804 
The Fermentations of Milk. 
fat be inoculated with rancid fat in which many bacteria may be 
present it does not become rancid. The best authorities agree that 
the development of rancidity is a purely chemical process, promoted 
by light and requiring the presence of air. It is nevertheless held 
that rancidity is promoted by the presence of lactic acid, and 
possibly hastened indirectly by the growth of microbes. 
Bitter milk, a not very infrequent product, is sometimes asso- 
ciated with the butyric fermentation of the milk sugar, whilst some- 
times it has a different origin. It is true that boiled milk or other 
milk which undergoes the butyric fermentation generally turns bitter 
at the same time ; but it is a question whether the bitterness is 
caused by the same organism which produces the butyric acid. Two 
years ago Weigmann found in bitter milk an organism which 
produced the bitter taste without any butyric acid, but still more 
recently Conn has isolated from an intensely bitter sample of cream, 
and made pure cultures of, a micrococcus which rendered the milk 
very acid and very bitter, at the same time producing butyric acid. 
So it would seem that both cases occur. As regards the bitter 
substance itself, it may in the milder cases be peptone or the soluble 
and diffusible albuminoid produced by the action of the microbes on 
some of the casein of the milk. But in those cases where the taste 
is intensely bitter, it is more likely that a specific bitter principle 
is produced as an alteration product of a little of the casein. 
“Blue milk” is, or at any rate used to be, a comparatively 
common trouble, sometimes in isolated dairies and sometimes 
spreading through a locality like an epidemic. Of course we do 
not allude to what used to be called (before the Adulteration Act 
days) “ London sky blue,” but to the intensely blue colour which 
occasionally invades genuine milk, appearing at first in spots. 
This was the first change in milk which was recognised as due to 
the invasion of an organism, Fuchs having fifty years ago described 
the organism causing it, and proved that the growth of the 
organism and consequent blueing of the milk could be prevented by 
the use of antiseptics. Modern experimenters have amply confirmed 
this result. Hueppe, in particular, has isolated and made pure 
cultures of the specific organism, which is called Bacillus cyano- 
genus, and rapidly gives rise to the blue patches when inoculated 
into any ordinary sample of milk. It is curious that when inocu- 
lated into sterilised milk, although the bacillus grows well enough, it 
produces only a greyish colour, but if a little lactic acid be added 
this colour turns bright blue. Lactic acid is therefore necessary to 
the formation of the blue colour, and as some lactic organism or 
other is always present in unsterilised milk, this condition is never 
lacking. 
Blue milk does not appear to be injurious to animals. Blue 
cheese (not, of course, the ordinary blue mould of cheese) has been 
attributed to the same cause as blue milk. 
Other pigment-producing organisms are quite capable of growing 
