Instructions: Yeast: profiles in leavening

Ah yeast. The bakers soul mate. Show me a baker or pastry chef worth his or her whites who does not love this ingredient. Every cell of yeast is filled with potential. In bread, yeast is everything. How it develops affects just about every facet of the final result. In pastry, it takes on another face entirely. Yeast-based coffeecakes, babas au rum, brioches, croissants depend on yeast for their unique dough and crumb structures.

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, active dry, instant, quick, rapid, bread machine yeast. Sheesh. Read on.

Fresh (compressed) 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 are not going through exhaustive 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 rehydration (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, does not impose this lag.

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 benevolent professional to obtain fresh yeast or purchase compressed yeast in the dairy case (when ever its available). Since fresh is not proofed like dry, its more difficult to tell if its viable. 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 do 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. It will eventually be replaced by instant yeast. When active dry is called for, you may confidently substitute instant if you like (see the substitution guide). As with fresh yeast, active dry is a live culture - with one notable exception. It must be proofed or reconstituted with water and a bit of sugar before use (it cannot be added to dry ingredients). Active dry yeast usually comes in a vacuum brick or can. Once 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: Probably the most confusing thing the yeast companies could have done for was to introduce a new product and call it "Instant" yeast. True, it was developed to shorten fermentation time (a nod to commercial baking), but they should have chosen a more distinctive approach. These so-called "instant" yeasts were invented in Holland and were later replicated by others. 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. Its 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 yeasts qualities become a detriment - 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 (I go for 1 1/2). 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 by 15% to 25% increments. You can use too little yeast, but you wont 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.

A yeast recipe collection:
All recipes call for "dry yeast". This can be either active dry or instant. Both will work. Note that recipes made with active dry will require a slightly longer fermentation time because the yeast is less potent (gram for gram).

Using yeast in scones or kamishbrot, lends a novel texture and crumb. In the pizza dough, you can see how a combination of baking powder and yeast makes for a tender, chewy crust.

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

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