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Kinds of Yeast
Yeast is a living organism: a
one-celled fungus. Various strains of yeast are present virtually
everywhere. Yeast feeds on carbohydrates, converting them to carbon
dioxide and alcohol in an organic process known as fermentation:
Yeast + Carbohydrates = Alcohol + Carbon Dioxide
When yeast releases carbon dioxide gas during bread making, the gas
becomes trapped in the dough's gluten network. The trapped gas leavens
the bread, providing the desired rise and texture. The small amount of
alcohol produced by fermentation evaporates during baking. As with most
living things, yeast is very sensitive to temperature. It prefers
temperatures between 90˚F and 110˚F (32˚C-43˚C). At temperatures below
34˚F (2˚C), it becomes dormant; above 138˚F (59˚C), it dies. Salt is
used in bread making because it conditions gluten, making it stronger
and more elastic. Salt also affects yeast fermentation. Because salt
inhibits the growth of yeast, it helps control the dough's rise. Too
little salt and not only will the bread taste bland, it will rise too
rapidly. Too much salt, however, and the yeast will be destroyed. By
learning to control the amount of food for the yeast and the
temperatures of fermentation, you can learn to control the texture of
your yeast-leavened products. Types of Yeast Baker's yeast is available
in two forms: compressed and active dry. (You may also encounter a
product called brewer's yeast; it is a nutritional supplement with no
leavening ability.)
Compressed Yeast
Compressed yeast is a mixture of yeast and starch with moisture content
of approximately 70%. Also referred to as fresh yeast, compressed yeast
must be kept refrigerated. It should be creamy white and crumbly with a
fresh, yeasty smell. Do not use compressed yeast that has developed a
sour odor, brown color or slimy film. Compressed yeast is available in
0.6-ounce (17-gram) cubes and 1-pound (450-gram) blocks. Under proper
storage conditions, compressed yeast has a shelf life of 2-3 weeks.
Active Dry Yeast
Active dry yeast differs from compressed yeast in that virtually all the
moisture has been removed by hot air. The absence of moisture renders
the organism dormant and allows the yeast to be stored without
refrigeration for several months. When preparing dough, dry yeast is
generally rehydrated in a lukewarm (approximately 100˚F (43˚C)) liquid
before being added to the other ingredients. Dry yeast is available in
1/4-ounce (7-gram) packages and 1- or 2-pound (450-gram or 1-kilogram)
vacuum-sealed bags. It should be stored in a cool, dry place and
refrigerated after opening.
Dry Instant Yeast
or quick-rise dry yeast is also available. It must be blended with the
dry ingredients in a bread formula, then activated with hot
(approximately 125˚F-130˚F [52˚C-54˚Cl) water. It dramatically speeds
the rising process. Instant yeast can be substituted measure for measure
for regular dry yeast. Instant yeast is still a living organism and will
be destroyed at temperatures above 138˚F (59˚C). The flavors of dry and
compressed yeasts are virtually indistinguishable, but dry yeast is
approximately twice as strong. Because too much yeast can ruin bread,
always remember to halve the specified weight of compressed yeast when
substituting dry yeast in a formula. Likewise, if a formula specifies
dry yeast, double the amount when substituting compressed yeast. All the
formulas in this text requiring yeast use regular active dry yeast.
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