PROPERTIES OF THE CURD OF MILK. 561 



are of less interest or importance to you. It is necessary only, to a 

 clear understanding of the kind of changes which take place when butter 

 becomes rancid, that I should exhibit to you the formulae by which these 

 acid bodies are severally represented : — 



Butyric acid = Cg Ug O4 



Caproic acid = C12 Ho O3 



Capric acid = Cis H14 O3 

 We shall see how these substances are produced from the solid and 

 fluid fats of butter, when we come to treat of the preservation of butter. 



§ 14. Of casein or the curd of milk and its properties. 



The casein or cheesy matter of milk may be obtained nearly pure by 

 the following process : — Heat a quantity of milk which has stood for 5 

 or 6 hours, jis if you intended to prepare clouted cream (p. 548), let it 

 cool, and separate the cream completely. Add now to the milk a little 

 vinegar and heat it gently. The whole will coagulate, and the curd will 

 separate. Pour off" the whey, and wash the curd well by kneading it 

 with repeated portions of water. When pressed and dried, this will be 

 casein sufficiently pure for ordinary purposes. It may be made still 

 more pure by dissolving it in a weak solution of carbonate of soda, al- 

 lowing the solution to stand for 12 hours in a shallow vessel, separating 

 any cream that may rise to the surface, again throwing down the curd 

 by vinegar, washing it frequently, and occasionally boiling it with pure 

 wat'='-r. By repeating this process two or three times, it may be obtained 

 almost entirely free from the fatty and saline matters of the milk. 



Casein thus prepared, reddens vegetable blues, and is therefore a 

 slightly acid substance. It is very sparingly soluble in water — 400 lbs. 

 of cold water dissolving only 1 lb. of pure casein (Rochleder). It dis- 

 solves readily, however, and in large quantity, in a weak solution of the 

 carbonate of potash or of soda, and to some extent even in lime-water. 

 These solutions are coagulated by the addition of an acid — of sulphuric 

 acid, of vinegar, or of lactic acid — and the curd readily separates on the 

 application of a gentle heat. If a large quantity of acid be added, a por- 

 tion of the casein is re-dissolved. This property of dissolving in weak 

 alcaline (potash or soda) solutions, satisfactorily explains what takes 

 place during the curdling of milk, as we shall hereafter see (p. 567). 



The casein of milk is identical in chemical constitution with the fibrm 

 of wheat, the legumin of the pea and bean,* and the albumen of the 

 egg or of vegetahle substances. Hence the opinion has naturally arisen 

 among chemists, that the cheesy matter contained in an animal's milk is 

 derived directly, and without change, from the food on which it lives. 

 The probability of this opinion will cx)me naturally under our considera- 

 tion in the following lecture. (See next lecture, " On the feeding of 

 stock.'') 



Casein possesses still one property more remarliable than any of its 



• In page 394 it is stated, on the authority of Dumas, that the legumin of the pea and bean 

 differs in composition from fibrin and albiin>en Since that sheet was published, it appears, 

 from the experiments of Rochleder (Annal. dor Cheni. und Pharm., xlvi., p. 162), that the 

 legumin which Dumas extracted from the almond, analysed, and supposed to be identical 

 with the legumin of the bean and pea, is not so, but is in reality a different substance ; an4 

 thatlhe legumin of peas does agree in composition with the casein of milk. 



