Condensed Milk and Milk Powder 97 



further disadvantage of causing the cans to bulge badly, owing to 

 the difference in pressure between the interior and exterior of the 

 .cans. This is especially noticeable in gallon-size cans, the ends of 

 which may become badly distorted, present an unsightly appearance 

 and their seams and 1 seals may be weakened' to> the extent of pro- 

 ducing "leakers." 



Fractional Sterilization. — In some factories fractional steril- 

 ization is occasionally practiced. The milk is heated in the sterilizer 

 to considerably 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 dur- 

 ing 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 manu- 

 facture. 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 temperatures 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 temperature 

 of 269 degrees F. In the evaporated milk, made from perfectly nor- 

 mal and sweet, fresh milk the casein curdles at much lower tempera- 

 tures, and the higher the ratio of concentration, the lower the tem- 

 perature required to precipitate the casein. It seems that the con- 

 centration of the milk intensifies the properties of milk to coagu- 

 late when subjected to heat. This factor is probably largely, though 

 not necessarily, wholly due to the increase of the per cent, of lactic 

 acid in the evaporated milk, due to the concentration. If the fresh 

 milk contains .17 per cent, lactic acid, a concentration of two and 



