THE EXAMINATION OF MILK 425 



verting the soluble lime salts into insoluble ones, interferes 

 •with the coagulation of the milk by rennet, and causes the 

 fat globules to partially coalesce. 



A large number of special appliances have been invented 

 for the sterilisation of milk. They may be divided into 

 two classes : those that are arranged for dealing with the 

 milk in bulk, and those that are adapted for dealing with 

 small quantities of it in bottles. As regards the former the 

 most effective, and at the same time the most economical, 

 forms of apparatus are those in which the milk is treated 

 with steam under pressure, the storage vessels being filled 

 and closed in such a way as to make it impossible for any 

 micro-organisms to get in during the process. For house- 

 hold purposes it is sufficient to have an inner vessel to 

 contain the milk, and an outer one in which water is placed. 

 If there be a free outlet for the steam, and if the inner 

 vessel be not allowed to touch the bottom of the outer one, 

 the milk will not actually boil, since its boiling-point is 

 about 1"5° F. higher than that of water. 



The ' pasteurisation ' process consists in heating the milk 

 to 65° to 70° C. for a short time, and then rapidly cooling 

 it so as to prevent the rapid increase of the bacteria that 

 remain, and to preserve the fresh flavour of the milk. This 

 process simply destroys the non-sporing organisms, and 

 where it is only necessary to preserve the milk for a limited 

 time this process may be employed with advantage. 



Various forms of apparatus have been invented for the 

 pasteurisation of milk in large quantities. These are 

 chiefly of two types : in the one such as Thiel recommends 

 the milk is made to flow over a heated corrugated surface ; 

 and in the other, the vessels containing the milk are 

 surrounded by water, the temperature of which can be kept 

 at a definite point for a definite period. If pasteurisation 

 is to be effectual the milk 'must not only be heated up to a 



