412 Bacteria in Relation to Country Life 



rancid so readily as butter made from unpasteurized 

 cream. In fact, butter made from sterilized cream does 

 not seem to become rancid at all as long as it remains 

 sterile. In preventing butter from becoming rancid, 

 antiseptics are as effective as sterilization by heat. 

 Butter made from pasteurized or sterilized cream may 

 be made to become rancid by the addition to it of a small 

 quantity of old butter. Furthermore, butter contain- 

 ing a large proportion of casein or milk-sugar becomes 

 rancid more readily than butter poor in these substances. 

 When air and light are excluded, the butter does not 

 deteriorate so rapidly. In hermetically sealed cans, it 

 keeps best when the latter are full, that is, when the air 

 is excluded. 



Numbers and kinds. — Bacteriological examinations 

 of fresh butter show large numbers of organisms, fre- 

 quently many millions per gram. The number is inti- 

 mately affected by the source and character of the cream. 

 Cream from the milk of clean cows and in a clean dairy- 

 house will yield butter with a much smaller number of 

 bacteria than filthy cream. Clean cream contains, for 

 the most part, only lactic-acid bacteria that rapidly 

 decrease in numbers in the butter, whereas filthy cream 

 contains other species that do not decrease as rapidly, 

 and, in some instances, actually multiply in the butter. 



A study of the various microorganisms found in 

 butter will show that different species are prominent in 

 the same sample at different times. Fresh butter neces- 

 sarily contains lactic-acid bacteria in predominating 

 numbers. There are usually present, also. Bacillus 

 fluorescens liquefaciens, derived from the wash-water, 



