Yeast is a single-celled organism classified in the kingdom Fungi, and requires moisture, oxygen, food, and appropriate temperatures in order to survive. Under these suitable conditions, the yeast will reproduce and generate alcoholic fermentation. During fermentation, yeast and bacteria consume sugars, and the resulting products are alcohol and carbon dioxide.
Generally speaking, bread dough is an ideal environment for the yeast, providing all the necessary conditions for its needs.
- Water is needed by the yeast, since the semipermeable yeast cell wall can only absorb small molecule nutrients in a dissolved state. It is well known by bakers that salt retards the activity of yeast fermentation, due to osmotic pressure exerted on the yeast cells by the salt. Salt, being hygroscopic (attracting moisture), draws water out of the yeast cell, reducing the amount available to the yeast, and this is why there is a decrease in fermentation from the presence of salt. Sugar acts the same way. It too is hygroscopic, and once a dough contains more than 10% sugar, a decrease is noted in the rate of fermentation.
- Oxygen, obtained mostly by the mixing of the dough, enables the yeast to metabolize nutrients and to multiply. Although yeast requires oxygen for its reproduction, in reality there is almost no reproduction occurring in bread dough, and the rise we see is almost entirely due to gas production during fermentation. Available oxygen is used up within a matter of minutes after dough mixing, and fermentation occurs in an anaerobic environment.
- Dough temperature is crucial for yeast activity. For commercial yeast, the optimum temperature for fermentation is 86° to 95°F or even higher. It is important to note, however, that a dough temperature in this range is inappropriate; although fermentation would be favored, it would occur at the expense of flavor development, which requires lower temperatures. Wild yeasts prefer a narrower temperature zone than commercial yeast, and in general perform better at slightly lower temperatures than commercial yeast.
During fermentation, food is provided to the yeast by the conversion of starches (by amylase enzymes) into sugar. The yeast ferments the sugar, and as a result of this fermentation, carbon dioxide gas and alcohol are produced. The CO2 is trapped by the gluten network in the dough, and provides volume to the baked loaf. The alcohol is largely evaporated during the baking of the bread. Another by-product of fermentation is heat.
A symbiotic relationship
There is an interesting relationship in what we call San Francisco Sourdough between the wild yeast, Candida milleri, and the dominant lactobacillus strain, Lactobacillus sanfranciscensis. C. milleri cannot utilize maltose during fermentation, while Lb. sanfranciscensis is happy to use it. And once it does, it excretes glucose. This is fortunate for C. milleri, because it is fond of glucose, and ferments this simple sugar readily. At the same time, competing bacterial species are inhibited by the presence of so much glucose, and this is to the benefit of Lb. sanfranciscensis, whose development is therefore favored. A last factor in this relationship pertains to acidity. Lb. sanfranciscensis produces a lot of acetic acid, which contributes significantly to the flavor we associate with sourdough bread. C. milleri is more tolerant of an acidic environment than many yeast varieties. The high level of acidity prevents competing yeasts from dominating the culture, much to the benefit of C. milleri.