1 88 Biological Chemistry. 



residue is acidified with sulphuric acid, and the distillation 

 of the fatty acid is then carried out in an apparatus of 

 standard dimensions. The amounts of liquid and the rate 

 of distillation are always kept as nearly as possible 

 uniform. The liquid to be distilled is usually 140 c.c., 

 and 110 c.c. are distilled off in one hour. The Reichert- 

 Meissl value is of special service in the examination of 

 butter, for which it is 2 0*6-3 3*1. This high number is 

 due to the relatively large amount of butyric acid yielded 

 on hydrolysis. For lard the value is only 0*68, and for 

 beef-fat it is also low. 



(d) The Acetyl Value. Certain fatty acids obtained by 

 the hydrolysis of fats contain a hydroxyl group. Substances 

 containing such a group readily form acetyl derivatives 

 when treated with acetic anhydride 



CH 3 -C(X 



K-OH+ >0 = R.O.COCH 3 + CH 3 -COOH 



CH 3 -C(K 



If the acetyl derivative thus formed is hydrolyzed, it 

 undergoes scission into the original hydroxyl derivative 

 and acetic acid 



R.O-CO-CH 3 + KOH = R-OH + CH 3 COOK 

 The number of milligrams of potassium hydroxide necessary 

 to combine with the acetic acid set free by hydrolysis from 

 the acetylated acids from 1 gramme of fat is known as the 

 acetyl value. 



The chemistry of the fats may be summarized as 

 follows : The fats are esters of glycerol in which all three 

 hydroxyl hydrogen atoms are replaced by an acidyl 

 radicle. They are sometimes called triglycerides. They 

 yield on hydrolysis (" saponification ") with alkalis the 

 alkali salts of the acids (" soaps ") and glycerol. Different 

 acids are yielded by fats from different sources, and the 

 mixtures thus obtained can be characterized by certain 



