CHAPTER XXXVI 

 PLANT PRODUCTS. FERMENTATION AND FUELS 



HAVING described the chemistry of plant life, we may now pro- 

 ceed to the chemistry of substances resulting from plant life and 

 growth. Foods will be taken up separately in the following chap- 

 ter. In the present chapter we shall restrict ourselves to two 

 other main branches, fermentation products and fuels. 



Enzymes. All fermentations are brought about either 

 directly or indirectly by the activities of animal or vegetable 

 organisms. The most familiar ferment, of course, is yeast. 



Yeast belongs to a low order of plants and consists of minute 

 cells. Its value lies in the fact that, while growing and multiply- 

 ing, it secretes within each cell small amounts of two very active 

 chemical substances which are dissolved in the cell contents. 

 These substances are known as zymase and invertase (or sucrase), 

 and belong to the class of organic materials called enzymes. 

 Enzymes produce remarkable chemical changes in organic ma- 

 terials by their mere presence (contact actions). These changes 

 are specific, each enzyme acting only on certain carbohydrates, 

 for example, and being quite inert towards others. 



> fFerrnentation^of^ Sugars^ When a cake of yeast is broken in- 

 to an aqueous solution oTglucose_or grape-sugar (p. 401), the small 

 amount of zymase present causes the gradual decomposition of 

 the sugar. The most favorable temperature is about 30. Bub- 

 bles of carbon dioxide soon begin to rise to the surface, and the 

 gas can be led off (Fig. 102) to exhibit its characteristic action 

 (p. 336) on limewater. At the same time alcohol C2H 5 OH ac- 

 cumulates in the liquid as the sugar disappears: 



C 6 Hi 2 6 -> 2CO 2 T 

 417 



