Thermal Limits of Bacteria. 157 



disease-producing properties to the milk through products of de- 

 composition, splitting up of proteids, etc., can only be possible 

 after an accurate determination of all properties which would 

 allow the recognition of the colony as a certain species belonging 

 to a large group. 



This determination of the representatives of a group is not simple, and requires 

 study and continued experimental work of days and weeks relative to fermentation 

 qualities, requirements of growth, pathogenic properties on test animals, ferment-like 

 characteristics, etc. These experiments are only of an optional value in practice on 

 account of the easy decomposition of milk as a food substance. Nevertheless through 

 continued experiments on these problems valuable data and results have been obtained 

 increasing our knowledge of the spread of typhoid fever and the methods for combating 

 this and other diseases. 



Therefore, it should be aimed to prevent the entrance into the 

 milk of directly or indirectly injurious bacteria by procuring the 

 milk in a clean and careful manner. Once such bacteria gain 

 entrance into the milk and multiply, their recognition and isola- 

 tion are too difficult for the practical inspector of milk to consider. 



Milk hygiene can produce practical results only if it is inau- 

 gurated at the place of production. 



The pathogenic bacteria of diseases of animals and man will 

 not be considered here, and only brief consideration will be given 

 to the army of saprophytes which gain entrance to the milk from 

 the air, straw, manure and the milk vessels. 



Although from the numerous possibilities of infection of milk 

 a definite bacterial flora can hardly be expected, nevertheless, cor- 

 responding with the nutritive material of the milk, and the methods 

 employed in its storage and transportation, as well as the subse- 

 quent treatment, conditions are created which are favorable to 

 some varieties of bacteria, while for others they are less favorable 

 or even harmful. Through the growth of a certain kind of bacteria 

 the conditions may be changed in such a way that the require- 

 ments of propagation for other groups are produced. Likewise 

 through symbiosis conditions may be developed which are re- 

 quired by certain species of bacteria, or under which certain 

 species may be destroyed, whereas without symbiosis probably 

 neither of the species could exist, since they are dependent upon 

 each other. The growth of certain species is therefore dependent 

 on numerous influences. 



According to the thermal limits in which bacteria can live, the 

 species may be separated into those which thrive at deg. C. (up 

 to 15-20 deg.) : psychrophile ; those which thrive at 10-15-40 deg.: 

 mesophiles ; and finally into thermophiles, whose thermal optimum 

 ranges between 40 to 70 deg. C., or even higher. 



The species of psychrophile propagate even in well cooled 

 milk and at low temperature, and at times change its taste. Ref- 

 erence should be made here to the Bacillus lactis saponacei and 

 the Bacillus sapolacticum, which give a soapy taste to the milk. 

 This defect of flavor is principally observed in cool weather, and 

 at the beginning or end of the winter. 



