Old dough leavening is exactly what it sounds like: a piece of dough held back from a previous batch of bread which is used as a starter for a new batch. There are a number of different kinds of old dough leavening, ranging from sourdough starters which have been in continuous use for decades at some bakeries, to dough left over from a batch of bread made with baker's yeast. Old dough leavening tends to create a loaf which is more complex, with a wider range of flavors, and many people enjoy using it.
In addition to being used for breads, this type of leavening can also be used for other foods, like muffins, yeasted cakes, and so forth. Like other leaveners, it will cause the dough to rise, making it light and fluffy, rather than dense. Because the leaven is older, it tends to have a slightly more acidic, tangy flavor, which will transfer to the baked goods it is used in.
Sourdough is an ancient form of old dough leavening. It is made by setting out a small amount of flour mixed with water and sometimes other ingredients like crushed fruits. This mixture attracts wild yeasts, which vary depending on the region where the starter is made. The baker carefully feeds the starter, encouraging the proliferation of yeasts inside, and then uses it to leaven a batch of bread. He or she can then either make a new starter, or reserve a chunk of dough from this batch of bread to use as leavening.
When old dough leavening is made from breads which use regular baker's yeast, a small piece of dough is simply reserved before the bread is baked, and carefully fed so that the yeasts do not die. Over time, the leavening may attract wild yeasts, fermenting slightly and becoming a unique starter. In other instances, the leavening grows weak when it is used too many times, and it may die.
Old dough leavening can be stored in the fridge for up to two days, slowing the development of the yeasts. It can also be frozen, after being wrapped in wax and then in foil, but it will need to be allowed to sit out at room temperature when bakers want to use it, allowing the yeasts inside to wake up. An old dough leavening is a truly unique leavener, harnessing the specific yeasts of a particular region and creating an individual flavor which will be distinctive. Some of this leavening is famous, and it has been carefully husbanded by its parent bakery for decades, ensuring that customers experience the flavors they are familiar with every time they buy bread.