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Requirements of Yeast
Appears in
Published 2004
As noted, yeast requires moisture, oxygen, a suitable temperature, and food in order to reproduce and generate fermentation. Generally speaking, bread dough is an ideal environment for yeast, providing all the necessary conditions for its needs.
- Moisture. Once water is added to the other ingredients in bread dough, the metabolic activity of the yeast commences. The cell membrane of yeast is semipermeable. Oxygen and nutrients are absorbed through the cell membrane, and enzymes and other substances are given off to the environment. Yeast can only absorb nutrients in a dissolved state, and it requires water in order to absorb these nutrients. Further, yeast can only absorb small-molecule nutrients, such as simple sugars, through the cell membrane, and it releases enzymes in order to break down large-molecule nutrients in the dough. Salt retards the activity of yeast fermentation due to the osmotic pressure it exerts on yeast cells. Salt, being hygroscopic (attracting moisture), draws water out of the semipermeable yeast cell, reducing the amount available to the yeast, and this is why there is a decrease in fermentation due to the presence of salt.
- Oxygen. Oxygen is obtained mostly by the mixing of the dough, enabling the yeast to metabolize nutrients and to reproduce. However, although yeast requires oxygen for its reproduction, there is virtually no reproduction of yeast occurring in bread dough, and the rise we see is almost entirely due to gas production during fermentation. It takes several hours for yeast to begin its reproductive cycle, and there is insufficient time between mixing and baking for reproduction to begin. Available oxygen is used up within a matter of minutes after dough mixing, and fermentation occurs in an anaerobic environment. There is an exception to this: When bread is made using a preferment, there is enough time during the maturing of the pre-ferment for yeast reproduction to occur. When yeast reproduction is necessary, as in a yeast manufacturing plant, oxygen is extremely important, and the machines that provide oxygen to the yeast can be among the most expensive ones in the facility.
- Temperature. Correct dough temperature is crucial for yeast activity. For commercial yeast, the optimum temperature range for fermentation is between 86° and 95°F, but it is important to note that dough temperatures in this range are inappropriate; fermentation would be favored at these high temperatures, but it would occur at the expense of flavor development, which requires lower temperatures. (Wild yeasts, such as those in a sourdough culture, prefer a narrower temperature zone than commercial yeast, and in general perform better at slightly lower temperatures than commercial yeast.) As temperatures get higher or lower, yeast activity is reduced. Between 32° and 50°F and between 116° and 131°F there is very little activity. At about 138° to 140°F, yeast reaches what is known as “thermal death point” and dies. Bakers often ask about freezing fresh yeast. When frozen, yeast cells (whether in a packet of compressed yeast, a frozen raw dough, or a sourdough culture) begin to die within a matter of days. Although fresh yeast can survive for a few weeks at temperatures as low as –4°F, it gradually loses its fermentation ability. Dry yeast, being dehydrated, is less affected by freezer temperatures and can be safely frozen for several months.
- Food. Food is provided to yeast during fermentation by the conversion of starches into sugar. Yeast cannot directly ferment starch and requires the amylase enzymes naturally present in flour, and also added at the mill or in the bakeshop, to convert the starches into fermentable sugars (see the discussion on malt). The small amount of sugar naturally present in flour is initially metabolized for fermentation. After this has been converted and consumed by the yeast, the amylases utilize the damaged starch particles. It is these damaged starch particles that provide almost the entire food source for the yeast’s fermentation. Whole, undamaged starch particles remain intact in the dough until it reaches the oven, at which time they become available to the amylases. The carbon dioxide that is a by-product of fermentation is trapped by the gluten network in the dough, and provides volume to the baked loaf. The alcohol that is produced during fermentation is largely evaporated during the baking of the bread; what alcohol does remain contributes to aroma and flavor. One other byproduct of fermentation is heat.
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