FAT METABOLISM 719 



many other ways, as for example by heating with steam or by the action of special 

 enzymes called Upases, which are widely distributed in plants and animals. 



The natural fats are usually a mixture of triglycerides, and their differences in 

 properties are dependent upon the relative amounts of fatty acids present. The three 

 most important in animal fats are tripalmitin, tristearin and triolein. It is essential in 

 the study of fat metabolism that we should know the most important methods ~by which 

 the proportion of fatty adds present in a mixed fat is determined. These methods 

 are as follows: 



1. The melting point. Olein is liquid at C.; palmitic acid melts at 62.6 C.; and 

 stearic at 69.3 C. The solidity of animal fats depends on the proportion of olein, 

 palmitin and stearin present. Mutton fat, for example, is much stiffer than pig fat 

 because it contains less olein and more stearin. The melting points of fats from 

 different parts of the body may also vary. 



2. The acid number indicates the amount of free fatty acid mixed with the fat, 

 and is determined by titrating a solution of a weighed quantity of the fat in alcohol with 

 a N/10 alcoholic solution of KOH, phenolphthalein being used as indicator. 



3. The saponification value indicates the total amount of fatty acid present, both 

 that which is free and that combined with glycerol. It is determined by heating a 

 weighed amount of fat with an exactly known amount of alcoholic KOH (determined 

 by titration with standard acid). After saponification is complete, titration of the 

 mixture shows how much alkali has been used to combine with the fatty acid. This 

 is the saponification value. 



4. The ester value indicates the amount of fatty acid combined with glycerol, and 

 is obtained by subtracting the acid value from the saponification value. 



Besides these there are two values, known as the iodine and the Beichert-Meissl val- 

 ues, that are of importance because they depend on certain -characteristics of the fatty- 

 acid radicles. 



5. The iodine value indicates the amount of unsaturated fatty acids present, 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 Eeichert-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 1 the 

 soap by mixing it with mineral acid and distilling the liberated fatty acid, the distil- 

 late 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 Reichert-Meissl 

 value helps us to distinguish between butter and margarine. 



Fat is insoluble in water but soap is soluble, forming a colloidal solution which 

 presents phenomenon "of surface aggregation of molecules. This consists in the con- 

 centration 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 there- 

 fore 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 emulsion to do more than shake the fat with weak sodium- 

 carbonate solution. With other substances not containing any free fatty acid, some 

 soap 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 Upases than 

 when they are present in an unemulsified state. Thus, emulsified fats are "digested" 



