WHEN EXPOSED A SHORT TIME TO THE AIR 571 



should be kept at least 12 months, if it is to acquire very powerful 

 coagulating properties. 



It is necessary further to remind you that when malt is steeped in 

 water for a few minutes, a substance, named diastase, is extracted from 

 it, which possesses the remarkable property of clianging starch into 

 sugar in a very short time, and in large quantity (p. 119). Now if this 

 diastase be exposed to the air for a length of time, it undergoes a change 

 similar to that experienced by the surface of animal membranes, and 

 acquires the property of transforming sugar into lactic acid. After un- 

 dergoing this change it still disscjives readily in water, and if a solution 

 of it be poured into one of sui»ar, the transformation of the latter into lactic 

 acid gradually proceeds. There exist, therefore, substances soluble in 

 water, which possess the same power as slightly decayed but insoluble 

 animal membrane, of converting sugar into lactic acid. 



During the protracted drying and decay of the salted stomach, the 

 change undergone at length by the surface of the membrane is such as to 

 produce a ([uantity of matter capable of dissolving in water, and which 

 also possesses the property of quickly converting the sugar into the acid 

 of milk. This matter, water extracts from the dried skin, and it forms 

 the active ingredient in rennet. 



I need not further explain to you upon what this activity depends — 

 since as you already know any thing which will rapidly change sugar 

 into lactic acid, will also, if gently warmed, rapidly curdle milk (p. 

 567). 



Thus the action of rennet resolves itself simply into a curdling of milk 

 by the action of its own acid. It is the same thing as when sour milk 

 in Switzerland is at once mixed with that from which the cheese is to be 

 made ; or it is only a more speedy way of bringing about the curdhng 

 that takes place when milk sours naturally and is then gently warmed 

 till the curd separates. 



But how, it may be asked, is the coagulation effected so much more 

 rapidly by the action of rennet than when the milk is left to sour of its 

 own accord ? It is because the whole of the animal matter in the rennet 

 is already in the state in which it easily transforms the sugar into acid, 

 and being intimately mixed with the whole milk in a warm state, it pro- 

 duces acid near every particle of the cheesy matter. From this 

 cheesy matter the acid formed takes away the soda that holds it in solu- 

 tion, and thus renders it insoluble or curdles the milk. In milk, on the 

 other hand, which is left to sour and curdle of itself, the casein must first 

 be changed by the action of the air before it can transform the sugar and 

 produce acid. This change takes place more or less slowly, and chiefly 

 at the surface of the milk where it is in contact with the air. The sour- 

 ing, therefore, must also proceed slowly, and the curdling of which it is 

 the cause. 



It is no objection to this explanation of the action of rennet, that neither 

 the milk nor the whey become sensibly sour during the separation of the 

 curd. The acid, as it is produced, combines directly with the soda pre- 

 viously united to the curd, and renders the latter insoluble — while, if 

 any excess of acid do happen to be formed, it is in great part taken up 

 and retained mechanically by the curd, and thus is not afterwards sen- 

 sibly perceived in the whey. 



