224 PHARMACEUTICAL BACTERIOLOGY 



hydrochloric and sulphuric acid. Acetic acid is the least active. Ren- 

 net does not occur in the stomach in the absence of free acid though the 

 proferment is present. 



Rennet is a highly potent ferment. It will coagulate from 4-800,000 

 times its weight of milk. It is most active in slightly acid solutions and 

 least active in alkaline solutions. Injecting minute doses hypodermically 

 causes the formation of an anti-coagulating ferment which, when added to 

 milk, prevents coagulation. 



b. Parachymosin. — The rennet of the human stomach and that of the 

 hog, differ from that of other animals, especially do they differ from that 

 of the calf and sheep. Thunberg found that the rennet of hog pepsin 

 does* not coagulate slightly acid mUk when neutralized by alkali, but will 

 do so when such milk is neutrahzed by means of carbonate of lime; where- 

 as calf rennet shows no such difference in behavior. It is therefore pre- 

 sumed that in addition to ordinary chymosin, the pepsin of man and of the 

 hog contains a modified rennet to which the name paracyhmosin has been 

 given. Parachymosin is more resistant to heat than chymosin as it is 

 still active at 75° C, while it is much more susceptible to alkalies. Heat- 

 ing hog pepsin to 70° C. destroys the chymosin and leaves the parachy- 

 mosin still active, or the parachymosin may be destroyed by means of 

 alkalies too weak to affect the chymosin. 



c. Vegetable Rennet. — A milk curdling ferment occurs in various plants. 

 Galium verum is used to curdle milk, also Pinguicula vulgaris, Drosera and 

 Carica papaya. The seeds of Wifharia coagulans, a plant found in India 

 and Africa, are used by the Hindus for coagulating milk, their religion 

 forbidding the use of animal substances. Germinating seeds of Ricinus 

 communis contain rennet in the form of the zymogen which is activated 

 by dilute acids. Rennet also occurs in figs, artichokes, thistle, and in 

 other plants. The fruit of Acanthosicyos horrida, of South Africa, con- 

 tains a rennet which is said to be soluble in 60 per cent, alcohol. Nu- 

 merous species of bacteria form milk curdling ferments, among others 

 Bacillus amylobacter, B. mesentericus vulgatus, B. prodigiosus and B. 

 cholercR. 



d. Pectase. — ^This enzyme causes the coagulation of pectin in plants 

 containing this substance. It is widely distributed almong higher plants 

 and also among cryptogams. It occurs in two forms, soluble as in carrots, 

 and in an insoluble form, as in acid fruits generally. The ferment is active 

 in the absence of oxygen and it does' not result in gas liberation, and is most 

 active at 30° C. Coagulation is most active in the presence of lime, 

 though the process is also initiated by barium and strontium salts. Boil- 

 ing or precipitating out the Ume, hinders coagulation. Acids destroy 

 the ferment. 



