BACTERIA IN BUTTER 207 
grow and hasten its spoiling. Unsalted butter does not keep long, 
and its destruction is largely due to bacteria. But if the butter is 
salted, as is the rule in most countries, the salt checks the growth of 
bacteria. As a result of this and the compact condition of the 
butter, together with its small amount of water, the bacteria do not 
find butter a favorable medium for growth, and they begin to 
diminish in numbers. A very few hours’ time shows a great re- 
duction in numbers, and this continues until, after a few weeks, the 
butter contains comparatively few. They do not entirely dis- 
appear, for some are found even in very old butter. The ordinary 
species of organisms, which have been active agents in the cream- 
ripening, play no further part in the changes which may occur in 
the butter. The following figures of the number of bacteria in 
butter will illustrate the facts. 
No. of Bacteria per Gram of Butter 
Two hours old One day old Four days old Thirty days old 
54,000,000 26,000,000 2,000,000 300,000 
It is well known that, if butter is not used immediately, certain 
changes occur in it which continue slowly for many weeks. The 
butter retains its fresh, delicate flavor and aroma for only a few 
days; but if itis kept cool and away from the light, it may remain 
sweet and good formany months. If, however, it is not kept very 
cold, further changes soon begin to appear which slowly progress 
and eventually ruin the butter. The most noticeable feature 
is the appearance of rancidity. ‘This changeis accompanied by the 
development of butyric acid and frequently by a considerable 
change in the consistency of the butter. It finally becomes 
strongly rancid and tallowy, totally ruined for use. The cause of 
this rancidity has been difficult to determine, apparently because a 
variety of factors contribute to it. It is probably due, in part, to 
chemical fermentation, produced by enzymes in the milk, and in 
part to the growth of bacteria. The rancidity is much more likely 
to occur if the butter is exposed to the light, and it develops more 
