126 EVAPORATED MILK SHAKING 



torted, present an unsightly appearance and their seams and 

 seals may be weakened to the extent of producing "leakers." 



Fractional Sterilization. In the early days of the manu- 

 facture of evaporated milk the product was sterilized by frac- 

 tional sterilization. This method has now been largely abandoned, 

 but is occasionally used when the milk happens to be in very 

 abnormal condition. The milk is heated in the sterilizer to con- 

 siderably lower temperatures than those stated above, and this 

 heating is repeated on two or three successive days. The prin- 

 ciple of this process is to kill all vegetative forms of bacteria 

 during the first heating. This gives the spores a chance to 

 develop into vegetative forms by the second and third days, 

 which forms are then destroyed during subsequent heating. This 

 system of sterilization is not practical for general use. It is too 

 great a tax on the capacity of the average factory and increases 

 the cost of manufacture. It should, therefore, be made use of 

 only in exceptional cases, when it is known that a certain batch 

 of milk could not be put through the higher sterilizing tem- 

 peratures without causing the product to become permanently 

 curdy. 



SHAKING 



Purpose. The purpose of shaking the evaporated milk is to 

 mechanically break down the curd that may have been formed 

 in the process of sterilization and to give the contents of the cans 

 a smooth and homogeneous body. 



The high temperatures to which the evaporated milk is sub- 

 jected in the sterilizer have a tendency to coagulate the casein. 

 In the case of normal, fresh milk the casein coagulates at a tem- 

 perature of 269 degrees F. In the evaporated milk, made from 

 perfectly normal and sweet, fresh milk the casein curdles at much 

 lower temperatures, and the higher the ratio of concentration, 

 the lower the temperature required to precipitate the casein. It 

 seems that the concentration of the milk intensifies the properties 

 of milk to coagulate when subjected to heat. This factor is 

 probably largely, though not necessarily, wholly due to the in- 

 crease of the per cent of lactic acid in the evaporated milk, due 

 to the concentration.* If the fresh milk contains .17 per cent 



