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+#### Leavening
+
+There are all kinds of ways the leaven breads. Here, we’ll be talking
+about two leavens: commercial yeasts and sourdoughs.
+
+##### Commercial Yeast
+
+The three most common types of commercial yeast are:
+
+**Fresh yeast:** Truth be told, fresh yeast, which is also known as cake
+yeast, is hard for home bakers to find these days. They are sold as
+little “cakes” that must be kept refrigerated, and they go bad after a
+few days. Many professional bakeries, however, still use fresh yeast,
+and so many bread formulas are written with fresh yeast in mind.
+Typically, breads that rise for 1.5 to 2 hours in bulk and 1 to 1.5
+hours shaped call for fresh yeast at 2% of total flour weight.
+
+Many recipes use much, much less yeast than that, however, and they
+ferment for much longer, which gives the bread more flavor.
+
+**Active dry yeast:** This is the kind of yeast that you’re most likely to
+find in your grocery store. Typically, you’ll want to use about 1 to 2
+tsp per loaf and, if the formula calls for fresh yeast, you’ll need to
+measure out 40% of that weight to convert to active dry.
+
+Active dry yeast needs to be “proofed” before using, which means it
+needs to be dissolved in about ¼ to ½ cup of lukewarm water (about 90
+degrees F or so).
+
+**Instant yeast:** This is what many amateur bakers prefer to use. In
+grocery stores you'll often find it labels "Rapid Rise Yeast" or
+"Bread Machine Yeast."  It looks like active dry yeast, but it retains
+many more living yeasts in each grain. As a result, there's no need to
+proof it – just add the yeast directly to the dry ingredients. If the
+formula calls for fresh yeast, measure out 1/3 of that weight for
+instant yeast. If the recipe calls for active dry yeast, cut the
+measurement by about 25 percent.
+
+##### Sourdough
+
+Before the 19th century, sourdough was really the only leavening
+available (unless you’re talking about salt rising bread, which uses
+bacteria alone – a leavening with which this book does not deal).
+Sourdough is really not that hard to work with – as some Internet sage
+once wrote, “People who thought the earth was flat made bread like
+this for thousands of years.”
+
+First, what is this stuff? Sourdough starter is a stable symbiotic
+culture of wild yeasts and bacteria. The yeasts break down starches
+into sugars, which the bacteria eat. The bacteria, on the other hand,
+create an acidic environment that kills off competitors to the yeasts.
+The yeasts were almost certainly already living on the grains when
+they were out in the field. As for the bacteria, that’s a trickier
+question, but the consensus seems to be that they come from us –
+studies have failed to isolate Lactobacillus Sanfranciscensis anywhere
+except in long-lived sourdough starters and on human teeth. The
+individual mix of yeasts and bacteria varies from starter to starter,
+and region to region. It's part of their charm; every starter is
+unique, and produces bread that tastes somewhat different from those
+produced with other starters.
+
+Sourdough starters work more slowly than commercial yeasts, which are
+much more concentrated that starters and have been carefully selected
+for their gas production. Typically, a sourdough loaf will rise for at
+least three to four hours in the bowl and will then need
+another two to three hours as a shaped loaf before it will be
+ready to bake.
+
+Not all sourdough breads are sour. The French pain au levain and
+Flemish desem breads are typically not very sour at all, while San
+Francisco-style sourdoughs and many German ryes are very sour, indeed!
+
+Different starters will produce different levels of sourness, but by
+far the most important factor in a sourdough bread's flavor is
+temperature. If the dough is allowed to ferment at 80 to 85 degrees or
+is allowed to rise slowly overnight in the fridge or in a cool room
+(35 to 50 degrees F), the bread will have a markedly stronger flavor
+than a sourdough that rose at room temperature (65 – 70 degrees F).
+
+Sourdough breads generally keep well, because their acid content slows
+down the staling process. In addition, the acid in sourdough both
+reduces the impact of bread on one's blood sugar and also neutralizes
+phytic acid in whole wheat breads. Phytic acid prevents the body from
+absorbing many nutrients.