Cooked: A Natural History of Transformation (32 page)

This lack of control has never sat well with
our species, which probably explains why the modern history of bread baking can be told
as a series of steps aimed at taking the unruliness, uncertainty, and comparative
slowness of biology out of the process. Milling white flour was the first such step.
Whole-grain flours, as I would soon learn, are much more complex and biologically active
than white flour. That’s because white flour consists chiefly of dead starch,
whereas the germ and the bran removed in milling it contain living cells. Whole grains
teem with enzymes and volatile oils that make their flours more perishable and
fermentation more difficult to manage.

Around the same time that the advent of
roller mills made white flour widely available in the 1880s, the introduction of
commercial yeast gave bakers an even more decisive gain in control. Now, instead of
having to rely on an unruly community of unidentified fungi and
bacteria to leaven bread, as had been the case for thousands of years, they could
enlist a single species of yeast to do the job on command. Called
Saccharomyces
cerevisiae
, this species had been (as its name suggests) found in beer,
selected over countless generations, and optimized for the role of putting gas in dough.
Commercial bread yeast is a purified monoculture of
S. cerevisiae
, raised on a
diet of molasses, then washed, dried, and powdered. Like any monoculture, it does one
thing predictably and well: Feed it enough sugars and it will promptly cough up large
quantities of carbon dioxide.

Though commercial yeast is alive, its
behavior is linear, mechanical, and predictable, a simple matter of inputs and
outputs—which is no doubt why it so quickly caught on.
S. cerevisiae
can be
counted on to perform the same way everywhere and give the same results, making it
supremely well suited to industrial production. Yeast could now be treated simply as
another ingredient rather than as a locally variable community of organisms in need of
special care and feeding. In fact, as microbes go,
S. cerevisiae
is notable for
not playing well with others, especially bacteria. Compared with wild yeasts, commercial
yeast cannot survive very long in the acidic environment created by lactobacilli.

While scientists have known about yeast
since Louis Pasteur first identified it in 1857, the intricate microbial world within a
wild sourdough culture like mine was a complete mystery until fairly recently—and
remains at least a partial mystery even today. In 1970, a team of USDA scientists based
in Albany, California, collected samples of sourdough starter from five San Francisco
bakeries and conducted a kind of microbial census. Why San Francisco? Because the city
was famous for its sourdough bread. The scientists were hoping to identify the local
microbes responsible for the bread’s distinctive qualities. Their landmark 1971
paper, “Microorganisms of the San Francisco
Sour Dough French
Bread Process,” helped to spur a revival in naturally leavened breads and almost
single-handedly established the (albeit still minor) field of sourdough
microbiology.
^*

The USDA team discovered that unlike what
happens in the straightforward fermentation performed by
S. cerevisiae
, no
single yeast species was responsible for what takes place in a sourdough culture.
Rather, the process depended on a complex, semisymbiotic association between a yeast
(
Candida milleri
^†
) and a previously unknown
bacterium. Assuming—wrongly, as it turned out—that the bacterium they had identified was
unique to San Francisco’s famed sourdoughs, they named it
Lactobacillus
sanfranciscensis
. It has since been found in bakeries all over the world. Oh
well.

Though not exactly dependent on each other,
the yeast and the bacteria are ideally suited to living together. Each microbe consumes
a different type of sugar, so they don’t compete for food. And when the yeasts
die, their proteins break down into amino acids that the lactobacilli need to grow.

At the same time, the lactobacilli produce
organic acids that shape the environment in ways agreeable to
C. milleri
(which
is acid-tolerant), but disagreeable to other yeasts and bacteria.
L.
sanfranciscensus
also produces an antibiotic compound that prevents competing
microbes from gaining a toehold in the culture, but which doesn’t trouble
C.
milleri
in the least. Thus the sourdough culture defends itself from
colonization by outsiders. This biochemical defense is a boon to us as well, since it
extends the shelf life of the bread.

Perhaps the USDA team’s most important
contribution was to
demonstrate that a sourdough culture functions as
a kind of ecosystem, with the various species performing distinct roles that lend
stability to the culture over time. Once established, the system exhibits more
cooperation than competition, so that no one organism ever dominates. Subsequent
research in other parts of the world has greatly expanded the list of species found in
sourdough cultures—at least twenty types of yeast and fifty different bacteria—but most
of them seem to fall into similar niches, organize themselves into similar
relationships, and perform similar functions. Same play, different actors. Presumably
these yeasts and bacteria coevolved with one another, which might explain why many of
them have been found nowhere
except
in sourdough cultures, their “natural
habitat.” Which in turn suggests these microbes probably coevolved with us: Their
culture depends upon our culture of bread making, and (until recently) vice versa.

In the microuniverse of a sourdough culture,
the baker performs in the role of god, or at least of natural selection. It may well be
that the requisite microbes are everywhere, but by shaping their environment—the food
and feeding schedule, the ambient temperature, the amount of water—the baker, wittingly
or unwittingly, selects which microbes will thrive and which will fail. Frequent
feedings and warm temperatures tend to favor the yeasts, for example, creating an
airier, milder loaf, whereas skipping meals and refrigerating the culture favors the
bacteria, leading to a more acidic environment, and a more strongly flavored bread.

