Elements of agricultural chemistry and geology / [Jas. F.W. Johnston].

  • James Finlay Weir Johnston
Date:
1844
Catalogue details

Licence: Public Domain Mark

Credit: Elements of agricultural chemistry and geology / [Jas. F.W. Johnston]. Source: Wellcome Collection.

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    connection with the separation of the curd. It is, in fact, the cause of the curdling. 2. But, in order to understand how this is, we must turn for a moment to the properties of the curd itself. When the curd is separated carefully from the whey, it may be washed or even boiled in water, without being sensibly lessened in quantity. Pure curd is nearly in- soluble in pure water. But if a little soda be added to the water in which the curd is heated, it will dissolve and disappear. Pure curd is soluble in a solution of soda. If to the solution of the curd in soda and water a quantity of the acid of milk be added, this acid ^\^ll com- bine with the whole of the soda,—will take it from the curd, which will thus be again separated in an insoluble state. The curd is insoluble in luater, rendered sour by the acid jof milk. These facts explain very clearly the curdling of milk. As it comes from the cow, milk contains a quantity of soda not combined with any acidfy which soda the curd is held in solution. As the milk becomes sour, this soda combines with the lactic acid produced, and thus the curd becoming insoluble separates from the whey—or the milk thickens and curdles. Now the effect which is thus produced by the natural formation of lactic acid in the milk, may be brought about by the addition of any other acid to it—such as vine- gar, or spirit of salt. And, in fact, vinegar is used now, in some countries, and in ancient times was used more ex- tensively for curdling milk; wdiile in some of the cheese districts of Holland spirit of salt (muriatic acid) is said to be employed for the same purpose. 3. But in most dairy countries rennet is the substance used for the curdling of milk. What is rennet, and how does it act ? a. The stomach of the calf, of the kid, of the lamb, and even of the young pig, when covered with salt, or stec])-