|Sourdough definition and references
(an attempt to drill down in this matter - this page will be amended sporadically)
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|Sourdough is dough which has microorganisms
(for example lactobacillus or yeast)
sourdough or sourdough starters, which
active or can be reactivited. With
of grain products and water, they are
of continuous acid generation.
Parts of a sourdough are used as storage leaven for new sourdoughs. The vitality of the microorganisms is only terminated with baking or hot-extrusion. The acid increase of sourdough is based exclusively on fermentation. Ingrediences influencing acid contents - except sourdough bread - are not used.
|(This is a translation from the sourdough section of the German "Guidelines for Bread and Small Baking Goods" from 1999)|
When water and flour are mixed and left alone for some time at an appropriate temperature, the mixture will get sour and start to bubble. Using this as a dough to make bread is probably the most traditional and oldest way to make fermented bread. The organisms found here differ from other Sourdough Types in such that the lactobacillus bacteria are mostly homofermentative.hydrations (usable for pumping) and longer fermentation times (5 days). The organisms established in this environment differ from Type I and are adapted to the paramenters, like lactobacillus pontis, lactibacillus panis.
*) I added Type 0 which seemed appropriate
|Microorganisms in sourdough cultures grow,
interact, ferment and die, producing fermentation
products during this cycle. In bread doughs,
they create taste precursors, CO2 for rising,
transform sugars and starches to be more
digestable for humans, change dough structure,
bread crumb, generate amino acids, alcohol,
several organic acids, antibiotics and who
knows what else. Even new strains are discovered
(Lactibacillus (Lb) frumenti in 2000, Lactobacillus mindensis  in 2002).
The main microorganisms involved in sourdoughs are Lb bacteria and yeast strains of non-sporulating, non-motile gram-positive forms. The yeast strains found in established sourdoughs (Type 1+) are acid tolerant, handicapped in such a way that they are unable to metabolize maltose.Lb's in Type 0 sourdoughs are mostly homofermentative which produce only lactic acid, whereas heterofermentative Lb bacteria present in Type 1+ can produce lactic acid, ethanol, acetic acid and CO2.
Lb SF bacteria are demanding in their nutrition and could not be isolated on regular growth media. Additions of yeast- and meat extract provided the environment for this bacteria to grow.
Originally, it was thought that the Lb sanfrancisco (SF) are only metabolizing maltose with glucose/fructose left for the yeast's to feast so the two organisms would not compete for food. Later research indicated, however that many of the larger number of Lb SF substrains discovered more recently are able to metabolise a wide variety of carbohydrates besides maltose, glucose and fructose. It appears that the LB SF's high capacity to metabolise maltose very rapidly is the cause for the prevalence of this organism in continuously propagated sourdoughs. This also may be the answer to the phenomen of LB SF isolated on various places on this planet (Germany, Italy, USA) in continously propagated sourdoughs independent from endogenious factors.
It is also thought that LB's benefit from yeast's waste products and remains and in return the LB's would provide protection for the yeasts by producing antibiotics against which, of cause, the yeasts are immune.
Unless the microoranisms are supplied continously with food and waste products eliminated, a culture will stop growing, and organisms will eventually die. The characteristics can be shown in a growth curve below.
To know about the individual growth phases and be able to relate them to the cultures on hand when dealing with sourdoughs is of significance.
The ideal point to propagate a culture (or make dough, for that matter) would be in Phases 4 and 5 because the highest germ counts are there which will assure strong rise, high acidification and good taste development.
Propagating from Phase 3 can have some benefit. In this phase, the heterofermentative Lb's, like Lb sanfrancisco, Lb brevis or Lb pontis produce the majority of CO2 (in addition to yeast CO2 production) and can be used in bread making processes to replace bakers yeast. Exact control of parameters (time, temperature, hydration) is a requirement in a production environment.
Propagating from a culture in Phase 6 does not assure vitality and and may lead to failures in an industrial production as well as in a home baker's environment.
In particular, in this phase, the growth of microorganims is inhibited by several factors, waste product accumulation and nutrient depletion. Fermentation products of LB and yeasts are (amongst others) acids and alcohols. Typically at a low pH around or below pH 3.6 (dependent on LB strain) activity stops.