1640 The Cornell Reading-Courses 



1. Variation due to breed. This fact is well known and needs no dis- 

 cussion. 



2. Variation due to age of the cow. As the cow grows older, the per- 

 centage of fat decreases. 



3 . Variation due to physical condition of the cow — whether she is 

 sick or well. It cannot be predicted whether the percentage of fat will 

 increase or decrease. 



4. Variation due to period of lactation. The percentage of fat increases 

 as the period of lactation advances. 



5. Variation between animals of the same breed. This is to be expected, 

 since individuals always vary. 



6 Variation in different parts of the udder. The first milk drawn is 

 very poor in fat and the last milk drawn is very rich in fat. Often there 

 will be a variation of one to nine per cent between the first and the last 

 milk. 



Milk fat compares well with other fats in digestibility. 



Casein 



Casein, popularly called " curd," exists in milk in a colloidal state, 

 that is, in a state of very fine suspension. The particles of casein are in 

 much finer suspension in milk than are the fat globules; the fat globules 

 may be seen with the high-power microscope, but the casein cannot be 

 seen at all. Casein is held in suspension by calcium salts, and when any- 

 thing forms in milk or is added to it which unites with the calcium salts 

 the casein is thrown down. This is what happens when milk sours naturally. 

 Acid forms in the milk, uniting with the calcium salts, and the casein 

 comes down in a mass. 



Casein forms a considerable part of the various kinds of cheese. Amer- 

 ican cheddar cheese is about one third casein. In either milk or cheese, 

 casein is a very excellent food. It is high in nitrogen content; and this 

 is one reason why whole milk and skimmed milk are excellent foods for 

 growing animals. 



Milk casein has many important commercial uses. It is used to finish 

 writing-paper, much of our finest stationery being finished with it. It 

 is used as a substitute for bone and celluloid in the manufacture of buttons, 

 backs of brushes and combs, and many toilet articles, and is employed 

 as a basis in the manufacture of cold-water paints and of glue. 



Milk sugar 

 Milk contains about 5 per cent sugar and this percentage is fairly con- 

 stant. The sugar is in solution; it is not quite so soluble as ordinary cane 

 sugar, and not so sweet. Milk sugar is used commercially in the manu- 



