Stepping back for a moment to apply just this rudimentary information, you can see how the action of this fungal cerevisiae yeast initiates a series of transformations in foodsand not just in bread doughby causing fermentation. (There are other fermentation types besides yeast-generated, such as bacteria-generated lactic fermentation, which creates acids in foodsthink pickles and cheeses.) What’s really going on during any kind of fermentation is that complex molecules are being broken down, metabolized, and converted into smaller units. This is first initiated by enzyme activity, a chemical rather than biological transformation in which starch and protein molecules are broken into smaller units, such as the simple sugar glucose, by natural enzymes. The released simple sugars then serve as yeast food; yeast is able to feed off of glucose, metabolize it, and expel it as ethyl alcohol and carbon dioxide. Other strains of yeast, different from Saccharomyces cerevisiae, can ferment available food, but the by-products could be toxins, fungal molds, or other undesirables. The benevolent carbon dioxide and ethyl alcohol outcomes are why Saccharomyces cerevisiae has emerged as the preferred fermenter for bread- and beer-making (winemakers prefer a related species called Saccharomyces ellipsoides), and why it is cultivated in huge quantities in scientifically controlled laboratories.
Bread Baking and the Yeast Sacrifice
In bread baking we can witness the results of fermentation within a few short hours, as the generated carbon dioxide causes dough to expand, trapping the carbonic gas in the intricate mass of gluten threads and starch mass. One of the fascinating and ironic aspects of yeast fermentation is that, in order for the yeast to complete its designated workto raise the dough into an aerated, light product, imbued with the flavor compounds of the ethyl alcohol and acids produced by both yeast and bacteriait must also die. Death arrives as soon as the internal temperature of the baking dough exceeds the “thermal death point” of all biological organisms (known as the TDP to bakers and cooks), which is commonly believed to be 139°F/59.5°C. Thus, even before the dough has completed its own transformational journey of changing into breadwhich doesn’t occur until the internal temperature passes at least 180°F/82°C and the starches have fully gelatinizedthe yeast has already fulfilled its leavening function and becomes denatured.
A Walk on the Wild Side of Yeast
To complete the picture, it’s important to touch upon the diverse community of naturally occurring wild yeast that is generically known as Saccharomyces exiguus. These yeasts find homes on the skins of fruit, grain, and vegetables, lying dormant until the skins crack and allow them to feed upon the sugars within. They occur as all sorts of subspecies, so it is safe to assume that what we call sourdough yeast, or wild yeast, is composed of many strains and in combinations that can vary from region to region and climate to climate. This is also true of various bacteria, which are, like yeast, dependent on climate conditions and the soil from which the ingredients are grown or where the wheat is milled into flour or the flour is transformed into dough. But, and this is what distinguishes wild yeast from commercially cultivated yeast, the various challenges of surviving in the wild cause these strains to be more resilient to otherwise growth-suppressing conditions, such as highly acidic environments. In doughs that we call sourdough starters, wild yeast and bacteria actually coexist, sharing the simple sugars created by enzyme activity while generating different by-products. Unlike commercially produced yeast, which can’t tolerate acidity very well, the exiguus wild yeast strains actually thrive, and thus live symbiotically with the acid-producing bacteria (at least until the simple sugars run out). The result, happily for those who like sourdough bread, is a complex, acidic flavor that is not achievable by commercial yeast fermentation alone.
Bread baking is really about the balancing act of time, temperature, and ingredients, so there are many ways in which to make dough. But the single most important ingredient is the leaven, the yeast, because it is the most responsive of all the ingredients to the manipulation of time and temperature in terms of creating flavor. It takes only a little yeast to raise an entire loaf. But yeast exists within a complex biosphere and, as a result, creates wonderfully complex possibilities for those who know how to channel its potential. In the world of pharmacology, for instance, scientists are only scratching the surface of its healing properties, and in the culinary world, even after 6,000 years, we are still finding new ways to use yeast to make bread, beer, wine, and all sorts of other delicious fermentable foods.