Putrid odour may also be due to the presence of ammoniacal gas in the byre, or to special toxins liberated by microbes which have found their way into the milk. It is most marked during the warm seasons of the year.
The occurrence of putrid milk can be prevented by disinfecting the dairy and the milking pails daily for a certain time.
Mucous, viscous, or thready Milk.—These terms are applied to a condition which usually appears twenty-four or thirty-six hours after the milk has been withdrawn. The milk seems thick and viscous, and can be drawn out into threads like mucus. It sticks to neighbouring objects, and adheres to milk vessels like molasses. It coagulates imperfectly on standing, gives little cream, and even this cream only furnishes a mawkish, ill-flavoured butter.
In certain parts of Switzerland the production of mucous milk is favoured, because it is employed in making cheeses.
The change is due to the presence of various microorganisms. Those which have been best studied are Schmidt-Mülheim’s micrococci, the Actinobacter polymorphus of Duclaux, the Bacillus lactis pituitosi of Löffler, the Bacillus lactis of Adametz, the Streptococcus hollandicus, and, finally, three others which are much commoner, Guillebeau’s bacillus, the Micrococcus Freudenreichii, and the Bacterium Hessii. These microorganisms act on the lactose, decomposing it and causing the formation of a kind of filamentous mucilage, which can be isolated by the addition of alcohol.
The mucilaginous change in milk can be prevented by ordinary methods of disinfection.
Red Milk.—Milk which becomes red some hours after withdrawal, or within forty-eight hours after milking, should be distinguished from milk which on withdrawal from the udder is tinted red in consequence of hæmorrhage within the udder itself. When the milk is of a hæmorrhagic tint the blood corpuscles are soon deposited on the bottom of the vessel if the milk is allowed to remain undisturbed.
The tint which the milk assumes is due to the growth of chromogenic organisms, the best known of which are as follows:—1. B. prodigiosus, which produces large red patches on the surface. It grows readily on potato and gelatine, which it liquefies. 2. The Sarcina rosea, which develops first of all in the cream and afterwards invades the milk. It grows in sterilised milk, on alkaline potato, and on gelatine. 3. The Bacterium lactis erythrogenes, which liquefies gelatine and produces a reddish coloration. Casein can be precipitated and peptonised by means of its cultures. It develops in the milk below the cream, the serum alone becoming red, and only when shaded from the light.
Blue Milk.—In this case the milk appears normal when withdrawn, but some days afterwards shows blue patches, which gradually increase in size, and by uniting produce a distinct blue tint at the surface.
This change is connected with the presence of the B. cyanogenus. The organism grows in sterilised milk, but in this case merely produces greyish patches, the blue tint only occurring when a certain quantity of lactic acid is added or when the ordinary lactic ferments are present.