434 



hydrogenation. At 200, in the presence of finely divided 

 nickel as a catalyst, these unsaturated compounds take up hydro- 

 gen and become saturated, the oil hardening to a fat in the 

 process. 



Many substances which were formerly waste products, such as 

 cotton-seed oil, are now treated in enormous quantites in this 

 way. The hydrogenation of low grade oils (for example, fish oil) 

 is also of great industrial importance in the manufacture of soap 

 (see p. 438) and candles. 



Digestion of Fats. At body temperature, the fats and oils 

 present in foods are all insoluble in water, and therefore can- 

 not be directly absorbed into the system. But fats, if already 

 emulsified (p. 110), as in milk, are hydrolyzed by a lipase (enzyme 

 for fat) in the gastric juice of the stomach, and are decomposed 

 into the acid and glycerine (p. 433). Fat in larger masses is 

 hydrolyzed by lipases in the bile and here the acid (insoluble in 

 water) is dissolved. The acid and the glycerine then diffuse 

 through the intestinal wall and finally recombine to form fat in 

 the blood. Some of this fat is deposited in the tissues and some is 

 oxidized (giving muscular energy and heat). 



Cooking (application of heat) does not affect the digestibility of 

 fat. However, when fat is heated too strongly, the beginning of 

 destructive distillation produces unsaturated compounds. These 

 are intensely irritating to the digestive organs as the way their 

 vapors bring tears to the eyes would lead us to expect. 



Digestion of Proteins. The proteins contained in foods, 

 of which the white of an egg (albumen) is a typical example, are 

 not affected by saliva, but, when mixed with the gastric juice of 

 the stomach, they are changed by the free hydrochloric acid it 

 contains into syntonin. This in turn is hydrolyzed by the pep- 

 sin (enzyme), also contained in the gastric juice, into peptones 

 which are soluble in water. These changes, only partly carried 



