On Food and Cooking

time. This frozen “par-baked” bread can be
thawed and finished with just a few
minutes in a hot oven. Yeast survival is no
longer important, because the yeast cells
have done their leavening and are killed
during the initial bake.
Both yeasts and bacteria grow fastest in
liquid starters, which allow the microbes
easier access to nutrients; in a semisolid
dough they grow more slowly and require less
frequent attention. Because growing microbes
consume nutrients rapidly, and produce acid
and other growth-inhibiting substances,
starters need to be divided and refreshed
frequently, two or more times per day. Adding
new water and flour dilutes the accumulated
acids and other growth inhibitors, and
provides a fresh supply of food. Aerating the
starter — whisking a liquid one, or kneading a
doughy one — supplies the oxygen that yeasts
require to build cell membranes for new cells.
The more frequently the starter is divided and