Sourdough

Sourdough is a type of bread that uses fermentation by naturally occurring yeast and lactobacillus bacteria to raise the dough. In addition to leavening the bread, the fermentation process produces lactic acid, which gives the bread its distinctive sour taste and improves its keeping qualities.

History

Sourdough is one of the most ancient forms of bread. It was the standard method of breadmaking for most of human history until the Middle Ages, when it was replaced by barm. Barm, in turn, was replaced in the late 19th and early 20th centuries by industrially produced baker's yeast.
The Encyclopedia of Food Microbiology states: "One of the oldest sourdough breads dates from 3700 BCE and was excavated in Switzerland, but the origin of sourdough fermentation likely relates to the origin of agriculture in the Fertile Crescent and Egypt several thousand years earlier", and "Bread production relied on the use of sourdough as a leavening agent for most of human history; the use of baker's yeast as a leavening agent dates back less than 150 years from the year of 2014". Sourdough remained the usual form of leavening down into the European Middle Ages until being replaced by barm from the beer-brewing process, and after 1871 by purpose-cultured yeast.
French bakers brought sourdough techniques to Northern California during the California Gold Rush, and it remains a part of the culture of San Francisco today. Sourdough has long been associated with the 1849 gold prospectors, though they were more likely to make bread with commercial yeast or baking soda. The "celebrated" San Francisco sourdough is a white bread characterized by a pronounced sourness, and indeed the strain of Lactobacillus in sourdough starters is named Fructilactobacillus sanfranciscensis, alongside the sourdough yeast Kasachstania humilis found in the same cultures.
The sourdough tradition was carried into the Department of Alaska in the United States and the Yukon Territory in Canada during the Klondike Gold Rush of 1898. Conventional leavenings, such as yeast and baking soda, were much less reliable in the conditions faced by the prospectors. Experienced miners and other settlers frequently carried a pouch of starter either around their neck or on a belt; these were fiercely guarded to keep them from freezing. However, freezing does not kill a sourdough starter; excessive heat does. Old hands came to be called "sourdoughs", a term that is still applied to any Alaskan or Klondike old-timer. The significance of the nickname's association with Yukon culture was immortalized in the writings of Robert Service, particularly his collection of "Songs of a Sourdough".
In English-speaking countries, where wheat-based breads predominate, sourdough is no longer the standard method for bread leavening. It was gradually replaced, first by the use of barm from beer making, then, after the confirmation of germ theory by Louis Pasteur, by cultured yeasts. Although sourdough bread was superseded in commercial bakeries in the 20th century, it has undergone a revival among artisan bakers and, more recently, in industrial bakeries. In countries where there is no legal definition of sourdough bread, the dough for some products named or marketed as such is leavened using baker's yeast or chemical raising agents as well as, or instead of, a live sourdough starter culture. The term sourfaux has been applied to such breads by the Real Bread Campaign, an initiative that has advised against the consumption of most modern breads because of such preparation techniques plus the inclusion of leavening agents, such as baking soda and baking powder, which may be linked to celiac disease.

Preparation

Starter

Sourdough leavening requires a pre-ferment, a live mixture of flour and water, containing a colony of microorganisms including wild yeast and lactobacilli. The purpose of the starter is to produce a vigorous leavening and to develop the flavour of the bread. In practice there are several kinds of starters, as the ratio of water to flour in the starter varies. A starter may be a liquid batter or a stiff dough.
To initially create the pre-ferment, water and flour are mixed and left in a warm place for a week or two. When wheat flour comes into contact with water, the naturally occurring enzyme amylase breaks down the starch into the sugars glucose and maltose, which sourdough's natural yeast can metabolize. With sufficient time, temperature, and refreshments with new or fresh dough, the mixture develops a stable culture. This culture will cause a dough to rise. The bacteria ferment starches that the yeast cannot metabolise, and the by-products, chiefly maltose, are metabolised by the yeast, which produces carbon dioxide gas, leavening the dough.

