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lightbulb socket
inside
near the center of the bottom (now the top) of the cooler, with an ordinary rheostat (dimmer switch) in the power cord. Use a 25-watt lightbulb and an accurate thermometer to measure the interior temperature. We use an outdoor thermometer. We’ve found that heating pads, incubators, or aquarium heaters are not as efficient as this system.

Steps in Proofing
    After activation, a sourdough goes through three proofing stages on its way to becoming a bread. First, the active culture is proofed to greatly increase the number of organisms; then the dough is given a lengthy proof after kneading, allowing both yeast and lactobacilli to multiply throughout the dough; and finally the shaped loaves are proofed to maximize both leavening and flavor.
ACTIVATING A DRY CULTURE
    Activation is a sort of pre-proofing step, which converts a dried, dormant culture into a mass of active sourdough organisms ready to be further prepared for use in the “culture proof” stage described below. Activation is achieved by simply adding flour and water to the dried culture (which is a mixture of concentrated organisms and the flour in which they were grown) and proofing for three to five days. This is something you’ll do only once, when you first acquire a dried sourdough culture.
    Start by thoroughly mixing all the dried culture (which contains approximately ¼ cup/35 g of flour) with ¾ cup (105 g) of unbleached all-purpose flour and ¾ cup (180 ml) of warm water in a 1-quart (1 liter) widemouthed canning jar. The culture should be the consistency of thick pancake batter. Place the jar in a warm place (about 90°F/32°C) and proof for about 24 hours. A proofing box is useful at this stage (see this page ); if you use one, check the temperature of the proofing box with a reliable outdoor thermometer. This high starting temperature promotes the growth of the lactobacilli and thus increases acid production. High acidity helps prevent contamination of the active culture by nonsourdough organisms present in most flour, most of which do not thrive in an acid environment.
    At the end of 24 hours, a few bubbles may appear in the culture as the first sign of growth and activity. Now reduce the proofing temperature to about 70°F (21°C). This lower temperature slows the bacterial growth and acid production—important at this stage as too much acidity inhibits the growth of the yeast. Continue feeding the culture every 12 to 24 hours for 3 to 5 days: feed with 1 cup (140 g) of flour and enough water (up to ¾ cup/180 ml) to maintain the thick pancake-batter consistency. It will be necessary to discard about half of the mixture before each feeding or the jar will overflow. Discarding dilutes the culture and helps reduce acid buildup. This is a good time to divide the culture into two jars—one for a backup in case of an accident with the other. After dividing into an additional jar, feed both jars.
    Sometimes the culture becomes quite active in the first 24 hours. This could be a sign of contamination, but if the culture has a pleasant odor and continues to respond when fed, it is a good culture and can be retained. If the odor is unpleasant, follow the directions for washing a culture below.
    When foam and bubbles increase the culture’s volume by about 3 inches (8 cm) within 2 to 3 hours of its last feeding, the culture is fully active and can be used or refrigerated until needed.
    During refrigeration, as the culture becomes semidormant, a layer of clear tan or brown fluid forms on the surface. This is the “hooch”: it’s perfectly normal, and it should be stirred back in when the culture is used.
    Culture Container
    A 1-quart (1 liter) widemouthed glass canning jar is recommended for activation and maintenance of cultures. These jars perform an essential function. Each time additional flour is added, the subsequent activity of lactobacilli makes the culture slightly more acidic. Repeat feedings in a larger container will