Yeast or the Alcohol Plant.—Sweet cider and other fruit juices are sometimes spoiled by a plant named yeast. This plant has the form of a football and is so small that a million of its kind together would not make a mass as large as the head of a pin. It floats about in the air and is present on the skins of fruits.
Yeast is also called the alcohol plant because whenever it grows in a sweet substance like fruit juice it changes part of it into a biting substance called alcohol. At the same time it gives off a gas. It is this gas which forms the bubbling or frothing in beer.
The millions of yeast plants in the yeast cake bought at the store, when put into the dough for bread, grow and form gas. This pushes the bits of dough apart and makes it light. The little alcohol formed is all driven off in the baking.
The alcohol which yeast forms by growing in sweet cider is in a few weeks changed to vinegar by other germs called the vinegar plants. Sour cider may make those who use it sick and drunk because it contains alcohol. Yeast makes wine out of grape juice.
PRACTICAL QUESTIONS
1. Where are germs found?
2. What is the form of microbes?
3. Name some diseases caused by germs.
4. What is mold?
5. Why should food be kept covered when not in use?
6. What causes meat to spoil?
7. How may fruit be kept from spoiling?
8. Where is yeast found?
9. What effect has yeast on fruit juice?
10. Why should you not drink sour cider?