EXPERIMENT STATION BULLETINS. 371 



After a week the taste of the butter exposed to sunlight was so bad that 

 it was difficult to determine whether it was worse at the end of three weeks. 

 It gave at three weeks a strong aldehyde reaction. The butter kept in the 

 incubator in diffused light did not taste as bad as that in sunlight. The 

 acidity of the incubator butter was higher than that kept in sunlight, but 

 this was due, largely, to lactic acid rather than a hydrolytic decomposition 

 of the butter fat. In the sunlight, as well as in the incubator, the butter 

 melted, the butter-milk and other nutrient matter settled to the bottom 

 and the clear fat on top became almost sterile. The acidity of the butter 

 kept at room temperature (not melted) was higher than either. At the 

 end of the experiment the acidity of the butter kept in the dark at room 

 temperature was the highest, that in sunlight the lowest. 



It follows, he says, that oxygen does not play the direct role in the oxi- 

 dation. The portion kept in darkness at room temperature was especially 

 rancid. It is shown that: 



(a) Acid number is highest where germ number is highest. 



(b) Other things being equal, sterile butter is more oxidized than non- 

 sterile. 



Since on the one hand micro-organisms increase the rancidity of butter, 

 and on the other hand, protect it from oxidation, the conditions for rancidity 

 of butter stand in opposition to those of oxidation, which further empha- 

 sizes the fact that oxygen cannot he the direct cause of rancidity in butter. 

 The role of oxygen can, therefore, only be to favor the development of the 

 organisms necess^iry for this process. The changes which ordinary butter 

 at an ordinary temperature undergoes are of an entirely different nature 

 from those caused by sunlight. The free fatty acids in ordinary rancid 

 butter are for the most part non-volatile; that decomposed by sunlight, 

 volatile. 



His conclusions are : 

 "(a) The air plays a direct role in the decomposition of butter only when 

 it is exposed to the sunlight, or to a high temperature. The butter is then 

 oxidized and acquires on this account a very disagreeable smell and taste — 

 but does not become rancid. 



(b) Butter becomes rancid only by the action of certain micro-organisms. 

 Since all of these require air, the rancidity proceeds from the surface to the 

 interior, as does the ripening of soft cheese. 



(c) The micro-organisms which, under ordinary circumstances, cause 

 rancidity in butter are: 



Oidium lactis, 



Cladosporium butyrii, 



B. fluorescens liq., 



B. prodigiosus (occasionally). 



All of these split butter fat. The volatile fatty acids are formed at first 

 by the bacteria, and later by the associative action of the two molds. From 

 this associative action also arises the butyric ester. 



(d) The use of salt diminishes the production of volatile fatty acids; 

 and lactose, the production of esters. Whether the combined action of 

 the two can completely prevent rancidity is yet to be determined. 



(e) On account of the frequent occurrence of B. fluorescens liq., and B. 

 prodigiosus in water, it is to be supposed that they gain entrance to the 

 butter from the wash water. Since B. fluorescens is much more frequent 

 in water than B. prodigiosus it is also found much more frequently in butter. 



(f) As to Oidium lactis and Cladosporium butyrii, they come, without 



