THE PHYSICAL PROPERTIES OF MILK 65 



ing for some time. The viscosity can be restored after milk has 

 been heated by addition of "viscogen," first recommended by Bab- 

 cock and Russell. Viscogen is prepared by mixing a concentrated 

 solution of cane-sugar with freshly slaked lime. After the mix- 

 ture has stood for some time the clear liquid is poured off. One 

 part of viscogen in 100 to 150 parts of milk or cream will have 

 the desired effect. Viscogen reunites the fat globules into clusters 

 which were broken up by heat. 



Under some conditions, such as the presence of slime-forming 

 bacteria or pathologic conditions of the udder, accompanied by 

 pus formation, the viscosity of milk may increase considerably. 

 The "tatt6 melk" of the' Norwegians is thick and slimy, due to 

 microbial activity, and is eaten with a spoon. The "lange wei" 

 of the Hollanders is also an example of sUmy milk due to the 

 action of a streptococcus. 



Viscosity is measured by allowing the milk or cream to flow 

 from a pipet and then comparing the time required for discharge 

 with the time required to discharge an equal volume of water 

 under the same conditions. Another method used is to let a few 

 drops of millv flow down a smooth piece of glass inclined at an 

 angle and then measure the hne formed in a definite period of time. 



Specific Gravity. — The specific gravity of milk from difi'erent 

 animals is not the same, and since the solids — especially the fat 

 — vary in different breeds of cows, we would naturally expect to 

 find a corresponding variation in specific gravity. The specific 

 gravity of mixed milk is more uniform than that from individual 

 cows. The specific gravity of cow's milk is usually given as 1.027 

 to 1.034 at 15° C. Human and goat's milk have about the same 

 figure as cow's milk. The specific gravity of the total solids of 

 cow's milk is 1.3 to 1.4; that of the plasma solids, 1.6. 



The specific gravity is conditioned by the presence of dissolved 

 substances, casein and fat. While dissolved substances and ca- 

 sein increase the specific gravity, fat decreases it. Milk rich in 

 fat, therefore, has a lower specific gravity than milk poor in fat. 

 The specific gravity of skimmed or separated milk always exceeds 

 that of whole milk, while cream has a lower specific gravity. 

 There is a fairly definite relation between the specific gravity of 

 milk, the percentage of fat, and the plasma solids, so that when 

 two of these factors are known the third can be calculated. The 

 values obtained by this method of calculation are useful for rapid 

 determinations, but are only approximately accurate. 

 - The table on page 66 shows the decrease of the specific gravity 

 when the percentage of fat increases. The specific gravity was 

 determined by Bowen at 20° C, and is expressed in terms of water 

 at the same temperature: 



