Fermentation

Fermentation draws a fine line between fresh and rotten. And if you’ve ever popped the lid on a bulging tub of soup, you’ll know that fine line usually veers towards rotten. But in notable cases, like yogurt, beer, cheese and bread, fermentation makes food desirable to eat. In bread, fermentation refers to the metabolic action of yeast and of the many strains of yeast, the strain used to ferment bread is Saccharomyces cerevisiae. Although food chemists are stumped to explain what draws yeast to a simple mixture of flour and water, what is known is that fermentation develops flavor by encouraging yeast or bacteria to feed on flour and produce alcohol and carbon dioxide as digestive byproducts. The carbon dioxide is trapped and later leavens the dough as the alcohol evaporates during baking. While we ferment grain in order to leaven it, we also ferment grain to release the simple sugars trapped within the complex starch molecules some of which feeds yeast, but most of which flavors the dough and caramelizes to create the beautifully, burnished crust of baked bread.

Fermentation begins the moment that yeast, water, and flour are mixed and continues until the dough is baked, where the heat of the oven will kill the yeast and end the fermentation process. Traditionally, bread recipes instruct for the dough to be fermented after mixing and again after shaping. This is the most time-consuming part of bread baking and is the one that bakers—both aspiring and professional—are eager to skip. Using dough conditioners and flavor “enhancers” allow one to do just that but the bread will never match the quality of one made with properly fermented dough as fermentation produces organic acids that strengthen the dough and aid in its presentation once baked.

Although fermentation occurs at temperatures between 30F and 130F, between 75F and 80F is the optimum range for bread fermentation. For certain, this would be a narrow range if gas production was our only goal, but flavor is an important consideration. In fact, it’s our primary consideration and this range encourages both gas and flavor development without favoring one at the expense of the other. At higher temperatures, fermentation is wildly chaotic, uncontrolled and produces a loaf with a pale, insipid crust, overtly acidic flavor, and a lingering boozy aftertaste. I wouldn’t even feed this “bread” to a pig. At cooler temperatures, the dough ferments slowly enough to allow the acids to perfume the dough and for the dissolved gases to create a pleasantly, lightened texture.  The yeast, still lively, also have enough vigor to give one last burst of activity when the dough hits the high heat of the oven and this oven spring contributes to the open-hole crumb prized by artisan bakers.

The French, observers and purveyors of all things romantic, proffer a richly descriptive vocabulary of fermentation. When selecting a flour to bake with, consider la tolerance, its strength, its  ability to support minor faults—over or under mixing or too much fermentation—without impairing the quality of the finished loaf. When dough is mixed and fermentation begins, it moves vigorously, sluggishly, or somewhere in between. This is la pousse, the visual and metaphysical processes that allow fermentation to transform flour and water into bread. Once mixed, la force improves the dough’s elasticity, its ability to stretch. Too much elasticity and the dough “bucks” like a wild horse. This tight and unworkable dough is said to have la ténacité. During le pointage, the first fermentation, the dough is allowed to rise before being shaped into loaves. The yeast feed voraciously on the small amount of sugar that is naturally present in flour, mostly due to milling damage.  L’appret, the second fermentation, occurs after the dough is shaped and the yeast, still hungry, continues to feed on sugars present in the starch, but only by the action of enzymes.

By themselves, yeast can only feed on simple sugars; they themselves cannot digest starch molecules. Instead, enzymes play a key role in fermentation by breaking down food into simple sugars making them vulnerable to yeasts, bacteria, and even our tongues. Over time, it is this degradation that makes food—be it wine or flour—fermentable. Because yeast lack this enzyme, flour millers add malted wheat or barley (grains that have sprouted and developed enzymes that convert starches into sugar) or enzymes that have been extracted from mold cultures. As food scientist Harold McGee notes, these enzymes “selectively increase the rate of chemical reactions that would otherwise occur very slowly” and without these enzymes, yeast would starve before they are able to fully ferment the dough enough to leaven it.  Each enzyme acts on a “substrate”, much like a key opens a lock, and amylase is the enzyme that acts on amylose, a category of sugars that include maltose, sucrose, glucose, fructose and dextrose. As amylase enzymes break down flour’s starch, the yeast consume the simple sugars from which they reproduce and exude a liquid that releases carbon dioxide and alcohol into the air bubbles of the fermenting dough.

A basic understanding of fermentation also furthers our knowledge of how sourdough breads acquire their prized flavor. It is true that a wild of strain of yeast differentiates sourdough from other breads leavened using commercial yeast, but this is only part of the story. The complex sour flavor prized by sourdough aficionados is not created by wild yeast, but by bacteria. These bacteria thrive in acidic environments and create lactic and acetic acid, the sour by-products from feeding off enzyme-released sugars in the dough, a process that takes longer to develop flavor and why these bacteria thrive in the cold temperatures of slow rise fermentation. (Unlike their wild counterparts, commercial yeast does not thrive in acidic environments and there is a limit to which commercial yeast can be used in slow-rise fermentation.)  In his baking text, Bread Alone, renowned baker Dan Leader likens slow-rise fermentation to the struggle of the wheat grain as it grows: “Ironically, the struggle of the living cells in the dough to thrive in moderate conditions produces a light and complex dough, not a tough one. It’s much like the struggle that forces the wheat plant to produce a good tasting grain. When the soil is just dry enough, but not too dry, the plant works harder, sending its roots deeper in search of moisture. The deeper the roots, the healthier the plant and its berry. It’s the same for a dough that ferments in a not-too-hot, not-too-cold environment. It will take longer, but the final bread will have the good flavor of the grain, a dense crumb with irregular air pockets, and a wonderfully satisfying chewiness.”

At its core, fermentation unleashes the hidden flavors of flour, making it the heart, the soul, the very essence of bread baking.