SPONTANEOUS CHANGES IN FATS 19 



The actual cause of this change is as yet but little understood, 

 though it appears probable that it is the result of the combined 

 action of a number of different factors such as oxygen, light, 

 moisture, bacteria and enzymes ; the complex fats, and possibly 

 also the small quantities of proteins and other impurities con- 

 tained in them, are thereby broken down into simpler bodies 

 such as the lower volatile fatty acids and aldehydes. It is 

 frequently true that a considerable quantity of free acid is 

 liberated in fats which have become rancid, and this is especi- 

 ally so in the case of fats such as butter which contain acids 

 of low molecular weight, as butyric acid, the smell of which 

 recalls that of rancid butter. It is, however, a fact that a fat 

 may be acid without being rancid ; cocoa butter, for instance, 

 has usually an acid reaction but very rarely becomes rancid. 



With regard to other constituents fouiid in rancid fats, 

 various authors have from time to time observed the presence 

 of hydroxy-acids, aldehydes, alcohols, and of esters of lower 

 fatty acids, but there appears to be a general consensus of 

 opinion that glycerine does not occur. 



Drying and Resinification. Most fatty oils on exposure to 

 the air tend to thicken, owing partly to polymerization and 

 partly to oxidation ; in some cases the oil actually dries up, 

 leaving a more or less hard mass or a thin elastic film. 



Those oils which only thicken, without actually becoming 

 hard or dry, are called non-drying oils. They are composed 

 for the most part of triolein (cf. p. 7) and contain only 

 small quantities of solid fatty acids ; to this class of oils belong 

 the following: olive oil, almond oil, arachis or pea-nut oil, 

 quince oil, cherry- plum- peach- and apricot-kernel oil, wheat- 

 meal oil, rice, tea-seed oil, and hazel-nut oil. 



Two further oils, namely castor oil and grape-seed oil, are 

 also included in this group of non-drying oils, but they have 

 a slightly different composition from the other members of 

 this group. They are characterized by possessing a consider- 

 able percentage of glycerides of hydroxylated fatty acids, such 

 as dihydroxystearic acid, a fact which is brought out clearly by 

 their high acetyl values (p. 34). 



In contrast with these non-drying oils are the so-called 

 drying oils, among the more important of which are the follow- 



2* 



