1 86 THE COAGULATION OF MILK 



occur in milk that has been rendered alkaline ; and the most favourable 

 condition is one of very slight acidity. The constitution of this enzyme is 

 still unknown. Its decomposing power is unusually high, one part by weight 

 being (according to Sbldner) sufficient to throw down at least one hundred 

 million parts of casein. As A. MAYER (III.) has shown, the optimum tem- 

 perature for the reaction is 37 C., the coagulation taking three times as 

 long at 25 0. Above 45 C. the enzyme is paralysed, and is destroyed at 

 70 C. Coagulation does not ensue immediately upon the addition of the lab, 

 but only after a lapse of from several minutes to some hours, according to 

 the temperature. 



The occurrence of this enzyme in nature is by no means rare. It was 

 found by Roberts in the stomach of birds, and in that of fishes by Benger, 

 and is also present in the cell-sap of various plants, e.g. of butterwort (Pin- 

 gulcula vulgaris and P. alpina), withania (Punceria coayulans), fig-tree (Ficus 

 carica), artichoke (Cynara scolimus), and others. It is also excreted by several 

 species of Schizomycetes, particulars of which will be given in the following para- 

 graph. For industrial purposes, however, there is only a single source (the 

 richest) worthy of consideration, viz., the stomach of the calf. The method of 

 production indicated above is now practised on a manufacturing scale, especially 

 in Copenhagen, whence most of the German and Dutch cheese factories derive 

 their supplies. The products are : Kennet solution, containing boracic acid 

 to improve the keeping quality ; rennet powder ; and, finally, rennet tabloids. 

 The efficacy and germ-content of these preparations were investigated by FRITZ 

 BAUMANN (1.). 



146. Lab-Producing 1 Bacteria. 



Milk may also curdle without previous acidification or addition of rennet. 

 HAUBNER (I.) in 1852 was the first to record this fact, and the first explanatory 

 research was made by DUCLAUX (IX.) in 1882. From his studies in this matter 

 the latter concluded that this precipitation of casein (occurring with an alkaline 

 reaction) is due to the activity of certain bacteria which excrete an enzyme 

 lesembling lab; and CONN (III.), in 1892, succeeded in isolating this enzyme 

 from cultures of such Schizomycetes. At first sight the identity of this lab with 

 the active ingredient in the rennet solution from the stomach of the calf appears 

 probable, but the discovery that the bacteria in question (which include many of 

 the species belonging to the potato bacillus group, in) are able to coagulate 

 boiled sterilised milk, goes against this view, rennet being incapable of producing 

 this reaction. 



The presence or absence of this power affords, in many instances, a valuable 

 means of differentiation between two species of bacteria. This is particularly 

 the case with the organism producing abdominal typhus in man, viz., Bacillus 

 typhi abdominalis, discovered by Eberth in 1880, and already referred to in 

 preceding chapters. This microbe is not endowed with the faculty in question, 

 whereas, as JAK. URY (I.) has shown, a number of the putrefactive bacteria 

 collectively termed Bacterium coli commune, and greatly resembling the typhus 

 bacillus in form, &c., rapidly produce coagulation in milk. According to the 

 researches of C. GORINI (I. and II.), Micrococcus pi-odigiosus also produces this 

 enzyme in large quantities. 



147. Casease. 



Not infrequently the precipitation of casein effected by such bacteria 

 disappears again after a short time. Duclaux ascertained that this new 

 alteration is due to a second (albumen-dissolving) enzyme, to which he gave the 



