330 DIGESTION OF MILK. [BOOK n. 



known that an infusion of calves' stomach, called rennet, has a 

 remarkable effect in rapidly curdling milk, and this property is 

 made use of in the manufacture of cheese. Gastric juice has a 

 similar effect ; milk when subjected to the action of gastric juice 

 is first curdled and then digested. If a few drops of gastric juice 

 be added to a little milk in a test-tube, and the mixture exposed 

 to a temperature of 40, the milk will curdle into a complete clot 

 in a very short time. If the action be continued the curd or clot 

 will be ultimately dissolved and digested. Milk contains, besides 

 a peculiar form, or peculiar forms of albumin, fats, milk-sugar and 

 various salines, the peculiar proteid casein. In natural milk 

 casein is present in solution, and ' curdling ' consists essentially 

 in the soluble casein being converted (or more probably as we 

 shall see presently, split up) into an insoluble modification of 

 casein, which as it is being precipitated carries down with it a 

 great deal of the fat and so forms the ' curd.' Now casein is readily 

 precipitated from milk upon the addition of a small quantity of 

 acid, and it might be supposed that the curdling effect of gastric 

 juice was due to its acid reaction. But this is not the case, for 

 neutralised gastric juice, or neutral rennet, is equally efficacious. 



The curdling action of rennet is closely dependent on tempera- 

 ture, being like the peptic action of gastric juice favoured by a rise 

 of temperature up to about 40. Moreover the curdling action 

 is destroyed by previous boiling of the juice or rennet. These 

 facts suggest that a ferment is at the bottom of the matter ; and 

 indeed, all the features of the action support this view. More- 

 over, as a matter of fact, a curdling ferment may be extracted by 

 glycerine and by the other methods used for preparing ferments. 

 The ferment, however, is not pepsin, but some other body ; and 

 the two may be separated from each other. 



It might be thought that the rennet-ferment, rennin we may 

 call it, acted by inducing a fermentation in the sugar of milk, 

 giving rise to lactic acid which precipitated the casein by virtue 

 of its being an acid. But this view is disproved by the following 

 facts which shew that the ferment produces its curdling effect by 

 acting directly on the natural casein itself. Casein may be pre- 

 cipitated unchanged, that is, capable of redissolving in water (the 

 presence of calcic phosphate being assumed) by saturating milk 

 with neutral saline bodies (such as sodium chloride or magnesium 

 sulphate) ; and by being precipitated and redissolved more than 

 once may be obtained largely free from fat and wholly free from 

 milk-sugar. Such solutions of isolated casein freed from milk- 

 sugar may be made to curdle like natural milk by the addi- 

 tion of rennin, shewing that the milk-sugar has nothing to do 

 with the matter. Moreover the precipitate thrown down from 

 milk by dilute acids, lactic acid included, is itself unaltered or 

 very slightly altered casein not curd, and with care may be 

 so prepared as to be redissolved into solutions which curdle 



