72 PLANT COMPOUNDS 



62. Pectins. — Pectins are amorphous, white substances 

 found in most fruit juices, soluble in water, but insoluble 

 in alcohol. By enzyme action they are derived from in- 

 soluble pectose which gives hardness to unripe fruit. The 

 pectins in turn are changed to pectic acid by enzymes as the 

 fruit ripens, and the calcium salt of pectic acid is what makes 

 the juice of fruits like apples, plums, and grapes, solidity 

 to a jelly. 



II. FIXED OILS AND WAXES 



63. General Definition. — The name oil is given to liquid 

 substances which are characterized by their slippery or 

 greasy feel, and by the fact that they leave a grease spot, 

 or permanent translucent spot on paper. The properties 

 are the same whether the oils are the so-called mineral 

 oils, hydrocarbons occurring naturally in the earth, or are 

 vegetable oils, called fixed oils or fats, or are volatile oils 

 also found in plants. 



The fixed oils are always esters of glycerine and fatty acids, 

 or glycerides of fatty acids. Glycerine being a trihydric 

 alcohol can combine with one, two, or three monobasic 

 fatty acids, and most fixed oils are mixtures of compounds 

 of glycerine with more than one fatty acid. The name oil 

 usually signifies a liquid compound, fat being the name 

 usually given to a fixed oil which is solid at ordinary tempera- 

 tures. When found in plants the fixed oils are sometimes 

 called vegetable oils to distinguish them from the so-called 

 animal oils which are found in animals. The present dis- 

 cussion will be confined to fixed oils of plants, or vegetable 

 oils, although the general characteristics are the same for 

 both classes of fixed oils. 



64. Properties. — Fixed oils when pure are colorless or 

 pale yellow. When impure they are frequently darker in 

 color, sometimes even greenish in shade from the presence 

 of small amounts of chlorophyl. With few exceptions they 

 have no particular odor or taste. They are lighter than 

 water, their specific gravity varying from 0.875 to 0.970. 



They will not distill unchanged. On heating they de- 

 compose into various compounds depending on the tern- 



