Carefully positioned on a warm, sunny corner of my desk, there is a squat glass tumbler half-filled with a thin paste of whole wheat flour and water. It’s been there since Sunday; lifeless and inert at first, but lately beginning to bubble and foam towards the rim of the glass, quietly inflating its transparent ziploc bag.
Soon it will be a proper sourdough culture. I had to make this one because my original sourdough culture sat neglected throughout my entire master’s degree, ultimately becoming hopelessly contaminated with some kind of foul-smelling mould. Reluctantly, I was forced to flush it down the toilet like a beloved childhood pet. This new culture is in its infancy — it will develop its own particular community of yeasts and bacteria, and eventually produce its own characteristic bread.
Baking with sourdough takes time and effort, much more so than using commercial baker’s yeast. You have to create and maintain a sourdough culture, feeding it fresh flour and water on a regular basis. You have to explain to visitors why there is a glass on your desk overflowing with fragrant, bubbling goo. You have to be patient, because sourdough bread takes a long time to make — hours or days for a single loaf — and is incredibly sensitive to variation in temperature, ingredients, humidity and other variables.
It wouldn’t be worthwhile if the end result were not superior in every respect to bread made with baker’s yeast. Yeasted breads are dull and flavourless by comparison, the worst of all being supermarket-style sandwich bread: fluffy, mildly sweet, full of preservatives and high-fructose corn syrup. Home-made breads made with baker’s yeast can be much better, but too often turn out uninteresting and excessively yeasty.
Using sourdough coaxes an entirely different range of flavours out of simple ingredients: many of the best and most flavourful breads contain only flour, water and salt. In The Man Who Ate Everything, Jeffrey Steingarten describes pain Poilâne, perhaps the world’s most famous sourdough, as follows:
…a thick, crackling crust; a chewy, moist, aerated interior; the ancient, earthy flavors of toasted wheat and tangy fermentation; and a range of more elusive tastes—roasted nuts, butterscotch, dried pears, grassy fields—that emanate from neither flour, water, nor salt, but from some more mysterious source. This is the true bread of the countryside, Poilâne writes, the eternal bread.
Sourdough breads aren’t always sour, so the name is inappropriate and demeaning, like calling wine “sourjuice.” Sourdough merely refers to a way of making many different kinds of bread, from sweet, rich panettone to pungent, dense pumpernickel. These breads are united only by the fact that they are leavened not with baker’s yeast (Saccharomyces cerevisiae) but with wild microbes, the various yeasts and bacteria that live on the surface of grains, berries and fruits, and which spring to life when you leave a glass of flour and water on your desk for long enough.
These sourdough communities can be biologically diverse, containing hundreds of coexisting strains of yeasts and bacteria. Most of the bacteria are from the genus Lactobacillus, which consists of lactic acid-producing microbes commonly found not only in fermented food products such as beers, yogurts and cheeses, but also in humans.
Some seem equally happy in either environment. For instance, one species of bacteria that lives in many sourdoughs — Lactobacillus vaginalis — was first isolated from trichomoniasis patients. Other species (e.g. L. reuteri, L.pontis and L. plantarum) are frequently found in both sourdough and the mammalian intestine. Who knows how they spread from one environment to the other? I came across a scientific article which nonchalantly mentions that traditional Italian panettone is made with “cow and horse dung,” which I find implausible (why would it be traditional for bakers to put manure in delicious Christmas bread?) and have been unable to verify, even after careful research (i.e. googling “traditional panettone dung”).
Whatever their provenance, these microbes endow sourdough bread with rich and varied flavours and textures. When you mix bread dough, enzymes in the flour break down starch into maltose, a disaccharide sugar made of two glucose molecules. Baker’s yeast is capable of digesting maltose on its own; in fact, it does this so efficiently that it tends to competitively exclude other microbes. In contrast, most yeasts found in sourdough (e.g. Candida milleri) cannot digest maltose. Instead, these maltose intolerant yeasts rely upon lactic acid bacteria to do their work for them. The bacteria break down maltose down into glucose; in doing so, they release some of the glucose into the dough, where the yeast feed upon it. Because the growth of the yeast can be limited by the activity of the bacteria, sourdough bread tends to take much longer to rise than yeasted bread, allowing time for flavourful chemicals to accumulate in the dough.
As the yeast and bacteria ferment sugars, they excrete a number of these chemicals as waste products. Lactic acid bacteria produce lactic and acetic acid, and yeast produce carbon dioxide. Both yeast and bacteria produce ethanol. In fact, they make quite a lot of it: during alcohol prohibition in America, scientists noted with concern that “ordinary bread from bakeries and housewives’ ovens” often had high enough alcohol content to be, strictly speaking, illegal.
These byproducts not only leaven and flavour the dough, but react with one another to produce volatile organic compounds. For instance, ethanol and acetic acid react to form ethyl acetate, the chemical that gives nail polish its peculiar odor. Ethanol and lactic acid form ethyl lactate, which I have never encountered, but which Wikipedia describes as “mild, buttery, creamy, with hints of fruit and coconut.” These esters, along with various alcohols, aldehydes, ketones, carbonyls and other byproducts of fermentation, give sourdough bread its complex, subtle flavours. They are why sourdough can have nutty, grassy, floral or medicinal notes; why, in short, it can be so much better – and so much more unpredictable – than yeasted bread. Everyone talks about pairing the right wine with a meal, but what about the right bread?
All of these flavours depend on innumerable variables; e.g., the flour used, the temperature of the dough, the quantities of water, salt and other ingredients, and, of course, the microbial composition of the sourdough culture. There’s no such thing as a recipe for sourdough bread: you and I can make the same cake relatively easily if we follow the same formula, but sourdough is too complex to replicate with fidelity. I can’t even make similar bread in different countries (while I’ve occasionally made good sourdough in Vancouver, in England it consistently turns out to be mediocre, even though I always smuggle small quantities of the same sourdough starter across the Atlantic in my checked luggage).
If it were easy, though, it wouldn’t be so satisfying to get it right.