CHARACTERISTICS AND ACTIVITY or LAB 185 



will also be thrown down only in the case of rennet curd, not in the curd 

 produced by acidification. 



Not only are fat and (in this instance) calcium phosphate carried down by 

 the coagulum, but also a large part of the organisms present in the milk will be 

 found in the fresh curd, so that the latter is relatively as rich in organisms as 

 the milk from which it was precipitated. Here, again, a considerable difference, 

 from a biological point of view, exists between the two classes of curd, and 

 exercises a decisive influence on their subsequent career. The flora of the 

 rennet curd from sweet (i.e. almost neutral) milk is much more diversified than 

 that of acid curd. The latter, having been thrown down from a sour milk in 

 a state of vigorous lactic fermentation, consequently contains only a limited 

 number of species, and these endowed with a particular fermentative power. 



Acid curd differs, therefore, from rennet, both in the method of production 

 and also in composition. Being devoid of flavour, both kinds are, however, un- 

 suitable for food ; their conversion into a form in which they both stimulate the 

 appetite and are also themselves more readily digestible, is the task of the 

 cheese-maker's art, fuller particulars of which, from the bacteriological point of 

 view, will be found in chapter xxxi. 



145. Characteristics and Activity of Lab. 



If the enzyme in question were exclusively a metabolic product of the animal 

 body, the foregoing details would suffice. However, since it is excreted by many 

 fungi as well, a few additional particulars will not be out of place in a work 

 dealing with technical mycology, 



When and in what manner the attention of mankind was first drawn to this 

 enzyme cannot be determined, since even the oldest authorities, e.g. the Bible, 

 speak of its employment as an ancient practice. Its method of action, however, 

 was unknown even down to the commencement of the nineteenth century. 

 The prevalent opinion, based on the curdling of sour milk, was that the 

 precipitation effected by rennet was indirect, an acid being first formed, which 

 then caused the precipitation of the curd. The elucidation of the true state of 

 the case was reserved for BEEZELTUS (I.) in 1840, and his discovery was soon 

 afterwards supplemented by the labours of FR. SELMI (I.), C. G. LEHMANN (I.), 

 HEINTZ (I.), and VOELCKEK (I.), who showed that the action of rennet is quite 

 independent of the formation of acid. 



The identity of this enzyme with the pepsin discovered by SCHWANN ( 18) 

 was disproved by 0. HAMMARSTEN (I.) in 1872. This observer was the first to 

 successfully separate these two gastric secretions, an operation which DESCHAMPS 

 (I.) had failed to effect thirty-two years earlier.. Hammarsten, however, did not 

 make use of the name chymosin, proposed by his predecessor, but adopted the 

 ancient appellation, lab. We are also indebted to the Swedish chemist for deep 

 researches into the activity of this substance. It acts on casein alone, not on 

 lactose or lactalbumen. 



A divergence of opinion still prevails as to the nature and course of this 

 reaction, and we will therefore merely refer to the investigations made on these 

 points by the under-named workers: A. DANILEWSKY and P. RADENHAUSEN (I.), 

 W. EUGLING (I.), F. SCHAFFER (II.), E. DUCLAUX (VIII.), M. ARTHUS and 

 C. PAG^S (I.), A. S. LEA and W. S. DICKINSON (I.), S. RINGER (I.), P. WALTHER 

 (I.), and A. FICK (I.). SCHREINER (I.), in 1877, showed that milk when boiled 

 is no longer coagulable by rennet, but the reason for this behaviour was not 

 ascertained until eleven years later, when FR. SOLDNER (I.) found that the lab 

 reaction can only proceed in the presence of soluble salts of lime, which 

 latter are precipitated by boiling. For the same reason coagulation does not 



