132 CHEMISTRY OF PLANT LIFE 



All natural fats are triglycerides, however. Most natural fats 

 are mixtures of several different triglycerides in each of which the 

 three (OH) groups of the glycerol has been replaced by the same 

 organic acid radical, as in the example of stearin shown above. 

 But recent investigations have shown that some of the common 

 animal fats, and perhaps some plant oils, may be made up of mixed 

 glycerides, i.e., those in which the different (OH) groups have been 

 replaced by different acid groups, as oleo-stearin, oleo-stearo- 

 palmitin, etc. 



THE ACIDS WHICH OCCUR IN NATURAL FATS 



The acids which, when combined with glycerol, produce fats 

 are of two general types. The first of these are the so-called 

 " fatty acids " having the general formula C n H.2n+'i COOH. 

 These are the " saturated " acids, i.e., they contain only single- 

 bond linkages in the radical which is united to the COOH group ; 

 hence, they cannot take up hydrogen, oxygen, etc., by direct 

 addition. The second type are the " unsaturated " acids belong- 

 ing to several different groups, as discussed below, but all having 

 one or more double-linkages between the carbon atoms of the alkyl 

 radical which they contain. Because of these double linkages, 

 they are all able to take on oxygen, hydrogen, or the halogen ele- 

 ments, by direct addition. When exposed to the air, for example, 

 these " unsaturated " acids, or the oils derived from them, take 

 up oxygen, increasing in weight, and becoming solid or hard and 

 stiff. Hence, natural oils which contain considerable proportions 

 of glycerides of these " unsaturated " acids are known as " drying 

 oils " and are largely used in the manufacture of paints, varnishes, 

 linoleums, etc.; while oils which contain little of these glycerides 

 are known as " non-drying," and are used for food, for lubrication, 

 or for other technical purposes in which it is essential that they 

 remain in unchanged fluid condition when exposed to the air. 



The following are some of the more important of the acids which 

 occur as glycerides in natural fats: 

 Saturated Acids: 



(a) Acetic, or stearic, acid series general formula, 

 CH 2n+ i-COOH. 



(1) Formic acid, H COOH, occurs free in nettles, ants, 

 etc, 



