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.