THE DAIRY" 195 



changed into lactic acid by micro-organisms which are very 

 widely distributed. 



In the udder, under normal conditions, milk is free from 

 micro-organisms, but, unless special precautions are taken, in 

 a short time after milking it becomes highly charged with 

 them. The micro-organisms find their way into the milk 

 from the air, the hands of the milker, the teats and hair 

 of the cow, and, too often, from the vessel in which the milk 

 is collected. 



Milk, as drawn from the cow, is at a temperature highly 

 favourable to the multiplication of the micro-organisms, and 

 the number present after any given time is largely dependent 

 upon the temperature at which the milk is stored. Thus 

 milk stored for fifteen hours at 15° 0. was found to con- 

 tain about 100,000 bacteria per cubic centimetre; another 

 portion, stored for the same period at 25°, contained 

 72,000,000 organisms per cubic centimetre ; while a third 

 sample, kept at 35°, contained 165,000,000 per cubic centi- 

 metre. 



The micro-organisms which find their way into milk are of 

 various kinds; generally the lactic organisms predominate, so 

 that, usually, the first evidence of change is the production of 

 lactic acid — i.e., the milk becomes " sour." As the quantity of 

 lactic acid increases the casein is coagulated, and the milk is 

 said to " curdle." This generally occurs when the amount of 

 lactic acid reaches about 0-7 per cent., or with less acid if the 

 milk be heated. 



Other bacteria also sometimes find their way into milk, 

 some of them dangerous to the health of the drinker. Out- 

 breaks of typhoid, cholera, diphtheria, diarrhoea and other 

 diseases have been traced to contaminated milk. Tuberculosis 

 has also been proved to be conveyable by milk. Milk, too, has 

 a great aptitude to absorb gases and vapours, and, in conse- 

 quence, readily acquires odours and flavours from the air. 



The . necessity of perfect cleanliness in the cow-house and 

 dairy is thus evident, if the milk is to be kept sweet and pure. 



