FOODS 55 



or by saturation with neutral salts like sodium chloride. This, how- 

 ever, is not coagulation, but precipitation. The precipitate may be 

 collected and dissolved in lime water ; the addition of rennet then 

 produces coagulation in this solution, provided that a sufficient 

 amount of calcium salts is present. 



In milk also, rennet produces coagulation, provided that a 

 sufficient amount of calcium salts is present. If the calcium salts 

 are precipitated by the addition of potassium oxalate, rennet causes 

 no formation of casein. The process of curdling in milk is a double 

 one ; the first action due to rennet is to produce a change in caseino- 

 gen ; the second action is that of the calcium salt, which precipitates 

 the altered caseinogen as casein. In blood also calcium salts are 

 necessary for coagulation ; but there they act in a different way, 

 namely, in the production of fibrin-ferment (see COAGULATION OF 

 BLOOD). 



Caseinogen is not coagulable by heat. We have already classed 

 it with vitellin as a phospho-protein (see p. 44). 



Caseinogen, as was originally pointed out by Hammarsten, is a protein 

 with acid properties : it is quite insoluble in water, but it forms soluble 

 salts with such metallic bases as potassium, sodium, and calcium. The 

 caseinogen as it exists in 'milk is combined with calcium as calcium caseino- 

 genate. When acetic acid is added to milk,- we therefore get calcium acetate, 

 and a precipitate of free caseinogen. On ' dissolving ' this caseinogen in an 

 alkali like soda or potash, we have the formation of sodium caseinogen ate or 

 potassium caseinogenate, as the case may be. The precipitate obtained in 

 milk by the addition of alcohol, or by ' salting out,' is not free caseinogen, 

 but calcium caseinogenate. When we add potassium oxalate to milk, we 

 get the reaction represented in the following equation: Calcium caseino- 

 genate + potassium oxalate = calcium oxalate + potassium caseinogenate. 

 When we add calcium chloride to oxalated milk, the following equation 

 represents what occurs : Potassium caseinogenate + calcium chloride = cal- 

 cium caseinogenate + potassium chloride. 



Calcium caseinogenate forms an opalescent solution in w r ater, and reacts 

 with the rennin ferment. The caseinogenates of magnesium, barium, and 

 strontium have similar characters. The caseinogenates of potassium, sodium, 

 and ammonium differ from the above by forming a nearly clear solution in 

 water, and they do not react with the rennin ferment. (W. A. Osborne.) 



The Fats of Milk. The chemical composition of the fat of milk 

 (butter) is very like that of adipose tissue. It consists chiefly of 

 palmitin, stearin, and olein. There are, however, smaller quantities 

 of fats derived from fatty acids lower in the series, especially butyrin 

 and caproin. The old statement that each fat globule is surrounded 

 by a membrane of caseinogen is, according to Eamsden's recent 

 work, correct. Milk also contains small quantities of lecithin, a 

 phosphorised fat ; of cholesterin, an alcohol which resembles fat in 

 its solubilities (see BILE) ; and a yellow fatty pigment or lipochrome. 



