HEAT 183 



essential that the pasteurized milk be cooled quickly and 

 stored at a low temperature. As milk is commercially han- 

 dled, not all of the acid-forming bacteria are killed; hence, 

 pasteurized milk sours as docs raw milk, but the process 

 is materially delayed. If heated at the higher pasteurizing 

 limits, all but the spore-forming bacteria will be destroyed. 

 Such milk will not sour, but decomposition changes will 

 occur; in which the casein and albumen will undergo a 

 change. 



Pasteurization of other liquid products is frequently em- 

 ployed. Beer is heated to low temperatures after it is 

 placed in bottles, to prevent the appearance of fermenta- 

 tions that might impart an undesirable taste and appear- 

 ance. The presence of alcohol and some of the extractives 

 from the hops tends to prevent the growth of the bacteria 

 that have not been killed by the heating. Wines are also 

 heated in a similar way to -overcome the turbidity that 

 sometimes results from bacterial changes. 



Such acid products as tomatoes, rhubarb, and grape juice 

 can be preserved by a ^hort exposure at the boiling-point. 

 This insures the destruction of all but the spores of bac- 

 teria, the development of which is prevented by the acid 

 reaction. To obviate subsequent infection, it is advisable 

 to treat such material after it has been placed in con- 

 tainers. Such a method has recently been introduced with 

 milk, but the attendant expense is such that it has not been 

 generally adopted by milk dealers, even though it would 

 insure complete immunity from milk-borne disease. 



Preservation by sterilization. The indefinite preserva- 

 tion of foods in closed containers has been rendered possi- 

 ble through the application of the process of sterilization. 

 On this principle rests the great development of the can- 

 ning industry, which has assumed such tremendous propor- 

 tions of late years. While it is impossible to raise the tern- 



