Sourdough definition and references
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What is Sourdough?

Sourdough is dough which has microorganisms (for example lactobacillus or yeast) from sourdough or sourdough starters, which are active or can be reactivited. With the addition of grain products and water, they are capable 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)

Another classification defines Sourdough Types I - III* [1]

Type 0*

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.

Type I

Sourdoughs grown at ambient temperatures (20 - 30 C, 68 - 86 F) and continous propagation (i. e.) a small amount of dough is taken from the current batch and used for the next batch. This method is used for thousands of years all over the world to make bread. The microorganisms found in this environment are heterofermentative, mainly lactobacillus sanfranciscensis and a dominating yeast like candida milleri.

Type II

Sourdoughs grown with continuous propagation in an industrialized environment at higher temperatures (like 40 - 50 C, 100 - 120 F), higher 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.

Type III

Sourdoughs initiated from artificially composed dried sourdoughs selected for their tolerance to drying.

*) I added Type 0 which seemed appropriate

Names in use for sourdoughs in various stages 

English German French Italian Text
storage leaven Anstellgut Chef Madre Sourdough one stores to be used later (can be dry material)
natural leaven Natursauer Chef Madre Sourdough one grows new from water and grain
first leaven sponge Anfrischsauer levain premiere 1. Girata Sourdough one is using to grow to more volume
second leaven sponge Grundsauer levain seconde 2. Girata Sourdough one is using to grow to more volume
ripe leaven sponge Vollsauer levain tout levien Impasto acidico maturo Sourdough one is using to make the final Sourdough to be baked

Sourdough Microbiology

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[2] in 2000, Lactobacillus mindensis [3] 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[4]. Later research indicated[5], 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[6], Italy[7], USA[8]) 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.

Microbiological Growth

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.

Growth Curve

Growth Curve Phases [9]

  1. Lag Phase: The organisms need to adjust to the new environment before growth can resume. This depends on the vitality of the inoculation. The time can be significant with dried cultures, which need to rehydrate. In this phase, the greatest danger exists that a spontanious flora from the organisms present in the flour will be able to alter the existing flora.
  2. Acceleration Phase: The organisms are now adjusted to the new environment and start to multiply. The end of this phase is reached when the highest multiplication rate is reached.
  3. Exponential growth Phase: The organisms are at their optimum growth rate, repeatedly doubling their numbers by binary fission. In general, the doubling time can vary from 20 minutes to several days, for sourdough organisms it is in the hour range.
  4. Transition Phase: The growth rate decreases from the optimal rate. Possible reasons are exhaustion of nurtients, waste products accumulating i. e. acidity increase.
  5. Stationary Phase: The number or newly generated and dying organisms is equal and it will not increase any further.
  6. Death Phase: Initially, the number of dying organisms exceeds the number of newly generated. Later on, no new organisms are generated. Existing organisms die in increasing numbers for various reasons as mentioned in Phase 4.

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[10]. 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.

Suggested rye flour contents in starter for rye bread, dependent on acid contents and pH of starter (ripe leaven sponge)*

The values are necessary to keep the pH below 4.5 to suppress amylase acivity.
acid contents of starter 9 12 15 18 21 24 27 30
pH value of starter 4.2 4.1 4.0 3.9 3.8 3.7 3.6 3.5
suggested rye flour contents in percent 50 45 40 35 30 25 20 15
*(information translated and adopted under fair use copyright from "Technologie der Backwarenherstellung" Schuenemann/Treu, 1999)
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