172 HOW PLANTS BENEFIT AND HARM MANKIND 



parts and pour them into the jars. Place covers on the jars. Put jar A 

 in the ice box on the ice, and jar B over the Mtchen stove or near a radiator ; 

 boil the jar C by immersing it in a dish of boiling water, and place it next 

 to B. After forty-eight hours, look to see if any bubbles have made 

 their appearance in any of the jars. K the experiment has been successful 

 only jar B will show bubbles. After bubbles have begun to appear at the 

 surface, the fluid in jar B will be found to have a sour taste and wiH smeU 

 unpleasantly. The gas which rises to the surface, if collected and tested, 

 win be found to be carbon dioxide. The contents of jar B are said to have 

 fermented. Evidently', the growth of yeast will take place only under 

 conditions of moderate warmth and moisture. 



Fermentation a Chemical Process. — In this process of growth the 

 sugar of the solution in which they live is broken up by a digestive fer- 

 m.ent or enzyme into carbon dioxide and alcohol. This may be expressed 

 by the foUowing chemical formula: CeHijOe = 2(C2H60) + 2(C02). 

 This means that the sugar forms alcohol and carbon dioxide. This pro- 

 cess, which we call fermentation, is of the greatest importance in the 

 brewing industry. 



Beer-Making. — Brewers' yeasts are cultivated with the greatest care ; 

 for the different flavors of beer seem to depend largely upon the condi- 

 tion of the yeast plants. Beer is made in the foUowing manner : Sprouted 

 barley, called malt, in which the starch of the grain has been changed to 

 grape sugar by digestion, is killed by drying in a hot kiln. The malt is 

 dissolved in water, and hops are added to give the mixture a bitter taste. 

 Now comes the addition of the yeast plants, which multiply rapidly under 

 the favorable conditions of food and heat. Fermentation results on a 

 large scale from the breaking down of the grape sugar, the alcohol re- 

 maining in the fluid, and the carbon dioxide passing off into the air. The 

 process is stopped at the right instant, and the beer is stored either in bottles 

 or casks. 



Bread-Making. — In bread-making the rapid growth of the yeast 

 plants is facilitated by placing the pan containing the mixture in a warm 

 place overnight. Fermentation results from the digestion of grape sugar 

 by the j'easts, this grape sugar being part of the starch in the flour which 

 is changed by the diastase present in the grain of wheat. The carbon 

 dioxide remains in the dough as the bubbles so familiar to the bread- 

 maker, the alcohol produced being evaporated during the process of baking. 

 J Yeast Saprophytes. — The above paragraphs show yeast plants to be 

 saprophytes. In order to grow, they must be supplied with food materials 

 that will build up protoplasm as weU as release energy. This food they 

 obtain from the organic matter in the fluids in which they happen to be. 



~ The Shelf Fungus ; a Saprophyte. — A near relation to the mush- 

 room is the bracket or shelf fungus. This fungus is familiar to any 

 one who has been in a forest in this part of the country. 



