Factors Affecting Yeast Fermentation

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Published 2003

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The rate of yeast fermentation is affected by several important factors. Fast fermentation is desirable when time is a constraint. Slower fermentation is desirable for developing flavor and for maturing dough. Bakers often adjust one or more of the following factors to optimize the rate of fermentation.

Temperature of dough. Yeast is dormant at 32°–34°F (0°–1°C) and begins to be quite active starting at about 50°F (10°C). As dough temperature rises above this, the rate of yeast fermentation increases. By about 120°F (50°C), fermentation slows, because yeast cells begin to die. Fermentation essentially stops at 140°F (60°C), when most yeast cells are dead. (These temperatures are estimates only; actual temperatures depend on the dough formula and on the strain of yeast in question.) Optimum fermentation is often given as 78°–82°F (25°–28°C). At lower temperatures (60°F/15°C or below), fermentation of bacteria, not yeast, is often favored. Since bacteria produce more acids when they ferment, dough that is retarded (refrigerated) can develop a strong sour taste. At higher temperatures than optimal (85°–100°F/30°–38°C), fermentation is fast, so bread dough rises quickly but the flavor is generally much less complex.
Amount of salt. Salt retards (inhibits) yeast and bacterial fermentation, with higher levels of salt slowing fermentation. While the typical amount of salt in yeast doughs falls within the narrow range of 1.8–2.2 percent (baker’s percentage), bakers can vary the amount of salt in a pre-ferment, making up the difference in the final mix. A pre-ferment contains yeast and a portion of other ingredients from the formula. It is fermented before the makeup of the final dough. For a short fermentation, the preferment is made with a low amount of salt or none at all; for a longer one, it contains more. High salt levels, in particular, limit the production of acids, because salt is especially effective at limiting bacterial fermentation.
Amount of sugar. Yeast typically consume about 3–5 percent (baker’s percentage) sugar in dough as they ferment. This means that up to about 5 percent, the more sugar added, the more yeast fermentation. Larger amounts of sugar—especially those above 10 percent—slow fermentation. For this reason, a common method for preparing rich, sweet doughs, which often contain 20 percent or more sugar, is to use a sponge or other pre-ferment. Because large amounts of sugar are not added to the sponge, yeast can ferment without inhibition.
Type of sugar. Sucrose, glucose, and fructose are all fermented rapidly; maltose is fermented slowly; lactose is not fermented at all. A mix of both fast- and slow-fermenting sugars is important with lean yeast doughs (those with low levels of added sugar). This provides for continued gassing through final proof.
The pH of dough. The optimum pH for yeast fermentation is an acidic 4 to 6. Above and below that pH, yeast fermentation slows. As yeast ferments, it produces acids that lower pH to this ideal range.
Presence of antimicrobial agents. Certain antimicrobial agents slow or stop yeast fermentation. For example, when calcium proprionate is added to commercial doughs to prevent mold growth in breads, it must be added properly so that it does not prevent yeast fermentation.
Presence of spices. Most spices, including cinnamon, have strong antimicrobial activity and can slow yeast fermentation. Instead of mixing cinnamon directly into dough, make cinnamon breads and rolls by sprinkling cinnamon and sugar onto dough, then roll and shape the dough like a jelly roll before baking.
Chlorine content in water. Chlorine is an antimicrobial agent, and high levels of chlorine in water can inhibit yeast fermentation. Most water supplies do not contain high levels of chlorine, however, so this is typically not an issue. Where chlorine content is high, though, water can be passed through a carbon filter to remove the chlorine. Or water can be allowed to sit out at room temperature overnight to allow chlorine to evaporate.
Addition of yeast foods. Ammonium salts such as ammonium chloride or ammonium phosphate are a source of nitrogen for growing yeast. Likewise, calcium salts such as calcium carbonate and calcium phosphate provide calcium for optimum yeast fermentation. Ammonium and calcium salts are added to many dough conditioners.
Amount of yeast. For the most part, the more yeast, the faster the fermentation. However, a large amount of yeast can add an undesirable yeasty flavor. A large amount of yeast can also exhaust dough—especially a lean dough—of sugars needed for fermentation during final proof and oven spring. That is why it is best to use a smaller amount of yeast when using a long fermentation time. A good starting point is 2 percent or less of yeast (baker’s percentage), although some bread formulas call for up to 6 percent yeast.
Type of yeast. Some yeast products sold to bakers contain fast-fermenting yeast, good for no-time doughs. This is particularly true of instant yeast, discussed in a following section. Quick-fermenting yeasts are not as desirable, however, when a long fermentation time is used, since there might not be enough yeast activity to survive through final proof.