214 BACTERIA 



butter is usually less than in cream. 1 Moreover, they are 

 soon reduced both in quality and quantity. Butter ex- 

 amined after it is several months old is often found to be 

 almost free from germs; yet in the intervening period a 

 variety of conditions are set up directly or indirectly through 

 bacterial action. 



Rancid butter is partly due to organisms. Putrid butter 

 is caused, according to Jensen, by various putrefactive bac- 

 teria, one form of which is named Bacillus foetidus lactis* 

 This organism is killed at a comparatively low temperature, 

 and is therefore completely removed by pasteurisation. 

 Ill- flavoured butter may be due to germs or an unsuitable 

 diet of the cow and a retention of the bad quality of the 

 resulting milk. Lardy and oily butters have been investi- 

 gated by Storch and Jensen and traced to bacteria. Lastly, 

 bitter butter occasionally occurs, and is due to fermentative 

 changes in the milk. Butter may also contain pathogenic 

 bacteria, like tubercle. The B. coli can live for one month 

 in butter. 



Cheese suffers from very much the same kind of " dis- 

 eases " as butter, except that chromogenic conditions occur 

 more frequently. The latter are, under certain circum- 

 stances, more the result of chemical than bacterial action. 

 Most of the troubles in cheese originate in the milk. 



Method of Examination of Butter. Several grams of the 

 butter should be placed in a large test-tube, which is then 

 two-thirds filled with sterilised water and placed in a water- 

 bath at about 45 C. until the butter is completely melted. 

 A small quantity may then be added to gelatine or agar and 

 plated out on Petri dishes or in flat-bottomed flasks in the 

 usual way. After which the tube may be well shaken and 

 returned to the bath inverted. In the space of twenty or 

 thirty minutes the butter has separated from the water with 



1 Hewlett asserts that butter may contain from two to forty-seven millions of 

 bacteria per gram. 



