352 BUTTER 



ester was derived from milk-sugar by fermentation ; but 

 Jensen suggests that it more likely comes from an inter- 

 mediate product, monobutyrin, obtained in the decom- 

 position of tributyrin. 



The conditions which govern and accelerate rancidity 

 are : access of air, light, warmth, and moisture, high 

 casein and milk-sugar content, and the presence of 

 various micro-organisms. 



The splitting of the fats apparently arises in part 

 through the action of moisture, carbon dioxide, and 

 lactic and other acids produced by fermentation of the 

 milk-sugar, and also to a considerable extent through 

 the agency of micro-organisms, especially Bs. fluorescens 

 liquefaciens, Oospora lactis> as well as various yeasts and 

 " mould " fungi. Penicillium glaucum and certain species 

 of Aspergillus are able to hydrolyse fats by means of 

 lipase, an enzyme which they possess. 



Free butyric acid arises through the decomposition of 

 butyrin in this way, and not as the result of butyric 

 fermentation : the latter is the work of anaerobic species 

 of bacteria (p. 285) which play very little or no part in 

 the development of rancidity in butter. 



The oxidation of oleic acid and the production of 

 aldehydes and ketones is to a certain extent due to the 

 direct chemical action of the air under the influence of 

 light, and the butter may develop a tallow-like, but not 

 rancid, character and taste by this means alone. Butter 

 does not become rancid when oxygen is completely 

 excluded. However, sterilized butter does not develop 

 the rancid flavour and odour even when exposed to 

 light and air ; so that the action of air would appear to 

 be chiefly important because of its assisting the growth 

 of bacteria and fungi of the aerobic class. 



