470 THE CHANGE OF LACTOSE INTO LACTIC ACID 



never sterile, nor did nature attempt or intend to furnish to the young 

 animal or infant an absolutely sterile germ-free food supply. The 

 milk as drawn by suction from the milk glands becomes mixed with 

 bacteria from the ducts, and the skin and the bacterial contents are 

 further much augmented, before the stomach is reached, by the 

 admixture with the secretions of the mouth, which contain very 

 numerous bacteria. 



The general statement that absolutely sterile milk can never be 

 obtained refers, of course, to the practical collection of the fluid from 

 the cow. A trained bacteriologist, after the liberal removal of the 

 foremilk and a larger amount of milk flowing subsequently, can 

 collect a few cubic centimeters of bacteria-free milk in a sterile test- 

 tube, as has been shown in Bergey's extensive bacterial milk analyses. 



What has been said, however, must not be taken to imply that 

 efforts should not be made to obtain milk in such a way as to keep 

 its bacterial contents as low as possible. This can best be accom- 

 plished by using the greatest care in collecting the milk, by cleaning 

 the udder, cleansing the milker's hands, tying the tail of the cow 

 during milking, receiving the milk into a clean (if possible sterile) 

 vessel washed out with boiling hot water, protecting the lacteal fluid 

 afterward from contamination with dust and dirt, and cooling it 

 rapidly and keeping it cool to prevent subsequent bacterial growth 

 and multiplication. 



Numerically, most bacteria in contaminated milk are derived from 

 fecal matter of the cow. \\iithrich and Freudenreich have ascer- 

 tained that the feces of the cow contain about 375,000,000 bacteria 

 per gram of moist substance, hence it is of the greatest importance 

 in collecting milk to guard against contamination with cow's dung 

 and against the dust and dirt derived from it. In practice, however, 

 it has been recognized that a certain amount of dust and dirt con- 

 tamination is unavoidable, and various bacteria from those sources, 

 in addition to those habitually present in the milk-ducts, must be 

 expected to be found, no matter how clean and ideal the environments 

 of the milch cow's stable. Renk and others have elaborated methods 

 to ascertain the amount of dirt which can be removed by sedimen- 

 tation from milk, and which can subsequently by exact chemical 

 methods be dried and weighed. European authorities appear to agree 

 that milk collected by cleanly methods should contain less than 10 

 milligrams of dry dirt per liter, i. e., less than one part to 100,000 

 parts of milk; but otherwise good market milk often contains an 

 amount many times in excess of this standard. 



THE CHANGE OF LACTOSE INTO LACTIC ACID. 



Even when milk is collected in a very clean and careful manner 

 it will undergo certain changes and generally become sour unless 

 permanently kept at a temperature very near the freezing point of 



