64 MILK 



the fluid becomes opaque and white. The surface of small glob- 

 ules is larger in proportion to the size than that of large globules. 

 Small globules, therefore, have a larger reflecting surface than 

 large ones. It follows that the smaller are the globules, the whiter 

 is the milk; the larger the globules, the more pronounced is the 

 tendency toward yellow, other conditions being equal. The color 

 of milk or cream is, therefore, not necessarily an indication of 

 richness in fat. 



The colloidal casein also contributes toward the opacity and 

 whiteness of milk. Separator milk, containing but 0.1 per cent, 

 fat, is still opaque and white, and a 3 per cent, solution of pure 

 casein in lime-water resembles skimmed milk in appearance. So- 

 lutions of casein in alkalies are more translucent than solutions 

 in alkaline earths. The cream separates almost completely in 

 hviman milk, still the plasma is white and opalescent, showing that 

 the casein is in part responsible for the color. 



Boiled milk is more yellow than raw milk, due to protein 

 decomposition. Addition of boiled milk to skimmed milk may, 

 therefore, restore the yellow color and cause it to resemble whole 

 milk. 



Odor and Taste. — Fresh clean milk has little odor. The 

 "cowey" odor usually given off by market milk is due to admixture 

 of cow manure and absorption of odors from the air. Milk, 

 especially when warm, absorbs odors rapidly and retains them 

 tenaciously. When milk is heated the odors are quickly removed, 

 a circumstance that accounts for the fact that pasteurized milk 

 has a sweeter smell than raw milk. Part of the odor is due to 

 volatile acids in the milk-fat. Cream, therefore, has a stronger 

 odor than whole or skimmed milk. 



The taste of clean milk is sweet and aromatic. The sweetness 

 is due to the milk-sugar, while the aromatic taste comes from the 

 fat. The "cowey" taste of market milk is due to manure and 

 ^absorbed impurities. 



Viscosity and Cohesion of Milk. — Cool milk has greater vis- 

 cosity and cohesion than warm milk; therefore, cold milk will 

 adhere to the walls of a vessel more closely and in larger quantity 

 than warm milk. Cold cream holds foam better and is easier to 

 whip than warm cream. Heating reduces the viscosity and ul- 

 timately destroys it if the temperature is high enough. Con- 

 sequently, the pasteurized product is not as viscous as raw milk 

 and more so than boiled milk. According to Bowen, the vis- 

 cosity of milk at 30° C. is about 1.7 times as great as that of water, 

 and at 0.° C. its viscosity is 2.6 times as great as that of water. 

 At 0° C. the viscosity of milk is about 2.6 times as great as at 

 30° C. Fresh milk has less viscosity than milk that has been stand- 



