334 CREAM 



essentially from lactose ; e.g., the sugar in mares' milk is easily sus- 

 ceptible to alcoholic fermentation. According to Eichmond, 1 the milk 

 of the gamoose, or Egyptian water-buffalo, contains a sugar distinct 

 from lactose. This, however, has been denied by Porchers. 2 



MILK PRODUCTS. 



Many valuable products are derived from milk, the most import- 

 ant being the following : 



1. Cream and skim-milk. 



2. Butter and butter-milk. 



3. Cheese and whey. 



4. Condensed milk. 



5. Koumiss. 



6. Kephir. 



Cream. The fat globules of milk, being lighter than the liquid 

 in which they are suspended, tend to separate and collect near the 

 surface of the milk, when the latter is allowed to remain motionless 

 under the action of gravity. The rapidity with which the separation 

 of the milk into two layers one rich in fat globules and the other 

 almost devoid of them occurs, depends upon many conditions. One 

 of the most important is the size of the globules. The milk of Guernsey 

 or Jersey cows quickly throws up its cream owing to the large size of 

 the fat globules, while that of Ayrshire cows is slow in yielding cream. 



Another factor of importance is the difference in the magnitude of 

 the forces acting upon the aqueous and fatty portions of the milk. 

 Under ordinary conditions this (depending upon gravitation) is practi- 

 cally constant (though differing to a small extent according to the 

 latitude) ; but, by imparting rapid rotation, centrifugal force of far 

 greater magnitude than the force of gravitation can be brought to bear 

 upon the milk. The separation of the lighter fat from the heavier 

 aqueous portion then becomes very rapid. This is the principle of 

 the milk separators which are coming so rapidly into use. For details, 

 the reader is referred to any modern treatise on dairy work. 



Another method of facilitating the separation of cream is known as 

 "deep setting," in which the milk, while yet warm, is placed in cans 

 about 18 in. deep, which are then surrounded with cold water or, 

 better, ice. In this case, the whole of the fat will be found on the sur- 

 face after about twelve hours. 



It is not quite easy to understand why the fat globules should 

 collect at the surface more quickly when the milk is thus cooled than 

 under ordinary conditions. As fat contracts and expands with changes 

 of temperature more rapidly than water, a low temperature would 

 tend to lessen the buoyancy of the fat globules, and on that account 

 tend to lengthen the time necessary for their coming to the surface. 



The writer is of opinion that the explanation of the action observed 

 is to be found in the gentle convection currents which are set up by 

 the cooling action of ice or water on the walls of the can. The milk 

 in contact with the walls of the vessel, as it cools, becomes heavier and 



1 Jour. Chem. Soc., 1890, Trans., 754. -Bull. Soc. Chim., 1903, 828. 



