The Growth Of Yeast; Experiments

A. Mix one tablespoonful of flour, one of sugar,1 and three-fourths of a yeast-cake to a smooth paste with four or five tablespoonfuls of cold water. Divide the mixture between three six-inch test-tubes (or three tumblers). Label the test-tubes a, b, and c respectively. Fill a with boiling water; half fill b with lukewarm water, and stand it in lukewarm water or in a warm place; half fill c with cold water and keep it at a temperature of 32° F. or below (by placing it in a bowl of cracked ice, or outside the window on a freezing day). In a fourth test-tube, d, put one-fourth of a yeast-cake mixed with water only; treat it like b.2 After fifteen minutes examine all four test-tubes. What do you see on the top of the liquid in b? What goes on in the liquid? Let a and c stand for a time where they will be about as warm as b; what change do you notice in either of them? Is there any foam on d? The quantity of foam produced is a measure of the vigor of the yeast. At what temperature does yeast thrive best? Will it grow at all at 32°? after being frozen and thawed? after being heated to 212°? Will it grow in water alone?

1 Adding sugar hastens the growth of the yeast; for bread-making the sugar in the flour is sufficient.

2 A mixture of molasses and water may be used instead of a flour mixture in this series of experiments.

Fig. 9.   Yeast cells.

Fig. 9. - Yeast-cells.

B. (To be done during the progress of Exp. A.) Prepare in a generating flask a mixture like the first used in Exp. A, using three or four times the quantity. Arrange an apparatus like that shown in Fig. 10. Or use test-tube b instead of a flask, standing it in a tumbler of warm water. What gas comes from the yeast mixture? In what other ways may this gas be produced? (pp. 107, 108.) What effect has it when introduced into batter or dough? As it is heavier than air, this gas may, with care, be poured into a tumbler from the bowl in which bread is rising, and tested by pouring lime-water into the tumbler.

Story of the yeast plant; what it needs in order to grow. if thawed, revive and grow. It needs water, and either protein, or some mineral matter containing nitrogen, to feed upon. Dried yeast-cells floating in the air revive if they fall where conditions are right for their development, and grow much as seeds do that fall on good ground. Floating spores develop into cells and grow and bud.

Like mushrooms and other fungi (singular fungus) which have no green coloring-matter (chlorophyll), yeast needs no light; and like these, it grows and multiplies fast. It is like green plants in that it grows only when kept warm and moist. It thrives best at 78° F. It may be forced to grow and bud unnaturally fast by a higher temperature, just as hothouse plants are; but at about 130° F. it loses its liveliness, and by heat greater than this it is killed. Cold checks its growth, but even after being frozen it will,

Fig. 10.   Apparatus for proving that growing yeast produces carbon dioxide.

Fig. 10. - Apparatus for proving that growing yeast produces carbon dioxide.

y = flask containing yeast mixture. w = vessel of warm water. I = beaker containing lime-water. t = glass tube.