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ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 103 
Grosser, Biochem. Zeit., 48, 427 (1913) tcok a sample of milk 
and filtered it through a Bechold filter under a six atmospheres 
pressure of nitrogen, thus removing the colloidal constituents. The 
filtered milk was analysed raw, and also after being boiled for 
15 minutes. The CaO content of the raw filtered milk was 23-4 
per cent. of the initial (unfiltered) sample, and after 15 minutes 
boiling, only 18-0 per cent., or a loss due to heating of 5-4 per cent. 
of CaO. Repeating this experiment with butter-milk he found no 
change in the CaO content of the raw and the boiled filtrate, and 
inferred that the phosphorus and calcium had been spilt off from 
the casein owing to the acidity. 
When milk is boiled the decrease in the calcium content is 
accompanied by a rise in the hydrogen ion concentration; see 
Milroy, Biochem. Jour., 9, 215 (1915). Approximately 8 per cent. 
of the total crude protein of fresh milk is not recovered by acid 
precipitation or heat coagulation. Cf. Palmer, J. Ass. Off. Agric. 
Chemists, 2, part I., 144 (1916). 
Rennet Coagulation. 
When rennet is added to milk a separation into a precipitate 
(curd) and liquid (whey) takes place ; the casein is transformed to 
a dyscaseose, “curd” being the insoluble calcium compound of © 
this, and to a caseose (soluble). Rennet can clot 400,000 times its 
weight of casein in milk (Hammarsten), but it is a most remarkable 
thing that shaking the rennet renders it inactive. Cf. Schmidt- 
Nielsen, Zeit. f. physivl. Chem., 60, 426-62 (1909). An aqueous 
Solution of rennet can be separated into fractions, having varying 
capacities for curdling milk by filtration through porous clay. See 
Korschun, Zeit. f. physiol. Chem., 37, 366-76 (1902). 
Rennet acts best at 41° C., and the curd is then very firm; a 
fluffy, soft curd results at temperatures between 15° C. and 20° C. 
(at which temperature only 18 per cent. of the milk is curdled), and 
also ai 50° ©. (where 50 per cent. of the milk is curdled). The 
optimum range of temperature is from 35° to 45°. 
Rennet is rendered inactive by prolonged storage, by shaking, by 
heating to temperatures over 60° C., and by addition of alkalies. 
- When milk is heated the action of the rennet is delayed, owing 
to the precipitation of calcium, but if CaCl» be added, the coagulation 
occurs in the normal way. Milk deficient in calcium salts requires 
a longer time for rennet-coagulation to take place. Cf. Sdéldner, 
Landw. Versuchs. (1888), p. 351; Ringer, Journal of Physiology, 
11, 464 (1890) ; Conradi, Wiinch. med. Wochensch., 48, 175 (1901) ;: 
Kreidl and Lenk, Biochem. Zeit., 86, 357 (1911). 
Interesting observations were made by Rupp, who observed 
that curdling by rennet was more rapid than with raw milk at 
temperatures between 55° C. and 65° C., but slowed down at about 
70° ©. to nearly double the time; in this case, however, a finer 
grained coagulum resulted. Cf. Bureau of Animal Ind. Bull., 
p. 166 (1913). 
Although boiled milk is not curdled by rennet, a coagulation 
occurs on the addition of small amounts of CaCl, Lindet (Comptes 
Rendus, 157, 381 (1913)), suggests that this is due to the interaction 
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