Sourdough starter won't work in every location. The famous San Francisco sourdough owes

its existence to the city's cool, humid climate and a strain of yeast present in the city’s air. If you

don't live in San Francisco or in similar sourdough-friendly climate, you can bake a compromise

sourdough bread. Add dry yeast to the starter to kick-start the action. A combination of dry and

natural yeasts leavens the resulting bread with some bacteria from the air for the sour flavor. The

starter is ready in a couple of days, several days makes it even more sour. But don’t expect a real

sourdough. The large commercial bakeries go even a step further in compromise—they prepare a

common yeast bread dough and add the sour taste, usually ascorbic acid. Read the ingredient list

next time you buy packaged sourdough bread. If ascorbic acid is listed, your sourdough bread is

a fake.

TASTINGS The sourdoughs of San Francisco

A group of San Francisco microbiologists investigated sourdough starters and

found that there appears to be some truth to ancient sourdough starters that people

treasure for decades without weakening or spoiling . They found that these

decades-old starters may have developed a community of two types of microbes,

yeast and bacteria, in perfect equilibrium with each other, including a sort of

antibiotic action that excludes other microbes and resists outside contamination.

As the starter ages, it becomes contaminated by other not-so-friendly bacteria and as a

result, your starter becomes too acidic or too off-flavored. Then you have to start over again from

scratch.

Baking the Bread—What Heat does

Baking seems simple to us: put the well-risen, proofed dough in the hot oven and take it out

when it is fully baked. If all went well (and there is no reason why it shouldn't), we place a still-

steaming, irresistibly-perfumed, brown-crusted, mouth-wateringly beautiful loaf of bread on a wire

rack, and we are ready to cut into it after a short cooling period. But the baking process is anything

but simple. There is a series of very complex chemical reactions and physical processes that happen

during bread baking, so complex that even food scientists who have studied the baking process for

decades are far from fully understanding it. For our purposes as home chefs we don't need to know

more about these complex reactions than the very basics which are simple.

In a nutshell, here is what happens in the oven. There are three stages of baking.

1. The first stage covers the first quarter of baking time, until the temperature of the dough

reaches 140°F (60°C). That is the temperature when the yeast cells die. Up to that point the rising

heat keeps the yeast more and more active to produce a great amount of carbon dioxide gas. All the

gas trapped in the dough now expands rapidly as we still remember from our physics class—heat

expands gases. Another thing happens, too. The by-product alcohol the yeast produce after gobbling

up the sugar evaporates and turns into gas in the hot oven. The result? Even more gases in the

dough.

As a consequence, the dough expands rapidly. Bread bakers call this process oven spring—

the bread dough springs up. Anticipating oven spring is the reason why you don't let the dough fully

double in the last rise. If you allowed the dough to rise too much, the expanding gases during oven

spring may rupture the barely solidified gluten structure, and the loaf may partially deflate. Also, if

you let the dough rise too much, its structure becomes too unstable, and even such last-minute

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