THEORY OF THE ACTION OF RENxNET. 569 



be poured upon the skins, and the infusion, when the skins are taken 

 out, may be kept for a length of time. Some even recommend that 

 the liquid rennet should not be used until it is at least two months old. 

 When thus kept, however, it is indispensable that the water should be 

 fully saturated with salt. 



In Ayrshire, and in some other counties, it is customary to cut the 

 dried stomach into small pieces, and to put it, with a handful or two of 

 salt and one or two quarts of water, into a jar, to allow it to stand for two 

 or three days, afterwards to pour upon it another pint for a couple of days, 

 to mix the two decoctions, and, when strained, to bottle the whole for 

 future use. In this state it may be kept for many months.* 



In all the methods above described, the exhausted skins are thrown 

 away. Where they are cut into pieces, as in Cheshire and Ayrshire, 

 they cannot of course be put to any second use, but where they are steeped 

 whole, there is ever}' reason to believe that they might be used with al- 

 most equal advantage a second or even a third time. Accordingly, it 

 has long been the custom in the north of England to re-salt the stomach 

 after it has been once steeped, and when long dried, as before, to use it 

 a second and even a third time for the preparation of rennet. When we 

 explain the mode in which rennet acts, you will see that the same skin 

 may, with good reason, be expected to yield a good rennet, after being 

 salted again and again for an indefinite number of times. 



In making rennet, some use pure water only, others prefer clear whey, 

 others a decoction of leaves — such as those of the sweetbriar, the dog- 

 rose, and the bramble — or of aromatic herbs and flowers, while others, 

 again, put in lemons, cloves, mace, or brandy. These various practices 

 are adopted for tli^e purpose of making the rennet keep better, of lessen- 

 ing its unpleasant smell, of preventing any unpleasant taste it might 

 give to the curd, or finally of directly improving the flavour of the cheese. 

 The acidity of the lemon will, no doubt, increase also the coagulating 

 power of any rennet to which it may be added. 



4°. Hoiv the rennet is used. — The rennet thus prepared is poured into 

 the milk previously raised to the temperature of 90° or 95° F., and is 

 intimately mixed with it. The quantity which it is necessary to add 

 varies with the quality of the rennet — from a table-spoonful to half a 

 pint for .30 or 40 gallons of milk. The time necessary for the complete 

 fixing of the curd varies also from 15 minutes to an hour or even an hour 

 and a half. The chief causes of this variation are the temperature of the 

 milk, and the quality and quantity of the rennet employed. 



But how does the rennet act in causing this coagulation? Before 

 we can answer this question it is necessary to encjuire what rennet 

 really is. 



§ 19. Theory of the action of rennet. 



It has been stated, and hitherto almost generally received, that the only 

 effective substance contained in. rennet is the gastric juice derived from 

 the stomach of the calf. To this persuasion is, no doubt, -to be ascribed 



• 



* A table-spoonful of this rennet, according to Mr. Aiton, will coagulate 30 gallons of milk, 

 nod will curdle it in five or ten minutes, whereas the English rennet requires from one to 

 three hours. This superiority he ascribes to the custom of leaving the curdled milk in the 

 stomach. He denies also that this milk gives nay harsh taste to the cheese. 



