50 



water they are killed, but their spores, which correspond to seeds, may 

 endure even this temperature tor some time, though higher heat kills 

 them speedily. 



Bacteria grow with the greatest readiness in milk and cream. 

 Hence they collect in milk and cream exposed to the air, and multiply 

 rapidly. 



A large number of different species of bacteria are found in milk 

 and cream. Different species have different effects. Many of them sour 

 and curdle milk at some temperature. A few induce changes that render 

 milk alkaline with or without the formation of a curd. When a curd is 

 formed, it differs in character with different species of bacteria. The sour- 

 ing of milk is more complex than has been supposed ; and while without 

 much doubt souring always depends upon the action of bacteria, any one 

 of a number of species, or several combined, may be the cause. 



The longer a specimen of milk has been exposed to the action of 

 bacteria, other things being equal, the greater will be the number of 

 bacteria present. Hence it follows that cream will usually contain a 

 very large number. The presence of these organisms, so far from being 

 injurious, is of a positive advantage to the butter maker, since it is by 

 their action that cream is " ripened." 



Vessels in which milk and cream are to be kept are a great source 

 of contamination by bacteria. The latter gather upon the sides and in 

 the joints, and develop in the minute portions of milk, grease, or other 

 matters from which it is difficult to free the wulls of the vessels com- 

 pletelv by washing. 



Two important points in the handling of milk and cream are 

 brought cut by these considerations : 



First.— The importance of keeping milk, so far as possible free 

 from bacteria by the exercise of the greatest cleanliness. 



Second. — The importance of cooling milk immediately after it is 

 drawn from the cow in order to prevent the souring as long as possible. 



Bacteria in Cream. 



Besides the ordinary souring of milk, there are many other changes 

 which are produced by bacteria, as the ripening of cream, the ripening 

 of cheese, butter becoming rancid and others less common. 



The chief object of the ripening of cream is to produce the butter 

 aroma and flavour which, though very evanescent, control the price of 

 the butter. This aroma and flavour the butter-maker owes to the bacteria; 

 for by their growth the materials in the cream are decomposed and 

 the compounds formed which produce the flavours and odours of high 

 quality butter. 



Different species of bacteria vary much as to the flavours which 

 they produce, some inducing good, some extra fine, and others a very 

 poor quality of butter. A majority of our common dairy species pro- 

 duce good, but not the highest quality of butter. Up to the present 



