It must not be forgotten that redness in milk may actually be due to the presence of blood from the udder of the cow. In such a case the blood and milk will be inextricably mixed together, and not in patches or a pellicle.
Blue Milk is due to the growth of Bacillus cyanogenus. This is an actively motile rod, the presence of which does not materially affect the milk, but causes the milk products to be of poor quality.
Yellow Milk. Bacillus synxanthus is held responsible for curdling the milk, and then at a later stage, in redissolving the curd, produces a yellow pigment.
Violet and Green Pigments in milk are also the work of various bacteria.
2. Various Unclassified Bacteria
In milk this is a comparatively small group, for it happens that those bacteria in milk which cannot be classified as fermentative or pathogenic are few. The almost ubiquitous Bacillus coli communis occurs here as elsewhere, and might be grouped with the gaseous fermentative organisms on account of its extraordinary power of producing gas and breaking up the medium (whether agar or cheese) in which it is growing. What its exact rôle is in milk it would be difficult to say. It may act, as it frequently does elsewhere, by association in various fermentations. Some authorities hold that its presence in excessive numbers may cause epidemic diarrhœa in infants.
Several years ago a commission was appointed by the British Medical Journal to inquire into the quality of the milk sold in some of the poorer districts of London. Every sample was found to contain Bacillus coli, and it was declared that this particular microbe constituted 90 per cent. of all the organisms found in the milk.[57] We record this statement, but accept it with some misgiving. The diagnosis of B. coli four or five years ago was not such a strict matter as to-day. Still, undoubtedly, this particular organism is not uncommonly found in milk, and its source is unclean dairying. In the same investigation Proteus
vulgaris, B. fluorescens, and many liquefying bacteria were frequently found. Their presence in milk means contamination with putrefying matter, surface water, or a foul atmosphere.
A number of water bacteria find their way into milk in the practice of adulteration, and foul byres afford ample opportunity for aërial pollution.