EXTRACTION 13 



The principle of the extraction is to treat the dried mixture 

 with a solvent which will dissolve only the fat and leave the 

 other substances unchanged. The solvents most commonly 

 used for this purpose are ether, light petroleum, carbon tetra- 

 chloride, and carbon disulphide, the two latter being used 

 chiefly on a commercial scale. 



It must be borne in mind that besides extracting fats, ether 

 will also dissolve essential oils, chlorophyll, cholesterol, lecithin, 

 and allied substances variously known as lipoids, lipins, etc. 



Moreover, other substances which are of themselves in- 

 soluble in ether may become soluble in the presence of fats. 



Whatever solvent is employed must be tested before use 

 to see that it leaves no residue on evaporation and is free 

 from moisture. 



A rough and ready method of extracting fat from a given 

 sample is to place the finely divided and dried material on 

 a filter paper folded into a funnel and to pour the fat-solvent 

 on to it. The filtrate will contain most of the fat, which may 

 be recovered by evaporating off the solvent. 



When it is desired to extract the fat quantitatively, the 

 operation is most conveniently carried out in a Soxhlet 

 apparatus (see below). 



Previous to extraction, the substance must be thoroughly 

 dried. For this purpose it must either be gently heated in a 

 current of dry air or else desiccated by means of alcohol or 

 anhydrous salts. 



The first method, which is the most convenient, should, 

 however, be used with caution, as many fats may undergo 

 chemical change during the process, as a result of which the 

 material extracted by ether after drying may be very different 

 from the substance originally present in the moist sample. 



The second method, which consists in treating the sample 

 to be dried with absolute alcohol for some hours and then 

 filtering and pressing, depends on the fact that the alcohol 

 withdraws the water without dissolving away any appreciable 

 quantity of the fat ; if treated two or three times in this way 

 the substance will be practically free from moisture and can 

 then be extracted under a Soxhlet with ether. The wet 



