CHAPTER XIX. 



THE PRESERVATION OF MILK. 



121. Dirt- and Germ-Content in Milk. 



THE sterilisation of milk is excessively difficult, because this 

 liquid is particularly liable to infection by very hardy germs. Even 

 when yielded by a healthy cow, the milk, on issuing from the 

 udder, is already infested with bacteria. When milking is ended, 

 a small quantity of milk is left behind in the lacteal ducts, and in 

 this there settle a number of bacteria which make their way from 

 outside, are favoured in their development by the high tempera- 

 ture, and become incorporated with the subsequent flow of milk. 

 In addition to these are the innumerable bacteria originating in 

 the dung and adhering to the udder. Both groups consist mainly 

 of species very tenacious of life, derived from the soil and entering 

 the alimentary canal along with the fodder. They pass through 

 the intestines unhurt, are conveyed with the dung on to the udder 

 and the hands of the milker, and then into the milk, where they 

 flourish exceedingly. The dirt, adhering to the cows, and itself 

 infested with bacteria, is partly disseminated as dust through the 

 air of the cowhouse, so that this also is impregnated with bacteria, 

 and yields up no small quantity to the milk. For investigations 

 on this point we are indebted to G. J. LEUFVEN (L), who held 

 sterilised flat glass dishes open, for a second, above the edge of a 

 milking-pail into which milk was being drawn from the cow, and 

 then introduced into the dishes some liquefied nutrient gelatin, 

 wherein the germs present in the basins developed into colonies, 

 and could therefore be counted. In this manner it was proved 

 that, in the space of one second, from 47 to 1210 germs, according 

 to circumstances, were deposited in an area of i square decimetre 

 (100 sq. c.m., or about 16 square inches). 



The amount of the germ-content is thus primarily determined 

 by the degree of contamination prevailing in the cowshed, a cri- 

 terion of which is afforded by the amount of dung constituents 

 present in the milk. This estimation was first attempted by 

 RENK (L), who found, for instance, in the market milk of Halle- 

 on-Saale, some 75 m.g. per litre ; in Berlin milk, 10 m.g. ; and in 

 Munich milk, 9 m.g. of such milk -dirt, the highest quantity 

 amounting, at Halle, to 0.362 grm. per litre. If the milk be 

 treated in the centrifugal machine, in order to remove the cream, 



