3/6 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



Crude fiber consists of cellulose, lignin, cutin, pentosans, and 

 other substances. Digested crude fiber appears to be equally 

 as good as starch, but the labor of chewing materials containing 

 much crude fiber, largely counteracts the value of the food. 



Seeds and tubers contain little crude fiber. Hay, straw, chaff, 

 and woody materials in general may contain considerable quan- 

 tities. For the crude fiber content of some materials, see the 

 tables of analyses. 



About 20 per cent, of the pentosans of hays and straws is in 

 the crude fiber, making up 10 to 15 per cent, of the crude fiber. 



Organic Acids. Organic acids are found in plants and plant 

 products, though often in very small quantities. They may be 

 present in the free state, but are usually present as salts of lime 

 or potash. In green plants the acids are found mainly in solu- 

 tion in the sap ; later on they are deposited into the cell tissues. 



The quantity of organic acids in ordinary agricultural plants 

 is very small. Appreciable amounts are found in fruits and some 

 vegetables. Tartaric acid occurs in appreciable amounts in 

 grapes, and is deposited as potassium acid tartrate in wine. 

 Small amounts are found in pineapples, cucumbers, and tomatoes. 

 Malic acid occurs in apples, from which it gets its name, but is 

 widely distributed, occurring in a number of fruits and vegetables. 

 Oxalic acid and succinic acid are found in many plants. 



Citric acid is present in lemons and limes, and in small amounts 

 in pears, beans, cherries, and other fruits. Tannic acid is not 

 present in food plants to appreciable extent, though it is found 

 in tea and coffee. It is used for tanning leather. Some plants 

 are grown for the tannic acid they contain. 



Lactic acid occurs in silage and sour milk. 



The organic acids have little food value, but affect the palat- 



