The Laurel's Kitchen Bread Book_ A Guide to Whole-Grain Breadmaking - Laurel Robertson [149]
When plenty of oxygen is available, yeast metabolizes its food completely, multiplying energetically and giving off carbon dioxide and water as waste products. This efficient metabolic process is called respiration, and its discovery by Louis Pasteur was what made the commercial manufacture of yeast possible: bubbling air through the nutrient solution keeps the yeast metabolism efficient and its waste products harmless. When there is not much oxygen—as in bread dough, where the oxygen is rather quickly used up—yeast adapts by changing its metabolism from aerobic respiration to anaerobic fermentation.
Fermentation burns the available carbohydrate food less efficiently, producing carbon dioxide and alcohol as byproducts. Beermaking capitalizes on this process, but with bread dough that is left too long, the accumulated alcohol will eventually kill the yeast. Deflating the dough, kneading, shaping, and so on, all remove alcohol through evaporation. They also move the yeast to new pastures, aerate the dough somewhat, and break up the accumulated carbon dioxide bubbles into smaller air sacs, making a finer-textured dough that can hold the gas better and produce a lighter bread.
Releasing carbon dioxide to leaven the dough is the flashiest thing yeast does, but it is not the only one. Many minute chemical by-products of fermentation give the bread flavor; and in some mysterious ways the action of the yeast both develops and mellows the gluten so that it can do its gasretaining work better. During the fermentation period, other changes take place: the starch and proteins of the dough continue to absorb water into themselves (one reason that longer-fermented bread keeps better), and there is a lot of enzyme activity.
One enzyme that does important work during the whole fermentation time is amylase, which we have discussed elsewhere. Another, probably even more important to us who eat the bread, is phytase. Like amylase, phytase is an enzyme that the new plant would use when it needed to gain access to nutrients stored in the seed. These essential nutrients are locked up by a substance called phytic acid, which safeguards them until they are needed by the growing sprout. With amylase the nutrients in question are sugars, which the enzyme releases from its storage form, starch. With phytase, the nutrients are minerals: phosphorus, zinc, calcium and others. As bread ferments, stored minerals are released, and this is one reason that leavened bread has nutritional advantages over unfermented wheat products, and one reason for choosing the longer fermentation times when that option is open to you.
STORING & USING BAKERS’ YEASTS
Baking yeast comes to us either in dried granules or moist pressed cakes, and technologists have put a good deal of energy into seeing that they are both exceedingly good at their job of raising bread.
The commonest kind of yeast for home baking is active dry yeast, the granular kind found in tiny flat foil packets at the grocer’s, or in bulk at natural food stores. This is the kind we call for in our recipes: it is available everywhere, convenient to keep, and very dependable. A typical recipe in this book calls for 2 teaspoons (1 packet) of active dry yeast (¼ ounce or 7 grams). When we wrote Laurel’s Kitchen, one packet measured a tablespoon, and when we started this book, it measured 2 ½ teaspoons; now it is 2—but it still weighs the same, and still raises two loaves. The yeast companies are always trying to improve their products, and maybe from their point of view, less bulk means less storage and shipping space.
For moist (or “cake,” or “compressed”) yeast, the equivalent amount is one square (⅗ ounce or 17 grams). When you buy either of them in bulk, or want to substitute one for the other, think of moist yeast as composed of about half water by weight, so that active dry yeast weighs 45 percent as much as cake yeast of the same leavening power.
The most important thing to keep in mind when you buy yeast is to be sure it is fresh. If active dry yeast is kept airtight