Many foods constitute nourishment for these organisms. It is because these plants exist in foods and live upon them that changes in foods result. The mold on bread and fruit, the odor from decaying meat and eggs, the liquefaction of decayed eggs, and the gas from fermenting canned fruit are caused by microorganisms existing and growing in these foods. The following experiments show the growth of molds on food and other materials:
EXPERIMENT 80: EFFECT OF AIR, LIGHT, AND DRYING UPON THE GROWTH OF MOLDS. —Place a piece of bread on a saucer. Allow it to remain uncovered, in a light place, at room temperature, for several days. Examine. What is the condition (moist or dry) of the bread? Have molds grown upon the bread?
[Illustration: From Household Bacteriology, by Buchanan. FIGURE 88.—SOME
SPECIES OF MOLDS.]
EXPERIMENT 81: EFFECT OF MOISTURE AND LIGHT UPON THE GROWTH OF MOLDS.— Sprinkle a thick piece of bread with water, place it on a saucer, and cover with a jelly glass or any glass dish. Leave in a light place at room temperature for several days. Examine. Is the bread moist or dry? Have molds grown upon the bread?
From the results of Experiments 80 and 81 what would you say has caused the molds to grow? What conclusion can you draw
from this concerning the growth of molds upon foods in damp and dry places and in damp and dry weather? How should bread be stored in dry weather? In damp weather? Give the reason for storing Dried Bread Crumbs as directed in Lesson L.
EXPERIMENT 82: EFFECT OF MOISTURE AND DARKNESS UPON THE GROWTH OF MOLDS.— Repeat Experiment 81, except the method of covering. Cover with an earthen dish so that the light is excluded. Let it remain at room temperature for the same length of time as given in Experiment 81. Have molds grown? How does the growth compare in quantity with that of Experiment 81?
EXPERIMENT 83: EFFECT OF MOISTURE AND LOW TEMPERATURES UPON THE GROWTH OF MOLDS.—Repeat Experiment 81, but place the bread on the lower shelf of the refrigerator. After several days, examine. Have molds grown? How do they compare in quantity with that of Experiment 81? What conclusion can you draw from this concerning the temperature at which food liable to mold should be kept?
EXPERIMENT 84: GROWTH OF MOLDS UPON CUT FRUIT.—Place pieces of apple, banana, lemon, or other fruits on separate saucers and cover each with a glass dish. Place some lemon or other fruit juice in a test tube and allow it to stand. After two days examine. Have molds grown on all the fruits? Do you notice any difference in the quantity of the molds on the different fruits? Have molds grown on the fruit juice?
EXPERIMENT 85: GROWTH OF MOLDS UPON WHOLE FRUITS.—Place whole fruits, such as apples and lemons, on saucers and cover with glass. After two days examine. Have molds grown upon the whole fruits? If so, how do the molds compare in quantity to those growing on cut fruit? Account for this difference. Apply the results of Experiments 84 and 85 to the "keeping" of fresh fruits.