Notes on Sourdough

Appears in

Bread: A Baker's Book of Techniques and Recipes

Published 2004

About

Methods are offered below for making three different styles of sourdough culture: white liquid levain, stiff-textured levain, and a rye culture. Before giving the actual methods for each type, a few considerations are in order:

Occasionally grapes, potato water, grated onions, honey, and so on are added to flour and water during the initial phases of culture development. While these can provide an additional nutritional boost, they are not required for success. Good-quality flour will be sufficient to supply the needed nutrients to the culture.
Bleached flour is never appropriate when developing or perpetuating sourdough. Vital nutrients necessary to sustain the microorganisms (not to mention the humans who later consume the bread) are lost in the bleaching process.
High-gluten flour is not a good flour choice when beginning a culture. Being higher in protein, it contains proportionally less starch, and much of the nutrient supply for the microorganisms comes from the starch.
Chlorinated water impedes fermentation and can be harmful during the fragile beginnings of culture development. Chlorine gas rapidly dissipates, however, and by keeping an open jug of water on the counter for several hours, most all the chlorine will dissipate. Filtered water and well water can, of course, be used to begin a culture.
All or part rye flour is often used in the beginning stage of developing a culture that will eventually become all white. Rye is quite high in nutrients and fermentable sugars, and can help get the culture off to a good start (there are about 200 times more sourdough-friendly microorganisms in whole grain flour compared to white flour). Similarly, some bakers soak bran in water overnight before commencing their culture; the drained water is mixed with the flour, as it is nutrient rich from the bran.
Yeast production is faster than the production of acidity during the early stages, at least in a bakery environment, where yeasts tend to be abundant. This is why we see evidence of a rise in the culture after a day or two, but taste little acidity. If correct feedings and temperature are maintained, acidity will become evident after a few days. In the home environment, it is not uncommon for acidity to develop before yeast, due to the lack of ambient yeast in the environment.
The bacteria in sourdough cultures come mainly from the genus Lactobacillus. In young and developing cultures, homofermentative bacteria develop, producing lactic acid. Older cultures have more complex flavors than young ones, largely due to the presence of heterofermentative bacteria, which produce both lactic and acetic acid.
The presence of both lactic and acetic acids, in balance, is most favorable in sourdough bread production. The lactic acid provides smoothness (somewhat akin to yogurt), while the acetic acid gives a pronounced sour bite (think vinegar). The development of lactic acid is favored in warm environments and loose dough conditions; acetic acids develop more readily in cool and stiff conditions. The baker can use that knowledge to impart desired flavors to his or her bread through manipulation of temperature and hydration.
Ideally, sourdough cultures should be refreshed daily and used to make bread daily (this is more proof that we live in a less-than-ideal world). “Refreshing” simply means feeding the culture with flour and water. In typical bread production, the sourdoughs are built in 1, 2, or 3 stages over a period of roughly 16 to 24 hours. Each build constitutes a refreshing, and when the baker removes a small portion of the ripe sourdough just before mixing the final dough, the removed sourdough is considered fully refreshed. One principle remains the same, whether the sourdough is developed using 1 build or 3 builds, and this principle is at the heart of sourdough bread production: Once the culture has been built and is at the point of maturity, a small portion must be removed and saved for future use.
It is a good practice to evaluate the maturity of the sourdough prior to each bread-making session. The signs of ripeness in a sourdough are similar to those of a yeasted pre-ferment. The rye or stiff white sourdough will be domed on the top, and just beginning to recede in the center. This is the time of perfect ripeness: The microorganisms have happily acidified the flour you fed them, and are now ready to make bread. If the culture is maintained in liquid form, look for evidence that the sourdough has not risen and then fallen from over-ripeness. If there is a “high-water mark” at the edges of the container and the culture has dropped, then it is overripe. The solution is to ripen the sourdough in a cooler environment, to let less time elapse between the building of the sourdough and the mixing of the bread, or to add a small percentage of salt in order to retard the activity of the wild yeast in the culture (see “Sourdough and Salt”).
I remember once asking a respected yeast microbiologist if he thought sourdough rye breads generated from rye culture were superior to rye breads made using white culture. He responded that, as long as a culture has a balanced component of heterofermentative bacteria, it does not matter at all whether the bread was derived from rye or white culture. From an empirical baker’s perspective, having used rye and white culture for many years, my experience is that rye breads are superior when mature rye culture is their foundation, perhaps because the microorganisms in the mature culture have a thorough familiarity with metabolizing rye flour. Bakers who only maintain a wheat-based culture might consider giving it a meal or two of rye flour before making rye breads.
The influence of cold storage on sourdough cultures, and the biological changes affecting cultures held under extended refrigeration, has been incompletely studied, but it is a topic of importance to bakers, particularly those who do not bake sourdough breads daily. Certainly the daily refreshing of a culture is impractical and onerous for most recreational bakers. How a culture changes and evolves while refrigerated is a very complex topic, and although it is now clear that sourdough microorganisms are able to exist in cold conditions, whether or not they will thrive is another matter entirely. Many factors affect how successfully a culture can weather frequent or extended periods of refrigeration, such as the type of flour used in refreshments, hydration, temperature of cold storage, usual feeding regimen, and above all, the relative health of the culture as it enters the cold phase. Common sense tells us that a culture that is refrigerated for a couple of weeks once or twice a year, and for the rest of the year benefits from frequent (daily) nutritional input is far more likely to weather a cold storage phase compared to a culture that spends the majority of its life under refrigeration. Another potential consequence of repeated lengthy storage at cold temperatures is some die-off of wild yeasts and lactobacilli, and the accumulation of off-flavors and aromas in the culture. While all the microorganisms don’t necessarily die off in these conditions, it is common for cultures thus kept to need so much remedial feeding before they are suitable for bread making that one could just as easily start over, make a new culture, and enjoy all the enthusiastic vigor that the new culture imparts to bread. Ultimately, using cold storage as a long-term strategy for sourdough maintenance is not the most effective means of keeping a culture in optimum health.
In Germany, the word for sourdough is Sauerteig, and it refers to a culture of rye flour and water. In France, the word for sourdough is levain, which refers to a culture that is made entirely, or almost so, of white flour. Slightly complicating things is that levain also refers to the last build that takes place before mixing the final dough, and to a type of bread, such as pain au levain. (The desem method of sourdough production, originally from Belgium, utilizes a whole-wheat culture, maintained in a cool environment, and almost always the bread is made without the addition of baker’s yeast.) Sourdough, Sauerteig, levain: what unites them is that each is a culture of naturally occurring yeasts and bacteria that have the capacity to both leaven bread and provide it with flavor. A German-style culture is made using all rye flour and water. A white levain culture may begin with a high percentage of rye or whole-wheat flour, or with all white flour. In any case, it eventually is maintained with all or virtually all white flour. While a rye culture is almost always of comparatively stiff texture, a levain culture can be either loose or stiff (ranging in hydration between 50 and 125 percent). Whichever method is used, the principle is the same. The baker mixes a small paste or dough of flour and water, freshens it with new food and water on a consistent schedule, and cultivates a community of microorganisms that ferment and multiply. In order to preserve and perpetuate the culture, a small portion of ripe culture is removed before the mixing of the final dough. This portion is held back, uncontaminated by yeast or other ingredients of the final dough, and used to begin the next batch of bread.