214 BACTERIA IN BUTTER. 



toward the interior. The rancidity is also* stimulated by the 

 action of light, takes place more readily at high temperatures, 

 and occurs more quickly in butter than in the oleomargarin 

 products. 



To what this rancidity is due has been long disputed. It 

 was at first regarded as a purely chemical process of direct 

 oxidation or decomposition of glycerids, but when the signifi- 

 cance of bacteria was recognized it was suggested that the 

 development of the rancidity was due to the growth of cer- 

 tain species of bacteria in the butter. Butyric acid was 

 regarded as a necessary factor, and it has long been known 

 that certain milk bacteria produce butyric acid (see Chapter 

 III.). Pasteur first proved this, but later others have shown 

 that this acid is an important by-product of a number of 

 species of milk bacteria. The inference that these bacteria 

 cause rancidity was a natural one. In more recent years 

 rancidity has been distinguished from butyric acid produc- 

 tion, and as the significance of enzymes has been recognized 

 it has been claimed that rancidity is due, not to the gro.wth 

 of bacteria in the butter, but rather to the action of certain 

 enzymes that produce a slow chemical change. 



A crucial experiment to settle the question seems to be 

 extremely difficult to devise. The most recent werk by 

 Jensen (239), however, has rendered it almost certain that 

 bacteria, rather than the presence of any chemical enzymes, 

 must be looked upon as the agents producing this change. 

 This fact has been indicated by a number of lines of experi- 

 ment. In the first place, the rancidity has been found to be 

 checked by anything that checks the growth of bacteria. 

 Butter can be made from pasteurized cream without the 

 presence of bacteria and is practically sterile. Such 

 butter, if protected from the contamination by bacteria, 

 does not become rancid, though it will become rancid 

 if inoculated with certain species of bacteria. The fact 

 that the rancidity develops from the surface inward 



