Omid, thank you for your detailed response. Apparently fermentation is lot more complex than I thought. Where does fermentation by natural sourdough starter fit in this scheme, if you don't mind? Thnk you
You asked, "Where does fermentation by natural sourdough starter fit in this scheme?" I do not know, but I can make some conjectures. If I understand your concern properly, this is one area that I usually struggle with. So, I do not know how reliable my answer is going to be. For me, the fermentative properties of sourdough cultures are usually more difficult to pin down and comprehend than the fermentative properties of Saccharomyces cerevisiae
(the bakerís yeast). From my perspective, when I use bakerís yeast, I already know the microorganism that I am dealing with (namely, S. cerevisiae
). In addition, I know its general behavior and metabolic requirements. On the other hand, when I use a sourdough culture, I do not know, with specificity, the fermentative agents that I am dealing with. Moreover, if it is a new culture, it takes me some time to observe and comprehend the cultureís general behavior under various circumstances.
If I am not mistaken, bakerís yeast is capable of only one type of fermentation, namely, the alcoholic or ethanol fermentation, which is basically a three-step process:
1) Glycolysis (conversation of glucose to pyruvate),
2) Decarboxylation (conversion of pyruvate to carbon dioxide and acetaldehyde via enzyme pyruvate decarboxylase),
3) Dehydrogenation (conversion of acetaldehyde to ethanol via enzyme alcohol dehydrogenase.)
Generally, when you utilize any form of bakerís yeast (dry yeast, cake/fresh yeast, liquid yeast), you are basically using one type of fermentative microorganism to inoculate your dough with. On the other hand, when you employ a sourdough culture as your inoculum, you are using more than one fermentative microorganismóin some cases as many as eighteen (18) different microorganisms
. And, keep in mind that the types of sugars required by different microorganisms vary by species and strains within species, although some may have the same sugar requirements. (To the best of my knowledge, bacteria and yeast can only catalyze the sugars that their DNAs code for. For instance, while bakerís yeast can ferment glucose, they are unable to ferment lactose, simply because the yeast lack the proper enzyme, lactase, to break down the lactose molecules. Enzymes are proteins and are coded for by genes. The bakerís yeast does not have the genes that code for lactase enzyme.)
Therefore, in understanding the fermentative properties of sourdough cultures, you are most likely dealing with:
1) Various types of glycolysis:
a. Embden-Meyerhof pathway (the classic glycolysis, used by yeasts and many bacteria)
b. Pentose phosphate pathway aka pentose phosphoketolase pathway (used by heterolactic acid bacteria)
c. Entner-Doudoroff pathway (used by some bacteria)
2) Various types of fermentation:
a. Alcoholic fermentation (used by yeast and at least one known bacteria, producing ethanol and carbon dioxide)
b. Lactic Acid fermentation
1. Homolactic fermentation (used by homolactic bacteria, producing lactic acid and no carbon dioxide)
2. Heterolactic fermentation (used by heterofermentative bacteria, producing lactic acid, ethanol, and carbon dioxide)
3. Homo-hetero-lactic fermentation (used by facultative heterolactic bacteria, able to switch between homo- & hetero-lactic fermentation)
In comparison, in understanding the fermentative properties of S. cerevisiae
, you are essentially dealing with:
1) Embden-Meyerhof pathway (the classic glycolysis), and
2) Alcoholic fermentation (products of which are ethanol and carbon dioxide)
These primordial microbes, the bacteria, as opposed to yeasts, show a great deal of diversity in their metabolisms and fermentative capabilities, making us non-specialists really toil at understanding them.
It is said that one distinctive feature of sourdough microorganisms is their ability to symbiotically (non-competitively) ferment sugars
. In principle, a sourdough culture is said to be composed of two types of microorganisms: lactic acid bacteria (for example, a species of Lactobacillus
) and wild yeast (for example, a suitable species of Candida
). Both microorganisms symbiotically metabolize the sugars in dough. For instance, while the bacteria concentrate on metabolizing certain types of saccharides, the wild yeast focus on digesting certain other types of saccharidesówithout competing with the bacteria over the resources
. The bacteria ferment specific sugars to lactic acid if they are homolactic fermenters; the bacteria ferment specific sugars to lactic acid, ethanol, and carbon dioxide if they are heterolactic fermenters; and/or the bacteria switch between homolactic and heterolactic fermentation if they are facultative heterofermentative bacteria. Meanwhile, the wild yeast, akin to S. cerevisiae
, ferment their prescribed sugars to ethanol and carbon dioxide.
As an example, let us briefly take a look at the well-known San Francisco sourdough culture. Basically, the culture is reported to contain two principal types of microorganisms: (1) heterolactic acid bacteria known as Lactobacillus sanfranciscensis
and (2) wild yeast known as Candida humilis
. Generally, the bacteria-yeast ratio is reported to be about 100:1. The bacteria normally outnumber the yeast cells, as it is the case with other sourdough cultures. L. sanfranciscensis
, which are obligate heterofermentative bacteria, require maltose (a di-saccharide composed of two glucose molecules) and glucose as the principal fermentable sugars. While the wild yeast are unable to metabolize maltose, they are able to metabolize glucose. The bacteria ferment the sugars via the pentose phosphate pathway (or the heterofermentative pathway), producing lactic acid, ethanol, and carbon dioxide. On the other hand, the yeast ferment the sugars via the Embden-Meyerhof pathway and alcoholic fermentation, producing ethanol and carbon dioxide. The sugar requirements of the bacteria and yeast and their use of different fermentative pathways do not promote them to compete over the resources, but create conditions under which they can coexist symbiotically. The bacteria cleave one maltose molecule into two molecules: glucose and glucose-1-phosphate. Next, the glucose is released into the dough and the glucose-1-phosphate is reduced to glucose-6-phosphate, which then goes through the pathway of heterofermentation to produce lactic acid, ethanol, and carbon dioxide. Meanwhile, the glucose molecules released into the dough can be ingested by the yeast in order to be fermented into ethanol and carbon dioxide. This is a very rough description and by no means accurate. In addition, it does not account for formation of acetic acid.
If anyone has a description or model of symbiotic fermentation by sourdough microflora, please share. Good day!