21. NATURE AND ACTION OF YEAST.--How yeast came to be discovered is not
definitely known, but its discovery is believed to have been purely
accidental. Some mixture of flour and liquid was probably allowed to
remain exposed to the air until it fermented and then when baked was
found to be light and porous. Whatever the origin of this discovery was,
it is certain that yeast was used hundreds of years ago and that its
action was not at that time understood. Even at the present time
everything concerning the action of yeast is not known; still continued
study and observation have brought to light enough information to show
that yeast is the agency that, under favorable conditions, produces
light, spongy bread out of a flour mixture.
22. It has been determined that yeast is a microscopic plant existing
everywhere in the air and in dust; consequently, it is found on all
things that are exposed to air or dust. In order that it may grow, this
plant requires the three things necessary for the growth of any plant,
namely, food, moisture, and warmth. Carbohydrate in the form of sugar
proves to be an ideal food for yeast, and 70 to 90 degrees Fahrenheit is
the temperature at which the most rapid growth occurs. When these
conditions exist and a sufficient amount of moisture is provided, yeast
grows very rapidly and produces fermentation.
The changes that take place when yeast causes fermentation can be
detected very readily by observing the fermenting of fruit juice. As
every housewife knows, the first indication of a ferment in fruit juice
is the appearance of tiny bubbles, which collect on the sides and the
bottom of the vessel containing the fruit and then gradually rise to the
top. These bubbles are a form of gas called _carbon-dioxide_, or
_carbonic-acid, gas_. If, after they appear, the juice is tasted, it
will be found to be slightly alcoholic and to have a somewhat sour or
acid taste. The gas, the acid, and the alcohol thus produced are the
three results of the action of the ferment.
23. When yeast is used in the making of bread out of wheat flour, the
changes just mentioned take place. To understand the action of this
plant, it will be necessary to remember that wheat contains a large
proportion of starch. This substance, however, cannot be acted on by the
yeast plant; it must first be changed into sugar. The yeast that is
added to the flour changes some of the starch into sugar and transforms
the sugar into alcohol and carbonic-acid gas. This gas, which is lighter
than the dough, rises, and in its efforts to escape expands the elastic,
glutinous dough into a mass of bubbles with thin walls until the dough
is two or three times its original bulk. The yeast plants, though, must
be well distributed throughout the dough; otherwise, there are likely to
be no bubbles in some places and large bubbles with thick walls in
others. The gas thus formed is prevented from escaping by the toughness
or the elasticity of the gluten, and the spaces that it leaves are what
produce a light, porous loaf. When the expansion has gone on long
enough, the formation of gas is checked and the ferment is killed by
baking the dough in a hot oven. During the baking, the alcohol is driven
off by heat, some of the starch is browned and forms the crust, and so
little acid is produced in the short time in which the yeast is active
that it is not noticeable.
24. Commercial Yeast.--When yeast plants are deprived of water and food,
they cease to multiply. However, under these conditions, they may be
kept alive so that when water and food are again provided they will
increase in number and carry on their work. Advantage has been taken of
these characteristics of yeast, for although at one time the making of
yeast was entirely a household process, it has now, like butter, cheese,
canned fruit, etc., become a commercial product. The first yeast put on
the market was collected from the surface of the contents of brewers'
vats, where it floated in large quantities; but as this was an impure,
unreliable product composed of various kinds of bacteria, it is no
longer used for the purpose of making bread. At present, yeast is
carefully grown as a pure yeast culture, or product. It is marketed in
such a way that when proper food, such as soft dough, or sponge, and a
favorable temperature are provided, the plants will multiply and act on
the carbohydrate that they find in the food. In fact, the purpose of the
well-known process of "setting" a sponge is to obtain a large number of
yeast plants from a few.
Commercial yeast is placed on the market in two forms--_moist_ and
_dry_. Each of these yeasts has its advantages, so that the one to
select depends on the method preferred for the making of bread as well
as the time that may be devoted to the preparation of this food.
25. Moist yeast, which is usually called _compressed yeast_, consists
of the pure yeast culture, or growth, mixed with starch to make a sort
of dough and then compressed into small cakes, the form in which it is
sold. The moist condition of this kind of commercial yeast keeps the
plants in an active state and permits of very rapid growth in a dough
mixture. Consequently, it proves very useful for the rapid methods of
making bread. It is soft, yet brittle, is of a grayish-white color, and
has no odor except that of yeast.
Since the plants of compressed yeast require very little moisture to
make them grow, an unfavorable, or low, temperature is needed to keep
the yeast from spoiling; in fact, it is not guaranteed to remain good
longer than a few days, and then only if it is kept at a temperature low
enough to prevent the plants from growing. This fact makes it
inadvisable to purchase compressed yeast at great distances from the
source of supply, although it may be obtained by parcel post from
manufacturers or dealers.
26. Dry yeast, the other form of commercial yeast, is made in much the
same way as moist yeast, but, instead of being mixed with a small amount
of starch, the yeast culture is combined with a large quantity of starch
or meal and then dried. The process of drying kills off some of the
plants and renders the remainder inactive; because of this, the yeast
requires no special care and will keep for an indefinite period of time,
facts that account for its extensive use by housewives who are not
within easy reach of the markets. However, because of the inactivity of
the yeast plants, much longer time is required to produce fermentation
in a bread mixture containing dry yeast than in one in which moist yeast
is used. Consequently, the long processes of bread making are brought
about by the use of dry yeast. If moist yeast is used for these
processes, a smaller quantity is required.
27. Liquid Yeast.--Some housewives are so situated that they find it
difficult to obtain commercial yeast in either of its forms; but this
disadvantage need not deprive them of the means of making good home-made
bread, for they can prepare a very satisfactory liquid yeast themselves.
To make such yeast, flour, water, and a small quantity of sugar are
stirred together, and the mixture is then allowed to remain at ordinary
room temperature, or 70 degrees Fahrenheit, until it is filled with
bubbles. If hops are available, a few of them may be added. When such
yeast is added to a sponge mixture, it will lighten the whole amount.
Before the sponge is made stiff with flour, however, a little of it
should be taken out, put in a covered dish, and set away in a cool, dark
place for the next baking. If properly looked after in the manner
explained, this yeast may be kept for about 2 weeks.
More certain results and a better flavor are insured in the use of
liquid yeast if it is started with commercial yeast, so that whenever
this can be obtained it should be used. Then, as just explained, some of
the liquid containing the yeast or some of the sponge made with it may
be retained for the next baking.
28. Quality of Yeast.--Of equal importance with the quality of flour is
the quality of yeast used in the baking of bread. Yeast is, of course,
accountable for the lightness or sponginess of bread, but, in addition,
it improves the flavor of the bread if it is of good quality or detracts
from the flavor if it is of poor quality. Since the condition of yeast
cannot be determined until its effect on the finished product is noted,
the housewife should take no chances, but should employ only yeast,
whether she uses commercial or liquid, that she knows to be good and
reliable. Compressed yeast may be easily judged as to quality. It should
be grayish white in color, without streaks or spots, and it should have
no sour nor disagreeable odor. If home-made yeast is used and the
results obtained are not satisfactory, it may be taken for granted that
a fresh supply should be prepared.