ZYMOLOGY FERMENTS AND FERMENTATIONS 223 



seeds, their function being to split up and render transfusable and assimi- 

 lable for the germinating seeds, the proteid granules. These have thus 

 far not received any careful study. 



The subject of auto-digestion of organs has received considerable 

 attention. Finely chopped fresh organs digested for a time at moderate 

 warmth undergo certain fermentation-like changes resulting in the forma- 

 tion of reducing sugars, leucin, tyrosin, etc., substances which do not occur 

 in living organs or in organs which have been boiled. Bacterial infection 

 is excluded by means of chloroform water (using ten volumes of the chloro- 

 form water), also by means of sodium fluoride and thymol, though these 

 latter agents are less suitable than the chloroform water. 



Autodigestion proceeds slowly, being a slow decomposition of the 

 albuminous matter. In this digestion, albumoses are formed but not 

 peptones; furthermore, nuclein is split up which is not the case in trypsin 

 fermentation. Autodigestion is no doubt due to a proteotytic ferment 

 which probably exists in the cells but which may be removed from the 

 tissues as its presence has been demonstrated in cell-free extracts. The 

 ferment of autodigestion is probably an autolytic product of tissue cells 

 causing a molecular change in albuminous matter. 



2. Coagulating Ferments. The curdling of milk was known in ancient 

 times and is the initial basic process in cheese-making. At one time it 

 was believed that pepsin had the power of curdling milk but that is 

 now known to be incorrect. Berzelius was the first to demonstrate that 

 the curdling of milk could take place without the presence of lactic acid, 

 thus disproving the contention of Liebig that lactic acid combined with 

 the alkali with a precipitation of casein. 



a. Rennet. Rennet or chymosin is the milk-curdling ferment used in 

 cheese-making, obtained from the fourth ventricle of the stomach of the 

 calf or sheep. It, however, also occurs in the stomach of all animals. 

 In serious pathological conditions of the stomach (as cancer, gastritis, 

 etc.) it may be partially or even wholly wanting. It also occurs in the 

 intestinal tract and in nearly all tissues and organs. 



While rennet has many of the properties of ferments generally, it 

 shows some exceptional peculiarities. In regard to its behavior with salt 

 solutions, it is precipitated by lead acetate only. It is destroyed by bile 

 and by even very weak solutions of alkalies. Dissolved in distilled water 

 it is destroyed upon exposure to a temperature of 40 C. This peculiarity 

 makes it possible to free pepsin from rennet, as pepsin is not affected by 

 this temperature and distilled water. 



Rennet is the product of a proferment or rennet zymogen which is 

 secreted by the cells of the stomach, acted upon by the free acid of the stom- 

 ach. Almost any acid will, however, activate the zymogen, especially 



