396 
MicrO'Organisms and their Action 
increased, and on an average amounts to 85 per cent. In the 
case of butter to which no salt has been added, from 13 to 14 
out of the remaining 15 per cent, is water, and from 1 to 2 per 
cent, is made up by the other constituents of milk, viz., proteids, 
milk-sugar, and ash. Butter made of sour material generally 
contains rather a large amount of casein, which is present in the 
precipitated state, and little or no milk-sugar, but in place of it 
lactic acid ; while salt butter contains from 1 to 5 per cent, of 
common salt and a corresponding lower percentage of fat. 
Pure butter-fat, obtained from butter by melting the latter and 
clarifying the fat by filtration, can be kept for a considerable time 
unchanged and undecomposed, provided it be protected from 
the action of air and light. Even one of these two agents alone 
will not readily act upon pure butter-fat, but under their 
combined action the fat attains a tallowy taste and appearance 
in a very short time, and when subjected to a chemical analysis 
is found to have undergone material changes in its composition. 
Other organic bodies which are in a state of decomposition will, 
when intimately mixed with butter-fat, also have the effect of 
causing its decomposition to a greater or less degree. 
Butter is such an intimate mixture of butter-fat and decom- 
posable organic matter, that decompositions occurring in the 
butter-milk left in the butter, even if present in very small 
proportion only, will extend themselves to the butter-fat. Indeed, 
it may be said that the fat has been already affected before the 
butter is made, in case sour milk or sour (ripened) cream formed 
the material for churning. The commonly called "nutty flavour" 
of butter made of slightly sour material must be assigned to 
traces of compounds formed by a slight decomposition of the 
fat, and as this flavour is not noticed with fresh butter made 
of sweet material, there is good reason to assume that the 
decomposition of the fat is the consequence of decompo- 
sitions of other organic constituents of the butter. How these 
decompositions are caused by micro-organisms we have seen 
in the preceding pages. If not directly, certainly indirectly, 
we must assign decompositions occurring in butter to the same 
causes. 
As a matter of fact, it was found that the butter-milk, in a 
butter made from sour cream and declared to be of bad keeping 
quality, contained a great number of micro-organisms, which 
are described as globular in shape and lively in motion ; while in 
the butter-milk of butter made from sweet cream and pronounced 
as of good quality these organisms were not present. The two 
kinds of butter were melted at a low temperature, and the 
separated scum of both mixed with sterilized milk, when it 
was observed that the scum of the former, containing the micro- 
