CHAPTER II. 



CASEIN : ITS ORIGIN, PEEPARATION AND PROPERTIES. 



THE milk of mammiferous animals, from which casein is de- 

 rived, is a normal secretion of the healthy lacteal glands of the 

 female animal. After the flow of colostrum .(the first milk 

 secretion after parturition) has ceased, normal milk is produced 

 for a longer or shorter time, known as the period of lactation. 

 As a general thing the milk of mammals is an opaque, whitish 

 or faintly yellow liquid, exhibiting a peculiar specific taste and 

 smell. It chiefly consists of water, fat, casein, albumen, milk 

 sugar and inorganic constituents, the most important being 

 the fat and casein. Cows' milk is the chief variety, and con- 

 sists in solid matter, according to Kirchner, of : 



Mean. Limits. 



Fat ... 3-4 per cent. 0-8 to 8*0 per cent. 



Casein 3'2 2-0 4-5 



Albumen . . 0'6 ,, 0-2 0'8 



Lactoprotein . Ol ,, 0'08 ,, 0-35 



Milk sugar . . 4'5 0*3 6'0 



Ash ... 0-7 0-6 0-9 



Milk must be regarded as an emulsion, i.e., a liquid contain- 

 ing other substances in suspension in a very finely divided state, 

 and we must assume that the fat globules the future butter 

 for instance, are surrounded by liquid envelopes. The liquid 

 fat globules attract substances dissolved in the milk and con- 

 dense the same around themselves, so that we must consider 

 every fat globule as surrounded by an envelope richer in dis- 

 solved substances than the milk itself. 



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