Yeast is obtained in a pure form for commercial purposes
from distilleries or breweries. The top yeast which floats
on the surface of the fermenting liquid, or wort, is skimmed
off and its impurities removed. It is then mixed with some
starch, pressed, cut into cakes, wrapped in tin foil, and sold
as " compressed " yeast. When mixed with corn meal and
dried, it is sold as " dry yeast." The- yeast cells of dry yeast
are not as active as those of compressed yeast and take longer
to raise bread.
Fermentation is a chemical change in an organic substance,
caused by the growth within that substance of some ferment,
as yeast. There are different varieties of ferments, each of
which causes a different form of fermentation to take place.
Alcoholic fermentation is caused by the growing of the
yeast plant, which feeds upon sugar, changing it to alcohol
and carbon dioxide. This is the fermentation that takes
place in bread making.
Acetic fermentation is caused by a ferment which changes
the alcohol formed during alcoholic fermentation into
acetic acid.
It occurs when alcoholic fermentation is allowed to go on
for too great a length of time, or when the mixture is raised
to a temperature above 90° F., as when hard cider turns to
vinegar or bread dough sours.
Lactic fermentation is the change that takes place when
milk sours. The lactic bacillus feeds upon the sugar of the
milk and changes it to lactic acid ; this acid causes the casein
to coagulate and the milk is " sour."
When milk is used for bread dough, it should be scalded
to destroy the lactic bacilli, thus preventing souring of the
milk during the rising, which might cause sour bread.
BREADS 137
Chemical reactions in bread making :
dextrose
starch water or sugar
C6Hio05 + a ferment + H20 = C6Hi206
carbon
Alcoholic fermentation: alcohol dioxide
C6H1206 + yeast = 2 C2H5OH + 2 C02
Acetic fermentation :
alcohol oxygen acetic acid water
2 C2H5OH + 4 0 + a ferment = 2 CH3COOH + 2 H20
Experiments with yeast.
EXPERIMENT 70. — Mix a small amount of yeast with water.
(a) Put a drop on a glass slide, cover with a glass, and examine
under the microscope, first with a low power and then with a high
power. Make a drawing.
(6) Stain the slide with a little iodine and examine again. What
color do the yeast cells become when they are stained with
iodine ? The starch cells ? What kind of starch was used in the
yeast cake ?
EXPERIMENT 71. — Make a solution of glucose and water, or
molasses and water. Add some yeast. Let stand an hour or two
in a warm place. Examine a drop under the microscope. How
does the yeast cell reproduce ?
EXPERIMENT 72. — Make a solution of glucose and water.
Add some yeast. Put equal amounts in three test tubes.
(a) Boil the contents of one tube.
(6) Put one tube in a cold place or on ice.
(c) Keep one tube at temperature of 80° F. At the end of an
hour or two, examine the contents of the tubes, noting any fermen-
tation that may have occurred. Draw conclusions as to the best
temperature for the growing of the yeast plant.
Warm (6) to a temperature of 80° F. and let stand for an hour
or two. Does cold destroy the life of the yeast? Does boiling
destroy it?
138 DOMESTIC SCIENCE
Page 59
Presented as published in 1913. Historical recipes may not meet modern food-safety standards. Cook from the modern interpretation, not the original instructions.
AI-modernized reading of the original text
Yeast is obtained in a pure form for commercial purposes from distilleries or breweries. The top yeast which floats on the surface of the fermenting liquid, or wort, is skimmed off and its impurities removed. It is then mixed with some starch, pressed, cut into cakes, wrapped in tin foil, and sold as " compressed " yeast. When mixed with corn meal and dried, it is sold as " dry yeast." The yeast cells of dry yeast are not as active as those of compressed yeast and take longer to raise bread.
Fermentation is a chemical change in an organic substance, caused by the growth within that substance of some ferment, as yeast. There are different varieties of ferments, each of which causes a different form of fermentation to take place.
Alcoholic fermentation is caused by the growing of the yeast plant, which feeds upon sugar, changing it to alcohol and carbon dioxide. This is the fermentation that takes place in bread making.
Acetic fermentation is caused by a ferment which changes the alcohol formed during alcoholic fermentation into acetic acid.
It occurs when alcoholic fermentation is allowed to go on for too great a length of time, or when the mixture is raised to a temperature above 90° F., as when hard cider turns to vinegar or bread dough sours.
Lactic fermentation is the change that takes place when milk sours. The lactic bacillus feeds upon the sugar of the milk and changes it to lactic acid ; this acid causes the casein to coagulate and the milk is " sour."
When milk is used for bread dough, it should be scalded to destroy the lactic bacilli, thus preventing souring of the milk during the rising, which might cause sour bread.
Chemical reactions in bread making :
dextrose
starch water or sugar
C6H10O5 + a ferment + H20 = C6H12O6
carbon
Alcoholic fermentation: alcohol dioxide
C6H12O6 + yeast = 2 C2H5OH + 2 C02
Acetic fermentation :
alcohol oxygen acetic acid water
2 C2H5OH + 4 0 + a ferment = 2 CH3COOH + 2 H20
Experiments with yeast.
EXPERIMENT 70. — Mix a small amount of yeast with water.
(a) Put a drop on a glass slide, cover with a glass, and examine under the microscope, first with a low power and then with a high power. Make a drawing.
(6) Stain the slide with a little iodine and examine again. What color do the yeast cells become when they are stained with iodine ? The starch cells ? What kind of starch was used in the yeast cake ?
EXPERIMENT 71. — Make a solution of glucose and water, or molasses and water. Add some yeast. Let stand an hour or two in a warm place. Examine a drop under the microscope. How does the yeast cell reproduce ?
EXPERIMENT 72. — Make a solution of glucose and water. Add some yeast. Put equal amounts in three test tubes.
(a) Boil the contents of one tube.
(6) Put one tube in a cold place or on ice.
(c) Keep one tube at temperature of 80° F. At the end of an hour or two, examine the contents of the tubes, noting any fermentation that may have occurred. Draw conclusions as to the best temperature for the growing of the yeast plant.
Warm (6) to a temperature of 80° F. and let stand for an hour or two. Does cold destroy the life of the yeast? Does boiling destroy it?