“Baking well really comes down to
managing fermentation,” according to Robertson. The flavor and quality of a
naturally leavened bread depends to a great extent on how skillfully the baker governs
this invisible microbial world. And if the baker fails to care for his culture? It may
take awhile, but once the sun of his attention goes dark, the culture eventually
dies.

The morning after starting my sponge, I woke up
eager to head down to the kitchen to see what, if anything, had happened overnight. When
I’d mixed the stuff the night before, the heavy paste of flour and water filled a
two-cup measuring bowl halfway to the top. Incredibly, it had doubled in volume
overnight, and I could feel it had lightened considerably, achieving a consistency
reminiscent of marshmallow. Through the glass I could see that the paste had become a
gassy foam, shot through with millions of air bubbles. I felt certain it would
float.

So into a larger bowl I measured out the
quantity of warm water called for in the recipe (750 grams), and then, using a spatula,
scooped out the sponge. It slid into the warm bath and then bobbed up to the surface of
the water like a raft, buoyant. I was in business! Next I added 900 grams of white flour
and 100 grams of whole-wheat flour. I mixed everything together by hand, squeezing the
flour and water through my fingers to make sure there were no unhydrated lumps of
flour—what bakers call “chestnuts.” The result was a dough wetter than
anything I had ever worked with before. This promised to be a challenge.
^*

Before any salt is added, the dough gets to
rest for twenty minutes or so. Called the “autolyse,” this period gives the
flour a chance to fully hydrate, the gluten to begin to swell and get itself organized,
the enzymes to begin cleaving complex starches into simpler sugars, and the fermentation
of those sugars to commence. Salt acts as a check on all these processes, which in its
absence would proceed too rapidly.
The goal is a long, slow
fermentation in order to build maximum flavor. As one nineteenth-century cookbook put
it, salt serves as the bridle on the wild horse of fermentation.

After I mixed in the twenty grams of salt,
the dough felt dull and sticky to the touch—a wet, heavy, lifeless clay. I covered the
bowl with a towel and went back to work, setting my phone to alert me in forty-five
minutes. “Bulk fermentation” was now under way—a period of between three and
four hours during which the principal development and fermentation of the dough takes
place.

A complex drama unfolds during the bulk
fermentation, one that the baker cannot see but can infer by the evolving texture,
smell, and taste of his dough. Within the dough, a spongiform structure is taking shape,
a three-dimensional lacework of air. The structure is the result of two separate
developments—one chemical in nature, the other biological—that in a dough made from
wheat flour happen, fortunately for the panivore, to coincide and intersect just so.

The chemical development is the formation of
gluten (the word means “glue” in Latin), an interesting if somewhat
problematic substance that is found primarily in wheat, and to a much lesser extent in
rye, another species of grass. To be precise, gluten as such is not found in wheat
itself, but, rather, its two precursors are, the proteins gliadin and glutenin, which
when moistened in water combine to form the mesh of proteins known as gluten.
Unprepossessing on its own, each of these proteins contributes a different but equally
important quality to a bread: extensibility on the part of gliadin, and elasticity on
the part of glutenin. As in the fibers of a muscle, these qualities exist in a
productive tension, the former allowing the dough to be stretched and shaped, while the
latter impels it to bounce back to something close to its original form. In fact, the
Chinese call gluten “the muscle of flour,” and all bakers speak in terms of
a dough’s “strength” or “weakness,” qualities that
correspond to the amount of gluten in it.

The pliable yet rubbery properties of gluten
make it the ideal medium for trapping air, which happens to be the crucial by-product of
the second, biological development under way in a wet mass of fermenting dough. While
the gluten network is forming and gaining strength, the community of yeasts and bacteria
introduced by the starter are dining on starches “damaged” during milling,
when some of them are broken into sugars. Various enzymes (some of which are present in
the flour, others produced by the bacteria and yeasts) go to work on the undamaged
starches and proteins, breaking them down into simple sugars and amino acids to feed the
microbes. Thus fed, the bacteria proliferate, producing lactic and acetic acids, which
help to strengthen the gluten while contributing new flavors. And, most important of
all, the yeasts are busy transforming each molecule of glucose they consume into two
molecules of alcohol and two of carbon dioxide. The carbon dioxide gas, which is a
by-product of alcohol production, would simply escape into the atmosphere if not for the
rubbery matrix of gluten, which stretches like a balloon to contain it. Without the
extensible and elastic gluten to trap the carbon dioxide, bread would never rise.

The properties of gluten have commended wheat
to humanity since the Egyptians first recognized what it could do. Before that, wheat
was just one edible grass among many, part of a crowded field that included millet,
barley, oats, and rye and, later, corn and rice. Barley barely registers in our eating
lives today, but before the invention of bread it was just as important a staple food in
the West. It grows more quickly than wheat, and in more places, from the tropics to the
Arctic Circle. Highly nutritious, it was the food of choice of the Roman
gladiators, who were in fact called
hordearii
, the barley
eaters. But though barley made nourishing porridges and flat breads (and beer, as I
would discover), no amount of leavening could raise it off an oven floor.