176 CONTROL OF THE MILK-SUPPLY. 
protect the public, but to keep the milk from souring. Milk 
distributers have found it difficult to furnish milk that will keep 
without preservatives, but have learned that the application of heat 
enables them to do so. For this reason pasteurization has become 
adopted by some large milk companies. 
Pasteurization is sometimes applied to cream, to enhance its 
keeping and enable it to find a market. The cream keeps well, 
but loses some of its consistency. It appears thinner than before 
treatment, and will not whip so well as ordinary cream. Its consist- 
ency may be restored by adding a little of a material called viscogen. 
This is made by adding a strong solution of cane-sugar to freshly 
slacked lime, and allowing the mixture to stand until the upper part 
of the mixture is clear. This clear liquid is poured off and added 
to the cream in the proportion of one part to one hundred or one 
hundred and fifty parts of cream. This restores the consistency to 
the cream, but, since it is an addition of a foreign substance, its 
use is illegal. 
In our larger cities a considerable part of the milk on the market 
is pasteurized; sometimes it is sold as pasteurized milk and some- 
times it is not so labeled. 
Preparations of Milk. The microorganisms that spoil milk 
will not grow in it if the water is removed, and several methods have 
been devised for producing a form of milk that will keep, all of which 
are based upon the removal of the water. Condensed milk is the 
oldest and has a wide use. It consists of ordinary milk evapo- 
rated to about one-third of its original bulk, to which is com- 
monly added a large amount of sugar. The sugar prevents the 
growth of bacteria, and this condensed milk, put up in cans, keeps 
well. In some forms of condensed milk the sugar is not added, but 
the product is preserved by sterilizing by heat. When subsequently 
diluted with water, condensed milk does not exactly replace the 
fresh article, because of the added sugar in the one type and the 
effect of sterilization in the other. A product known as concen- 
trated milk has recently been placed on the market. In this case 
the milk is first skimmed and then subjected to a heat of 140 F. 
till enough water is evaporated to bring the milk to about one-fifth 