Refreshment or feeding of the starter

As it ferments, sometimes for several days, the volume of the starter is increased by periodic additions of flour and water, called refreshments or referred to as a feeding. As long as this starter culture is fed flour and water regularly, it will remain active.
The ratio of fermented starter to fresh flour and water is critical in the development and maintenance of a starter. This ratio is called the refreshment ratio. Higher refreshment ratios are associated with greater microbial stability in the sourdough. In San Francisco sourdough, the ratio is 40% of the total weight, which is roughly equivalent to 67% of the new-dough's weight. A high refreshment ratio keeps acidity of the refreshed dough relatively low. Acidity levels of below pH 4.0 inhibit lactobacilli and favor acid-tolerant yeasts.
A starter prepared from scratch with a salted wheat-rye dough takes about 54 hours at to stabilise at a pH between 4.4 and 4.6. 4% salt inhibits L. sanfranciscensis, while C. milleri can withstand 8%.
A drier and cooler starter has less bacterial activity and more yeast growth, which results in the bacterial production of more acetic acid relative to lactic acid. Conversely, a wetter and warmer starter has more bacterial activity and less yeast growth, with more lactic acid relative to acetic acid. The yeasts produce mainly CO₂ and ethanol. High amounts of lactic acid are desired in rye and mixed-rye fermentations, while relatively higher amounts of acetic acid are desired in wheat fermentations. A dry, cool starter produces a sourer loaf than a wet, warm one. Firm starters tend to be more resource-intensive than wet ones.

Intervals between refreshments

A stable culture in which F. sanfranciscensis is the dominant bacterium requires a temperature between and refreshments every 24 hours for about two weeks. Refreshment intervals of longer than three days acidify the dough and may change the microbial ecosystem.
The intervals between refreshments of the starter may be reduced in order to increase the rate of gas production, a process described as "acceleration." In this process, the ratio of yeasts to lactobacilli may be altered. Generally, if once-daily refreshment-intervals have not been reduced to several hours, the percentage amount of starter in the final dough should be reduced to obtain a satisfactory rise during proof.
Faster starter processes, requiring fewer refreshments, have been devised, sometimes using commercial sourdough starters as inoculants. These starters generally fall into two types. One is made from traditionally maintained and stable starter doughs, often dried, in which the ratios of microorganisms are uncertain. Another is made from microorganisms carefully isolated from petri dishes, grown into large, homogeneous populations in fermentors, and processed into combined baker's products with numerically defined ratios and known quantities of microorganisms well suited to particular bread styles.
Maintaining metabolically active sourdough with high leavening activity typically requires several refreshments per day, which is achieved in bakeries that use sourdough as sole leavening agents but not by amateur bakers that use the sourdough only weekly or even less frequently.

Local methods

Bakers have devised several ways of encouraging a stable culture of microorganisms in the starter. Unbleached, unbromated flour contains more microorganisms than more processed flours. Bran-containing flour provides the greatest variety of organisms and additional minerals, though some cultures use an initial mixture of white flour and rye or whole wheat flour or "seed" the culture using unwashed organic grapes. Grapes and grape must are also sources of lactic acid bacteria, as are many other edible plants. Basil leaves are soaked in room-temperature water for an hour to seed traditional Greek sourdough. Using water from boiled potatoes is said to increase the activity of the bacteria by providing additional starch.
The piped drinking water supplied in most urban areas is treated by chlorination or chloramination, adding small amounts of substances that inhibit potentially dangerous microorganisms but are harmless to animals. Some bakers recommend unchlorinated water for feeding cultures. Because a sourdough fermentation relies on microorganisms, using water without these agents may produce better results. Bottled drinking water is suitable; chlorine, but not chloramines, can be removed from tap water by boiling it for a time, or simply by leaving it uncovered for at least 24 hours. Chlorine and chloramines can both be removed by activated carbon filters.
Adding a small quantity of diastatic malt provides maltase and simple sugars to support the yeasts initially.
The flavor of sourdough bread varies from place to place according to the method used, the hydration of the starter and the final dough, the refreshment ratio, the length of the fermentation periods, ambient temperature, humidity, and elevation, all of which contribute to the microbiology of the sourdough.