Baker's yeast is the common name for the strains of yeast commonly used as a
leavening agent in baking bread and bakery products, where it converts the
fermentable sugars present in the dough into carbon dioxide and ethanol. Baker's
yeast is of the species Saccharomyces cerevisiae, which is the same species
commonly used in alcoholic fermentation, which is called brewer's yeast. Baker's
yeast is also a single-cell microorganism found on and around the
human body. The use of steamed or boiled potatoes,
water from potato boiling, or sugar in a bread dough provides food for the growth
of yeasts; however, too much sugar will dehydrate them. Yeast growth is
inhibited by both salt and sugar, but more so with salt than sugar. Fats, such
as butter or eggs, slow down yeast growth; however, others say the effect
of fat on dough remains unclear, presenting evidence that small amounts
of fat are beneficial for baked bread volume.
Saccharomyces exiguus is a wild yeast found on plants, fruits, and grains that
is occasionally used for baking; however, in general it is not used in a
pure form but comes from being propagated in a sourdough starter.
History It is not known when yeast was first
used to bake bread; the earliest definite records come from Ancient
Egypt. Researchers speculate that a mixture of flour meal and water was left
longer than usual on a warm day and the yeasts that occur in natural
contaminants of the flour caused it to ferment before baking. The resulting
bread would have been lighter and tastier than the previous hard
flatbreads. It is generally assumed that the earliest forms of leavening were
likely very similar to modern sourdough; the leavening action of yeast would have
been discovered from its action on flatbread doughs, and would have been
either cultivated separately or transferred from batch to batch by means
of previously mixed dough. Also, the development of leavened bread seems to
have developed in close proximity to the development of beer brewing, and barm
from the beer fermentation process can also be used in bread making.
Without an understanding of microbiology, early bakers would have
had little ability to directly control yeast cultures, but still kept locally
interesting cultures by reusing doughs and starters to leaven later batches.
However, it became possible to isolate and propagate favored yeast strains in
the same manner as was done in the beer industry, and it eventually became
practical to propagate yeast in a slurry with a composition similar to beer wort,
usually including malted barley and wheat flour. Such cultures would become
the ancestors of modern baker's yeast, as, in general, they were carefully
maintained to avoid what would later be discovered to be bacterial
contamination, including using preservatives such as hops as well as
boiling the growth medium. In the 19th century, bread bakers
obtained their yeast from beer brewers, and this led to sweet-fermented breads
such as the Imperial "Kaiser-Semmel" roll, which in general lacked the
sourness created by the acidification typical of Lactobacillus. However, beer
brewers slowly switched from top-fermenting to bottom-fermenting
yeast and this created a shortage of yeast for making bread, so the Vienna
Process was developed in 1846. While the innovation is often popularly credited
for using steam in baking ovens, leading to a different crust characteristic, it
is notable for including procedures for high milling of grains, cracking them
incrementally instead of mashing them with one pass; as well as better
processes for growing and harvesting top-fermenting yeasts, known as
press-yeast. Refinements in microbiology following
the work of Louis Pasteur led to more advanced methods of culturing pure
strains. In 1879, Great Britain introduced specialized growing vats for
the production of S. cerevisiae, and in the United States around the turn of the
century centrifuges were used for concentrating the yeast, making modern
commercial yeast possible, and turning yeast production into a major industrial
endeavor. The slurry yeast made by small bakers and grocery shops became cream
yeast, a suspension of live yeast cells in growth medium, and then compressed
yeast, the fresh cake yeast that became the standard leaven for bread bakers in
much of the Westernized world during the early 20th century.
During World War II, Fleischmann's developed a granulated active dry yeast
for the United States armed forces, which did not require refrigeration and
had a longer shelf-life and better temperature tolerance than fresh yeast;
it is still the standard yeast for US military recipes. The company created
yeast that would rise twice as fast, cutting down on baking time. Lesaffre
would later create instant yeast in the 1970s, which has gained considerable use
and market share at the expense of both fresh and dry yeast in their various
applications. Types of baker's yeast
Baker's yeast is available in a number of different forms, the main differences
being the moisture contents. Though each version has certain advantages over the
others, the choice of which form to use is largely a question of the
requirements of the recipe at hand and the training of the cook preparing it.
Dry yeast forms are good choices for longer-term storage, often lasting more
than a year at room temperatures without significant loss of viability. In
general, with occasional allowances for liquid content and temperature, the
different forms of commercial yeast are considered interchangeable.
