DOMESTIC PASTEURISATION 537 



3. All cans, pails, bottles, and other utensils to be thoroughly- 

 sterilised in steam before use. 



4. The prepared milk to be placed in sterilised bottles and 

 sealed up. 



The quality of the milk to be pasteurised is an important point 

 All milks are not equally suited for the purpose, and those con- 

 taining a large quantity of contamination, especially of spores, are 

 distinctly unsuitable. Such milks, to be purified, must be sterilised. 

 Dr Russell has laid down a standard test for the degree of con- 

 tamination which may be corrected by pasteurisation by estimating 

 the degree of acidity ; a low acidity {e.g. cyT. per cent) usually 

 indicating a smaller number of spore-bearing germs than that 

 which contains a high percentage of acid. 



Lastly, while the heating process is, of course, the essential 

 feature of eflficient pasteurisation, it must not be forgotten that 

 rapid and thorough cooling is almost equally important. As we 

 have seen, pasteurisation differs from complete sterilisation in that 

 it leaves behind a certain number of organisms or their spores. 

 Cooling inhibits the germination and growth of this organismal 

 residue. If, after the heating process, the milk is cooled and kept 

 in a refrigerator, it will probably keep sweet from three to six 

 days, and may do so for three weeks. 



Domestic pasteurisation. — For domestic use there are various 

 forms of apparatus. Many of them are constructed on the water- 

 bath principle of one vessel containing milk, inside another con- 

 taining water. Some are steam jacketed pans with revolving 

 stirrers ; others have automatic arrangements for the purpose of 

 avoiding a boiling temperature, and adapted for use with fire, gas 

 burners, or spirit lamps. 



In the opinion of those best able to judge, the Aymard Milk 

 Steriliser is one of the most satisfactory and efficient of household 

 sterilisers. It is made of joUed copper heavily coated with tin, and 

 constructed on the water-bath principle, the milk being placed in 

 the inner vessel. Hot water to the depth of about \ inch is placed 

 in the lower saucepan, covered with the lid and allowed to boil. 

 Next the lid is removed, and the milk chamber (containing the 

 requisite quantity of milk) placed in position, the large lid on the 

 top. Steam will be given off freely from the lid ; in the i pint 

 size in about ten minutes. To avoid any formation of scum, the 

 milk must be cooled by placing the upper portion of the steriliser 

 in a basin of cold water, care being taken not to remove the inner 

 lid until the milk is cooled. This will take, for the i pint size, 



