There, again is the basic equation that defines the fermentative process. In most fermenting systems there are two (desired) principle organisms involved: yeast and lactic acid bacteria. As stated earlier, fermentation is the process whereby these organisms consume a food source (simple sugars) to produce the necessary energy to support themselves long enough to reproduce (the only true “goal” of these simple life forms).



Yeast are single celled fungi. Like all of the higher organisms yeasts are eukaryotes, meaning their genetic material is organized into a membrane bound nucleus. Yeast can exhibit both anaerobic (without oxygen) and aerobic (with oxygen) life cycles. Since bread doughs contain very little oxygen it is the anaerobic life-cycle that is of interest to the bread baker. (In beer making the early stages of fermentation find the yeasts in an aerated wort and thus exhibit their aerobic life-cycle. The yeast are able to rapidly multiply in this environment, however, no alcohol is produced. Once the oxygen is quickly used up the yeast switch to their anaerobic life-cycle and the production of alcohol begins).



Yeasts are fascinating creatures and are present in multiple forms within their population. They can exist in a haploid (one set of genetic material – think of human sperm or egg) form or a diploid (two sets of genetic material) form with the diploid form being the most common in any given population. The diploid cells reproduce through mitosis in a process called budding (see diagram below). This mitotic budding results in two identical daughter cells each with a full set of genetic material.




The haploid cells can “mate” with other haploid cells of the opposite mating type producing a diploid cell (again, think of a human sperm (a haploid cell) fusing with a human egg (haploid) to form a diploid embryo). And, under stressful situations, diploid cells can undergo meiosis through a process known as sporolation and produce 4 haploid spores.



The other creatures of importance for fermentation are the lactic acid bacteria (LAB) and several different species are present in any given fermenting environment. Not surprisingly, lactic acid is the principle by-product of LAB, but they also produce small amounts of CO2. Species of LAB can be either homofermentative or heterofermentative. Homofermentative forms have only a single terminal “waste” product: lactic acid, whereas heterofermentative LAB have more than one end product: lactic acid and acetic acid.



LAB function optimally in the range of 5.0 -5.5pH. This is, of course, ideal for bread production where the pH of unbleached flour is 5.8-6.0 and a fermenting dough is 5.7-4.7ph. The reason for the increase in acidity (falling pH value) of a fermenting dough is due to the excretion of lactic and acetic acid by the LAB and the acetic acid from the yeast.



In a fermenting system, a dough in our case, the yeasts and LAB will attain a stable balance of coexistence which is generally believed to result from an equilibrium reached through non-competition for scarce food resources. It is even thought they exhibit symbiosis (wherein the species are mutually beneficial to one another, not just coexisting).



In the early stages of a developing fermentative environment there is a wide range of species of yeast and bacteria. In these early stages the LAB are dominated by the homofermentative varieties which preferentially consume the simple sugars. Yeasts are only able to directly metabolize monosaccharides (simple sugars – sugars that are composed of a single unit): glucose and fructose. The sucrose (a di-saccharide [glucose+fructose]) is rapidly converted to glucose and fructose by the enzyme invertase found on the surface of the yeast cell’s membrane. It is due to this early competition for a scarce resource (mono saccharides) that leads to the increasing prevalence of the heterofermentative LAB that are able to metabolize a di-saccharide(a two unit sugar): maltose (glucose+glucose). With the introduction of the heterofermentative LAB comes the increasing concentration of acetic acid (remember they can produce both lactic AND acetic acid).



Selection for yeast species in the final system is determined by a yeast species sensitivity and resistance to acetic acid. One of the great myths in sourdough production is that you can have your sourdoughs contaminated by bakers yeast (Sacc. cerevisiae). It has been shown that due to Sacc cerevisiae`s very low tolerance for acetic acid it will be completely eliminated from a starter within two feedings. This indicates that mature sourdough cultures are quite resistant to fungal and bacterial contamination.



Once the population of yeast and LAB has matured into a well developed and complex colony they become remarkable resistant to invasions from other yeast, fungal and bacterial species. Being a closed system (ie, no fresh influx of food sources) they population can rapidly change and deteriorate should the conditions change dramatically. This can include temperature changes, or changes to the viscosity of the solution owing to an increase or decrease in water content. Also, to maintain a healty culture, the culture must be regularly “fed” with fresh flour and water to ensure an amply supply of fermentable sugars but also, so the yeast and bacteria do not become “poisoned” by their own waste.



In the next section we will focus on the most common food source for fermenting cultures that are used to leaven bread: flour.