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Sourdough is one of the main ways to leaven bread in the baking process, whilst yeast is the other. The main difference in bread when made from sourdough as opposed to yeast, is its sour taste. In westernized countries, where wheat-based breads are most common, yeast is typically used to raise the bread. Sourdough was originally replaced by barm used to ferment beer, before yeast could be cultured and used as a leavening agent. Sourdough is still used but far more scarcely.

Sourdough starter is essentially a piece of dough extracted from a previous batch of dough. The culture needed to leaven the bread exists in this piece of dough, which is added to fresh dough and kneaded. The dough is then set aside so that it can leaven and the process of extracting a piece of dough to be used as a future sourdough starter is repeated. For this sourdough starter to maintain its culture, flour and water must be added every week, but if this is done then it can last for an indefinite period of time.

The sourdough starter should equate to around a quarter of the new, fresh sourdough mix, although this can vary in different recipes. The dough is separated according to requirements and then is leavened before baked.

A sourdough starter can have its roots in bread from many years ago, and so each baker’s sourdough can taste very different. The process used to make the starter, how often it is refreshed, and the moisture in the air, can all serve to create different tasting sourdoughs.

Biology and chemistry of sourdough

The starter contains the yeast saccharomyces exigus, which lives in the starter and exists in symbiosis with lactobacillus. The starter consists of the usual water and flour, and a starter that can be bought from a shop. When the starter has been prepared, it should be refreshed regularly. Most sourdoughs use a fairly fluid starter, although firmer dough can be used.

A new starter is made with flour and water. As the flour comes in to contact with the water, the starch is broken down in to compounds that the yeast metabolizes. Bacteria in the mix feed on the products metabolized by the yeast and develops the culture needed to make sourdough.

Using unbleached flour as opposed to processed flour creates a more successful culture, whilst whole meal flour contains the most minerals and organisms. Using water in the mix that has been used to boil potatoes can increase the leavening ability of the starter because of its starch content. Diastatic malt can also be used.

The culture in the starter will leaven a wheat flour bread but to achieve the same effect with sourdough is harder to do. This is because the lactobacteria outnumbers the yeast content, inhibiting the yeasts effects, resulting in a poorer quality bread.

Sourdoughs are categorized into two types; Type I and Type II. Each type contains different yeasts and has different pH levels. The types are fermented at different temperatures for varying lengths of time.

Preparing sourdough products

Sourdough starters can be used in two different ways. The most traditional method is to mix the starter into the dough, allowing it to leaven and then bake, in a similar fashion to the process that is followed when baker’s yeast is used. Using this method does take a substantially longer period of time though because of the amount of time the dough has to be left to leaven before baking. Care also has to be taken according the consistency of the starter being used, as liquid starter have a high bacteria content and will impact the resulting bread differently if not managed properly.

Alternatively, sourdough starters can be used to make quick breads and involve the use of baking soda or powder to neutralize the acid content; a reaction that creates the gases needed to raise the bread. This technique is most commonly used in Alaska.

History of sourdough

Although sourdough has its roots as far back as Ancient Egypt, it was still a method used to leaven bread up until the Middle Ages, when it was replaced by barm used in fermenting beer, and then finally yeast.

In Northern Europe, all-rye breads are particularly popular, and use sourdough in the leavening process. Baker’s yeast is not as successful as sourdough in leavening rye bread because the gluten content in rye is not high enough. Sourdough inactivates the amylase content in the rye so that it cannot break down the starch, as the starch is what provides structure to the resulting bread. In Southern Europe, where wheat and rye breads at one time used sourdough, now use baker’s yeast to leaven their breads. This change seems to have fallen in line with rising living standards in these regions.

Sourdough still remains popular in San Francisco as it was during the Gold Rush. The popularity of the bread spread to Alaska and Canada were they are still made today. The San Francisco sourdough is still famous throughout the US. It has been made for over 150 years and some bakers in the area are able to trace their original starters back to territorial times. The San Francisco sourdough is a sour white bread and is typically eaten with seafood or with soups.

Although the popularity of sourdough has dropped off, there are many eateries in the area that continue to sell it. Commercially sold sourdough uses a bread improver as opposed to a yeast and bacterial culture.

Sourdough breads

Sourdough bread is not the only bread that uses a starter in its preparation. Many other recipes use a sourdough starter, for example, some Amish bread uses a starter that is mixed with milk and sugar, before further leavening with baking powder. Pumpernickel traditionally used a sourdough starter, although nowadays they use yeasts with citric acid to neutralise amylase. Other breads use a sourdough starter for flavour as opposed to a leavening agent. Some French and Italian breads use sourdough starters in this way.