Types of Sourdoughs

Sourdoughs can be started as follows:

During spontaneous fermentation

By adding a piece of mature sourdough (mother sponge)

By adding a defined starter culture

Most sourdoughs used in both wheat and rye bread baking are still initiated by adding a piece of mature or ripe sourdough, also called mother sponge, but there is a tendency to use defined starter cultures with specific fermentation patterns or production of antimicrobial substances. This tendency increases as these cultures become commercially available.

1. Spontaneous Fermentation

When dough made from flour and water is left for 1 to 2 days at ambient temperature, a spontaneous fermentation will take place due to the naturally occurring microorganisms in the flour. The dough will become acidified due to lactic acid fermentation. During the fermentation there is a successive favoring of the gram-positive LAB from the flour at the expense of the gram-negative bacteria that dominate the microflora of the flour (21,32). The microflora of some spontaneously fermented rye sourdoughs were dominated by homofermentative Lactobacillus spp. and Pediococcus spp. (32,33). The level of LAB in sourdoughs was up to 3 x 109 colony forming units (CFU)/g and the number of yeasts about 106 to 107 CFU/g. However, spontaneous sourdoughs do not always succeed and may result in products with off-flavor.

2. Mature Sourdough

Sourdoughs used by artisan bakers and in bakeries have traditionally been based on spontaneous fermentation, during which the sourdough has been kept metabolically active and probably microbially stable for decades by the daily addition of flour and water, the so-called freshening of the dough based on back-slopping (Fig. 1). The fermented sourdough is used for bread production, but part of it is used as starter by initiating a new sourdough. The terminology for sourdough and starter in different countries is listed in Table 2.

In commercial rye bread baking, bakeries can use their own adapted sourdough or, if they have quality problems due to unstable process control, they can add a commercial sourdough as a starter. Most bakeries in Germany and Denmark regularly add commercial sourdoughs composed of a well-adapted microflora derived from natural sourdough fermentation. Examples of commercial sourdoughs are the Sanfrancisco sour for wheat bread production (34) and the Bocker-Reinzucht-Sauer® for rye bread production. Some prod-

Figure 1 A schedule for production of sourdough and sourdough bread.

Table 2 Terminology for Sourdough in Different Countries

English

German

French

Spanish

Italian

Sourdough for

Sourdough, Sauerteig leaven

Levain Masa madre natural (masa agria)

Le chef Pie

Lievito naturale

(impasto acido) Madre, capolievieto bread production

Sourdough used as starter for a new sourdough

Mother Anstellgut sponge, Reinzuchtsauer®

starter

ucts sold as sourdough have no living microorganisms, and these products will not contribute to a natural acidification and development of flavor compounds in the dough.

3. Starter Cultures of Pure Strains of LAB

Starter cultures for sourdough fermentation are pure cultures of dried or freeze-dried LAB, or a mixture of LAB and sourdough yeast. They should be mixed with flour and water, and kept for several hours for multiplication and fermentation of the microflora. This fermented dough can then be used as a sourdough. The microorganisms have been selected due to their ability to acidify dough in a short time and result in acceptable bread flavor when used in bread baking. Cultures containing Lactobacillus sanfranciscensis, L. plantarum, L. brevis, and L.fructivorans or L. brevis, L. pontis, and S. cervisiae are available (35). Use of defined starter cultures with specific properties gives rise to new interesting opportunities for controlling and regulating sourdough fermentation. The term ''starter culture'' is sometimes used in the literature for a mature sourdough that has to be mixed with flour and water to ferment, or for commercial sourdoughs.

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