearest tenths or hundredths. Please leave a comment if you have questions regarding the use of yeast. I'll answer as soon as possible.

Note that newer recipes at 'Round the Chuckbox call for instant yeast. Older recipes, including those borrowed from the Armed Forces Recipe Service, use active dry yeast. Unless I decide to experiment with fresh yeast, my recipes will continue to use instant yeast.

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