194 CIVIC BIOLOGY 



a slide). Mount scrapings in a small droplet of water and examine 

 under a microscope. Be particular to scrape especially in the little crack 

 around the stem. Can you see from results why stems should not be 

 pulled out in picking fruit ? 



Experiment 2. Pull out the stems and make slight punctures and 

 scratches through the skins of a number of apples or i:)ears, set them 

 aside with an equal number of perfectly sound fruits, and examine from 

 time to time for signs of decay. 



Experiment 3. Plant scrapings from the skins of the various fruits 

 in vials of dilute fruit juice (filtered cider, the juice from canned fruit, 

 diluted with half water if too sweet), plug with cotton, and examine 

 later for growth of yeasts and molds. 



If microscopes are not at hand. Experiments 2 and 3 can be done 

 perfectly well without them. What do these experiments mean with 

 reference to honest hand picking and packing of fruits ? If one decayed 

 fruit wets or touches another, what is likely to happen? Contagion? 



Uses. In making^ bread we use the carbonic acid which the 

 yeast plants give off to form bubbles in the dough. These 

 bubbles are hardened in baking, the alcohol is driven off, and 

 the bread remains light. In making alcohol we use the sugar 

 of fruits or the starch of potatoes, barley, corn, rye, which 

 has been changed to sugars by digestive ferments; then either 

 the wild yeasts that were on the fruits or the pure-culture 

 yeasts that we add to the fruit juice mash or wort ferment 

 the sugars, and the alcohol may be distilled off by heat. 



If the yeast fermentation has been too slow, or if the mate- 

 rial is allowed to stand after alcoholic fermentation is complete, 

 other microorganisms, with which yeast is always associated, 

 begin to turn the alcohol into acetic acid, and we have sour 

 bread, sour beer, and vinegar. This process may be roughly 

 represented by the equation 



C^H^O + O, = C,H,0, + H^O 



Alcohol Oxygen Acetic acid Water 



Then if vinegar is exposed to the air, another organism may 

 chano-e the acetic acid further into carbonic acid and water, 

 and the decomposition of the starch or sugar is complete. 



