The lactic ferments are numerous, comprising the lactic bacilli of Hueppe and Grotenfeld, the micrococci of Hueppe and Marpmann, and the bacilli and micrococci of Freudenreich. These different agents act on the lactose of the milk, decomposing it into carbonic and lactic acids, which coagulate the milk.
Another group of microorganisms which were well studied by Duclaux comprises those acting on the casein, among others Tyrothrix tenuis, filiformis, turgidus, scaber, virgula, etc.
These organisms secrete principles having similar effects to those of rennet, and are capable of coagulating enormous quantities of milk. After a certain time, they also secrete a second diastase, viz., casease, which acts in the ripening of cheese.
Clotted Milk.—This term is used in dairies to indicate milk which coagulates in lumps immediately after being withdrawn from the udder, or which coagulates spontaneously a few hours later.
The change may be of a chemical nature, depending on conditions of keep or feeding. More frequently, however, it is related to a latent non-pathogenic infection of the udder, or to immediate infection of the milk after removal by lactic ferments contained in the milk vessels or the atmosphere.
It is necessary, according to circumstances, either to modify the diet or disinfect the milk vessels, and immediately pasteurise the milk.
Milk without Butter.—Less commonly the diseased condition is indicated by marked diminution in the quantity of cream.
Churning only produces a poor kind of butter, particles of which do not readily cohere. This peculiarity is due to the presence of microorganisms, which have not yet been fully identified. It can be prevented by disinfection of the milking vessels, as well as of the dairy itself, and by the use of centrifugal separators.
Putrid Milk.—This milk is characterised by its odour. It cannot be used for making butter. In fact, as soon as the cream separates, little bubbles of gas form at various points and break, leaving small cavities. These little separate cavities reunite very rapidly, and the cream becomes reabsorbed as fast as it is formed. Afterwards oily drops formed of butyric, capric, and caprylic acids appear in the depressions and give the milk a repulsive odour (rancidity).
This change is seen during mammitis, but most commonly results from uncleanliness in byres and dairies. In the latter case putrefaction occurs about twenty-four hours after milking, and is due to the growth in the milk of Bacterium termo, lineola, etc. These organisms are present in the dust which falls into the milking pails in the byre; when milk so contaminated is stored in the dairy the changes occur.