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 temperature, and become in- 

 corporated 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 (I.), 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. cm., 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 criterion of which is afforded by 

 the amount of dung constituents present in the milk. This estimation was first 

 attempted by RENK (I.), who found, for instance, in the market milk of Halle- 

 on-Saale, some 75 mg. per litre; in Berlin milk, 10 mg. ; and in Munich milk, 

 9 mg. 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, the dirt is separated, and collects, along with casein, bacteria, &c., as 

 a coating, sometimes granular, at others mucinous, on the walls of the inner 

 drum, and is generally known as milk-sludge. It consists of about 26 per cent, 

 of albuminoids, 67 per cent, of water, &c., and is relatively much richer in 

 bacteria than the milk. O. WYSS (I.) was the first to investigate this mud, and 

 the quantity separated was found, in a case examined by NIEDERSTADT (1.), to 

 amount to 43 grms. per hectolitre (slightly more than 30 grains per gallon). 



The connection between the content of dirt and of germs in milk was shown 

 in particular by UHL (I.), a few of whose figures are now given : 



Dirt. Number of Germs. 

 Sample No. Mg. per Litre. Per c.c. 



1 36.8 12,897,600 



3 ' . .20.7 7,079,820 



6 -5-2 3.338,775 



