Conditions Affecting Butter-Fat. 459 



The three factors most active in the production of rancidity 

 in butter-fat are air, light, and warmth ; and decomposition 

 occurs most rapidly when all three conditions are favourable. 

 In the development of rancidity there are observed, first of all, 

 several changes as regards colour, odour, taste, and general 

 appearance : the sample begins to acquire a lighter colour, the 

 change appearing first on the surface and parts most exposed 

 to light, from which points the action spreads slowly downwards, 

 until finally the whole mass has become bleached. With this 

 loss of colour is noticed also the development of a characteristic 

 so-called " lardy " smell and taste, and as the rancidity becomes 

 greater the smell increases in pungency, whilst the taste, which 

 at first was not markedly acid, becomes exceedingly burning and 

 unpleasant. In addition, there are changes in the consistency 

 of the butter-fat. Originally firm and solid in texture, it 

 assumes a granular appearance, and, after many months, finally 

 turns into a semi-solid, pasty mass. 



On determining the various chemical and physical constants 

 of the fat at various stages of its rancidity and comparing them 

 with those obtained when it was in a fresh state, certain charac- 

 teristic changes are noticed. (Browne, loc. cit.) With the de- 

 velopment of rancidity we usually observe a decided increase in 

 the amount of free acid, in the saponification and Reichert 

 numbers, as well as in the specific gravity, with a decrease in 

 the insoluble fatty acids and iodine absorption — the latter being 

 of a very marked description. The constituent in butter- fat 

 which is most susceptible to chemical change is oleic acid ; this 

 being an unsaturated- compound absorbs oxygen with great 

 avidity, yielding either oxy-compounds or decomposing into 

 simpler bodies of lower molecular weight. This instability, and 

 the chemical changes consequent upon it, serve to explain the 

 alterations in the constants mentioned above. 



The free acid which soon makes its appearance is due not 

 simply to oleic acid and its decomposition products but to 

 other acids as well. In the development of rancidity there 

 seems to be a gradual breaking up of all the glycerides, the 

 acid part of the molecule spitting off from the glycerol residue, 

 which itself probably undergoes change, and in the case of 

 the complex oleins, the changes which so soon affect the oleic 



