202 ANNUAL REPORT OF THE Off. Doc. 



In the case of the pure butter- fat, the changes are purely chemical, 

 since fat does not sustain bacterial life. Pure fat in the absence of 

 light and air, keeps indefinitely, but undergoes a progressive series 

 of changes when exposed lo light and air. The nature of these 

 changes has been studied by many chemists, the investigations of 

 Dr. O. A. Browne in the laboratory of The Pennsylvania Agricultural 

 Experiment Station being probably the most exteosive. These in- 

 vestigations show that the fat first exhibits a tendency to bleach, 

 to become tallowy in flavor and odor and later, to assume a sharp 

 flavor and pungent odor. The chemical changes accompanying these 

 physical modifications are chiefly the result of an oxidation of the 

 oleic acid of the butter-fat; for the quantity of oleic acid steadily de- 

 creases as rancidity increases. The acidity of the fat increases with 

 the rancidity, though not necessarily, in proportion to the develop- 

 ment of undesirable odor and flavor. It was formerly supposed that 

 the acidity and the undesirable qualities of rancid butter were due 

 to the liberation of free butyric acid; but this is now shown to be an 

 incorrect surmise. Butyric acid is water-soluble, while the acidity de- 

 veloped early in the formation of rancid butter is proven to be due 

 to insoluble fatty acids. The bad odor and flavor may be due to alde- 

 hydes, which develop in larger quantity as rancidity increases, and 

 are like acids, formed by oxidation. The glycerol, which is an es- 

 sential part of all fats, is likewise diminished as rancidity advances. 

 Schmid suggests that the aldehydes found in rancid butter-fats are 

 formed from the glycerols. Scala, however, found that pure oleic 

 acid developed these compounds when exposed to the air; he sepa- 

 rated the aldehyde from rancid olive oil and identified it as oenan- 

 thylic aldehyde, which he regards as the principal malodorous ma- 

 terial of all rancid fats. Browne has shown that these aldehydes 

 are changed, probably to volatile acids, in the process of determining 

 the volatile fatty acid number of the rancid fats. In the later stages 

 of rancidity, the volatile acids in the fats themselves rapidly in- 

 crease. As a consequence of both facts, the volatile fatty acid num- 

 ber of pure butter-fat increases as rancidity advances. 



The chemical changes suffered by the fat when it turns rancid in 

 contact with the water, casein, milk, sugar, salt and ash present in all 

 butters, have not been so fully studied. They are, in part, of the 

 same nature as in the case of the pure fat. Bacterial life and the de- 

 velopment of moulds are, however, possible in the presence of the 

 casein, sugar and ash, and the former is probably present in all cases. 

 One fact^ clearly established by recent investigations is, that when 

 the rancid butter becames mouldy, the quantity of volatile fatty acids 

 decreases; renovated butters, sold as such, commonly show a low^ 

 volatile acid number, while their total acidity is high. 



As the fat becomes more rancid, whether in the pure condition or 