Cream yeast is the closest form to the yeast slurries of the 19th century, in
essence being a suspension of yeast cells in liquid, siphoned off from the
growth medium. Its primary use is in industrial bakeries with special
high-volume dispensing and mixing equipment, and it is not readily
available to small bakeries or home cooks.
Compressed yeast is, in essence, cream yeast with most of the liquid removed.
It is a soft solid, beige in color, and best known in the consumer form as
small, foil-wrapped cubes of cake yeast. It is also available in larger-block
form for bulk usage. It is highly perishable; though formerly widely
available for the consumer market, it has become less common in supermarkets
in some countries due to its poor keeping properties, having been
superseded in some such markets by active dry and instant yeast. It is
still widely available for commercial use, and is somewhat more tolerant of
low temperatures than other forms of commercial yeast; however, even there,
instant yeast has made significant market inroads.
Active dry yeast is the form of yeast most commonly available to noncommercial
bakers in the United States. It consists of coarse oblong granules of yeast, with
live yeast cells encapsulated in a thick jacket of dry, dead cells with some
growth medium. Under most conditions, active dry yeast must first be proofed
or rehydrated. It can be stored at room temperature for a year, or frozen for
more than a decade, which means that it has better keeping qualities than other
forms, but it is generally considered more sensitive than other forms to
thermal shock when actually used in recipes.
Instant yeast appears similar to active dry yeast, but has smaller granules with
substantially higher percentages of live cells per comparable unit volumes. It is
more perishable than active dry yeast but also does not require rehydration,
and can usually be added directly to all but the driest doughs. In general,
instant yeast has a small amount of ascorbic acid added as a preservative.
Some producers provide two or more forms of instant yeast in their product
portfolio; for example, LeSaffre's "SAF Instant Gold" is designed specifically
for doughs with high sugar contents, and such yeasts are more generally known as
osmotolerant yeasts. Rapid-rise yeast is a variety of dried
yeast that is of a smaller granular size, thus it dissolves faster in dough,
and it provides greater carbon dioxide output to allow faster rising. There is
considerable debate as to the value of such a product; while most baking
experts believe it reduces the flavor potential of the finished product,
Cook's Illustrated magazine, among others, feels that, at least for
direct-rise recipes, it makes little difference. Rapid-rise yeast is often
marketed specifically for use in bread machines.
Deactivated yeast is dead yeast which has no leavening value and is not
interchangeable with other yeast types. Typically used for pizza and pan bread
doughs, it is used at a rate of 0.1% of the flour weight, though manufacturer
specifications may vary. It is a powerful reducing agent used to increase
the extensibility of a dough. For most commercial uses, yeast of any
form is packaged in bulk; however, yeast for home use is often packaged in
pre-measured doses, either small squares for compressed yeast or sealed packets
for dry or instant. For active dry and instant yeast, in general a single dose
is about 2.5 tsp or about 7 g, though comparatively lesser amounts are used
when the yeast is used in a pre-ferment. In general, a yeast flavor in the baked
bread is not noticeable when the bakers' percent of added yeast is less than
2.5%. Use in research
Because it is readily available and easy to culture, baker's yeast has long been
used in chemical, biological, and genetic research. In 1996, after 6 years
of work, S. cerevisiae became the first eukaryote to have its entire genome
sequenced. It has over 12 million base pairs and around 6000 genes. Since then,
it has remained in the forefront of genetic research. For example, most of
our knowledge of the cell division cycle was worked out from experiments with
yeast. Baker's yeast contains enzymes that can
reduce a carbonyl group into a hydroxyl group in fairly high yield, thus making
it a useful bio-reagent in chemical syntheses. It is known to reduce
organometallic carbonyl compounds in very high yield. Baker's yeast can also
be used to produce ethanol via fermentation for use in chemical
synthesis, although doing so in some places requires permits. Baker Yeast
also used in the bio-remediation of Arsenic from the ground water that
contains toxic Pollutants. Industrial production
The baking industry relies on industrial production of its ingredients, including
baking yeasts. Much effort has been put into developing and marketing yeasts
that will perform reliably in mass production. Since the end of the
nineteenth century, baker’s yeast has been produced by companies that
specialize in its production. In 2012 the companies that dominated the
market included: Lesaffre Group, AB Vista, DSM, GB Plange and AB Mauri,
producing hundreds of thousands of metric tons of yeast each year in
industrial plants. See also
Dough Notes
References