688 METABOLISM 



5. The iodine value indicates the amount of unsaturated fatty acids pres- 

 ent, or the number of double bonds. It depends on the fact that iodine, 

 like many other substances, is capable of directly attaching itself to the 

 fatty-acid chain wherever double bonds exist. 



6. The Reichert-Meissl value indicates the amount of volatile soluble 

 acid present in the fat. It is determined by first of all saponifying the 

 fat, then decomposing the soap by mixing it with mineral acid and dis- 

 tilling the liberated fatty acid, the distillate being collected in a known 

 amount of standard alkali and titrated. It is a value that is not of very 

 great use in physiological investigations, but it is so in connection with 

 food chemistry. Since volatile acids are present in butter, the Keichert- 

 Meissl value helps us to distinguish between butter and margarine. 



Fat is insoluble in w r ater but soap is soluble, forming a colloidal solu- 

 tion which presents the phenomenon of surface aggregation of molecules. 

 This consists in the concentration of the soap both at the free surface of 

 the liquid, where a skin may form, and at the interfaces between the 

 soap solution and any undissolved particles present in it. This pellicle- 

 formation around the particles prevents them from running together so 

 that they remain suspended, thus forming an emulsion. An emulsion 

 may therefore be formed either of neutral fat of any other physically 

 similar substance. When fat itself is used, there is usually enough free 

 fatty acid admixed with it to make it unnecessary in forming the emul- 

 sion to do more than shake the fat with weak sodium-carbonate solution. 

 With other substances not containing any free fatty acid, some soaps 

 should be added. To preserve the emulsion it is often useful to add some 

 mucilage. In the emulsified state, neutral fats are much more readily 

 attacked by lipases than when they are present in an unemulsified state. 

 Thus, emulsified fats are "digested" by the relatively small amounts of 

 lipase present in the stomach, whereas neutral fats themselves are not so. 



Fatty acids also exist in nature in combination not with the triatomic 

 alcohol, glycerol, but with monatomic alcohols such as cholesterol. These 

 cholesterol fats differ from the glycerol fats in being very resistant to- 

 wards enzymes and microorganisms. They are therefore used for pro- 

 tective purposes in the animal economy; for example, they occur in the 

 sebum, the secretion of the sebaceous glands, where they serve to moisten 

 the hairs and skin. They are also present in cells, in which it is prob- 

 able they take an important part in forming the skeleton of the cell. 

 Cholesterol is absorbed from the intestine ; it is always present in the blood 

 both in plasma and in corpuscles ; and it is an important constituent of bile, 

 from which it may separate out in the bile passages and form calculi 

 (gallstones). 



In the cells themselves the lipoids are represented mainly by compounds 



