Leaven, strictly signifies sour dough, which acquires its acidity, when preserved after kneading flour with yeast, in order to ferment a larger quantity of paste.— It is a Very imperfect substitute for yeast; and, as it communicates to the bread an astringent taste which few persons relish, it ought to be used only where barm cannot be procured. As, however, the latter ferment is sometimes difficult to be obtained, especially during the winter, we shall communicate the most simple methods of preparing as well as of preserving it, under the article Yeast.