Sourdough - Biology and Chemistry of Sourdough

Biology and Chemistry of Sourdough

See also: Lactic acid fermentation

A sourdough is a stable symbiotic culture of lactic acid bacteria (LAB) and yeast in a mixture of flour and water. Typically, the LAB metabolises sugars that the yeast cannot metabolise and the yeast metabolises the products of the LAB fermentation. Broadly speaking, the yeast produces the gas that leavens the dough and the LAB produces lactic acid, which contributes flavor.

The yeasts Candida milleri or Saccharomyces exiguus usually populate sourdough cultures symbiotically with Lactobacillus sanfranciscensis. The perfect yeast S. exiguus is related to the imperfect yeasts C. milleri and C. holmii; while Torulopsis holmii, Torula holmii, and S. rosei are synonyms used more frequently prior to 1978. C. milleri and C. holmii are physiologically similar, but DNA testing established them as distinct. Other yeasts reported found include C. humilis, C. krusei, Pichia anomaola, C. peliculosa, P. membranifaciens, and C. valida.

L. sanfranciscensis prefers to consume maltose, while C. milleri is maltase negative.

There have been changes in the taxonomy of yeasts in recent decades. Lactobacillus species' phylogenetic groupings have also been undergoing reclassification, first being studied in 1991 by Collins, et al. In 1995, Hammes and Vogel phylogenetically grouped L. sanfranciscensis to L. casei-Pediococcus. In 2003, Hammes and Hertel grouped it to L. buchneri. In 2007, Dellaglio and Felis grouped it to L. fructivorans.

LAB are anaerobic, which means they can multiply in the absence of oxygen. Hammes and Vogel in 1995 distinguished three metabolic groups of LAB:

  • Group A. Obligately homofermentative. They metabolise hexoses via the Embden–Meyerhof–Parnas (EMP) pathway to produce two molecules of lactic acid (C3H6O3), (>85%) but no carbon dioxide (CO2). They cannot tolerate oxygen. "They grow at 45 °C but not at 15 °C." "They are represented by L. delbrueckii and L. acidophilus."
  • Group B. Facultatively heterofermentative. They metabolise hexoses to lactic acid, and pentoses to lactic and acetic acids. They can use oxygen and will "produce more oxidized fermentations (e.g. acetate) if O2 is present." They "grow at 15 °C and show variable growth at 45 °C." They are "represented by L. casei and L. plantarum."
  • Group C. Obligately heterofermentative. They metabolise hexoses via the EMP pathway to produce lactic acid, acetic acid, and CO2; and pentoses via the phosphogluconate pathway to lactic and acetic acids. They are represented by L. fermentum, L. brevis, L. kefiri, and L. sanfranciscensis.

Lactobacillus sanfranciscensis was named for its discovery in San Francisco sourdough starters, although it is not endemic to San Francisco. In general, San Francisco sourdough is the same as a Type I sourdough. Type I sourdoughs have a pH range of 3.8 to 4.5 and are fermented in a room-temperature range of 20 to 30 °C (68 to 86 °F); Saccharomyces exiguus leavens the dough, Lactobacillus sanfranciscensis and L. pontis highlight a lactic-acid bacterial flora that includes L. fermentum, L. fructivorans, L. brevis, and L. paralimentarius. In Type II sourdoughs Saccharomyces cerevisiae is added to leaven the dough, L. pontis and L. panis highlight the flora. These sourdoughs have a pH less than 3.5 and are fermented within a temperature range of 30 to 50 °C (86 to 122 °F) for several days without feedings, which reduces the flora's activity. This process was adopted by some in industry, in part, due to simplification of the multiple-step build typical of Type I traditional sourdoughs.

Dutch wheat sourdough investigations found that, even though S. cerevisiae exerted infection pressure on sourdough's microbial ecosystem, it had died off after two refreshment cycles. Continuously maintained, stable sourdough cannot be unintentionally contaminated by S. cerevisiae. 4% salt inhibits L. sanfranciscensis, while C. milleri can withstand 8%.

A Belgian study of wheat and spelt doughs refreshed once every 24 hours and fermented at 30 °C (86 °F) in a laboratory environment provides insight into the three-phase evolution of first-generation-to-stable sourdough ecosystems. In the first two days of refreshment, atypical genera Enterococcus and Lactococcus bacteria highlighted the doughs. During days 2-5, sourdough-specific bacteria belonging to the genera Lactobacillus, Pediococcus, and Weissella outcompete earlier strains. Yeasts grew more slowly and reached population peaks near days 4-5. By days 5-7, "well-adapted" Lactobacillus strains such as L. fermentum and L. plantarum had emerged. At their peaks, yeast populations were in the range of about 1-10% of the lactobacilli populations or 1:10-1:100. One characteristic of a stable dough is the heterofermentative have outcompeted homofermentative lactobacilli.

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