52 Agricultural Bacteriology. 



bottles. The water is heated to 150 F. and kept at 

 this temperature for twenty minutes. The bottles of 

 milk are then removed and cooled at once. 



In the pasteurizing process the bacteria that do not 

 form spores are killed, but the spores are not destroyed. 

 If the heated milk is not cooled rapidly and kept cold, 

 these spores will germinate and by their rapid growth 

 the milk will soon be rendered unfit for use. Only 

 small quantities of milk should be treated at one time, 

 no more than can be used in twenty-four hours. The 

 change which pasteurized milk undergoes will be dis- 

 cussed under milk fermentations, p. 58. 



In efficient pasteurization at least 99 per cent of the 

 bacteria should be killed. The better the quality of the 

 raw milk, the better will be the pasteurized milk. It 

 is not a process by which poor milk that has abnormal 

 odors or tastes can be made unobjectionable. 



Condensed milk. Condensed milk is prepared by 

 adding to the milk a quantity of cane sugar and evapo- 

 rating a part of the water in a vacuum until the milk 

 has the consistency of a thick syrup. This milk will 

 keep for the reason that its concentration is so great 

 that the bacteria tha<t it contains are unable to grow. 

 If it is diluted with water, they begin to grow and the 

 milk soon spoils. Unsweetened condensed milk is also 

 made. This has the consistency of thick cream and 

 keeps because the cans of milk have been sterilized in 

 steam sterilizers in exactly the same manner as canned 

 corn and peas are treated. 



Milk is also preserved by drying it until a powder is 

 obtained. The drying is done in a number of ways all 

 of which have been patented by the inventors. The 

 milk powder is used largely by bakers. 



