Sweetened Condensed Milk — Condensing 77 



orating milk under reduced pressure, or in vacuo, are : economy of 

 evaporation, rapidity of evaporation, low temperature and large 

 capacity of apparatus. All of these features are essential in the suc- 

 cessful condensing of milk. 



Rapid evaporation cannot take place until the milk is brought 

 to the boiling point and is kept there until evaporation is completed. 

 Under atmospheric pressure and at the seal level, the boiling point 

 of water is 212 degrees F., the boiling point of milk is very slightly 

 higher, about 214 degrees F. Evaporating of milk under atmos- 

 pheric pressure in an open kettle, however, is a relatively slow 

 process, requiring a long time, much fuel and large apparatus. Fur- 

 thermore, exposure of the milk to 212 to 214 degrees F. long enough 

 to complete evaporation would render the product unsuitable for 

 market. The properties of some of its ingredients are altered, the 

 product would assume a dark color and a marked cooked flavor as 

 the result of the effect of heat. All of these objections are mini- 

 mized and partly avoided by lowering the boiling point of milk. 

 These objections, however, do not apply to evaporation under atmos- 

 pheric pressure by film treatment, as is the case with the Continuous 

 Concentrator described in Chapter XIV, page 133. 



Relation of Pressure to Boiling Point. — The temperature 

 at which milk boils depends on the pressure to which it is exposed. 



