GERMS IN RELATION TO MILK 



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■other harmful germs so that it rots or putrefies when old. A low- 

 temperature (4b F.) also retards the development of the lactic 

 -■acid germs and they are killed when the milk, or cream becomes 

 very sour (when the formation of lactic acid reaches 0.8-2.0 per 

 cent), by means of the lactic acid they themselves produce. The 

 action of these lactic acid germs is taken advantage of in the ripen- 

 ing of cream for butter by adding them in great numbers, either by 

 the use of sour cream or milk, or by laboratory methods by which 

 they can be obtained in pure culture — that is, free from admixture 

 with other varieties of germs (see page 59). Lactic acid germs are 

 not found in milk when it leaves the udder, but enter the milk when 

 it is exposed to air. They are thought to reside on the skin of the 

 cow, in dust, in the air or surroundings of the barn. But milk 

 utensils — unless sterilized by steam or boiling water — are the chief 

 means of supplying milk with lactic acid germs. This happens 

 "because sour milk germs lurk in the corners and rough surfaces and 

 crevices of milk vessels. So, from the milk pail to the shipping can 

 •or milk bottle, each and every utensil adds its quota of lactic acid 

 germs — unless the utensils have been thoroughly washed and heated 

 for some time to the boiling point. Ordinary market milk at 50 F. 

 ,sours in 120 hours; at 6o° F. it sours in 66 hours; at 98 it sours 

 in 16 to 18 hours. 



There are two types of lactic acid germs : ( 1 ) The more com- 

 mon (B. lactis acidi or Streptococcus lacticus *) constitutes on the 

 average about 90% of all the germs in milk. It grows best without 

 air (anaerobic), and so milk sours best in deep vessels. (2) The 

 less common type of lactic acid germ is that which causes gassy 

 milk and cheese (B. lactis aerogenes) and is derived also from the 

 dirt of the cow and does not come from the udder. Certain germs 

 capable of producing disease in man also sour milk. 



Of these the colon bacillus is derived from manure of the cow. 

 And the germs causing inflammation of the udder (streptococci) in 



* Streptococcus lacticus rightly forms a third group of lactic acid bacteria. It 

 is the commonest organism souring milk, it differs from lactic acid bacteria (which 

 -are oval, in pairs or short chains) in appearance and is indistinguishable from 

 -disease-producing streptococci (S. pyogenes) but is harmless to man occurring in 

 aincontaminated milk from healthy cows. 



