MILK POWDERS. 401 



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 put, and on applying heat the milk boils without 

 frothing over. By 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 told easily 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, and by an apparatus for drawing 

 samples to test the density without breaking the vacuum. 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., I gallon of milk is evaporated to 2J pints. 

 The condensed milk is cooled as rapidly as possible and filled into 

 tins which are soldered down. 



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. 



Milk Powders. There are several methods of preparing milk 

 powders ; in the Just-Hatmaker process the milk is evaporated 

 on rollers heated by high-pressure steam above the temperature 

 of boiling water, and the dried layer of milk is taken off con- 

 tinuously by a knife set at a suitable angle. Other processes 

 employ evaporation on rollers or discs heated to a lower tem- 

 perature in vacuo, and the milk is preferably previously con- 

 centrated in a vacuum pan to one-third of its bulk. The milk 

 may also, after preliminary concentration, be dried in shallow 

 trays in vacuo, or on bands of wire gauze or other material. 

 Another excellent method is to atomise the milk by spraying, 

 and to evaporate this in a current of hot air. 



In order to ensure that the milk powder is soluble in water, 

 a small amount of alkali sodium carbonate, sodium phos- 

 phate, or saccharate of lime is sometimes added. To prevent a 

 separation of the fat in large particles on remixing the milk 

 powder, the milk is often homogenised before drying ; this has 

 the further advantage that the fat has less tendency to become 

 rancid. 



Preservatives. In order to check the growth of micro- 

 organisms in milk, and thus make it keep for a longer time than 

 it otherwise would, preservatives are frequently added. The 



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