198 NORMAL MILK : ITS ADULTERATIONS, ETC. 



Condensed Milk. 



For couvenience of transport, milk is deprived of tte bulk of 

 its water by evaporation under diminished pressure in a vacuum 

 apparatus fitted with a condenser, or by heating to a low tempera- 

 ture and exposing a large surface to evaporation ; this is termed 

 condensed or evaporated milk. It is made in two forms : 

 sweetened condensed milk, which is a preparation of milk and 

 cane sugar ; and unsweetened condensed milk, which consists 

 of milk evaporated jjer se. 



The methods of manufacture are similar. In the manufacture 

 of sweetened condensed milk 1|- lbs. of cane sugar are added to 

 each gallon of milk, and the mixture heated to such a tempera- 

 ture that it will commence to boil at once on being admitted to 

 the vacuum pan. It is allowed to flow in slowly, the pump 

 being kept working the whole time, and no heat is applied till 

 all the milk is in the pan. By this procedure the gases of the 

 milk are drawn out, and on applying heat the milk boils without 

 frothing over. By carefully regulating the supply of heat to the 

 pan, and cold water to the condenser, the milk can be boiled at 

 an even rapid rate till sufficiently concentrated, a point which 

 can be easily told by an experienced operator. The whole oper- 

 ation is controlled by looking through a glass sight-hole let into 

 the upper portion of the pan. The finished product has a density 

 of about 1'28 and weighs one-third of the original milk ; it only 

 occupies three-elevenths of the original volume — i.e., 1 gallon of 

 milk is evaporated to 2J- pints. 



Commercial glucose is sometimes substituted for a part or the 

 whole of the cane sugar. 



Machines employing heated discs or rollers which dip into 

 the milk, and carry up thin layers on being rotated, or in which 

 the milk is exposed in shallow trays, are also used to condense 

 milk. 



Milk may be also concentrated by freezing and removing the 

 ice deposited. 



Composition of Sweetened Milk. — The following analyses 

 (Table L.) will show the composition of sweetened condensed 

 milks. 



The first three are condensed whole milks — i.e., the milk has 

 been evaporated without previous removal of the cream ; the 

 last four are condensed separated milk, the separated milk 

 having been mixed with cane sugar and evaporated. 



These milks are not usually sterilised, as the large amount of 

 dissolved matter and the small amount of water renders them 

 unsuitable for the development of micro-organisms ; they keep 

 for a long time without appreciable change. 



