76 PHYSIOLOGICAL CHEMISTRY. 



menoid matters is caused by the action of ferments, which are without 

 influence on the remainder. The exact nature of the change which 

 takes place in coagulation has not been determined, and it probably 

 cannot be successfully investigated until the real constitution of the 

 albumenoid substances is known. 



Another important feature of the albumenoid substances is their con- 

 nection with "catalyses" or " catalytic transformations." These are 

 chemical changes, either combinations or decompositions, which take 

 place under the influence of a body acting in a hitherto unexplained 

 way, apparently by mere contact and without being itself either 

 decomposed or combined. Such a body is a ferment. It produces its 

 effect in very small quantity ; and it may cause important transfor- 

 mations in a large amount of other material without its own substance 

 being perceptibly diminished. Thus the starchy matter of plants is 

 converted into glucose by the influence of a nitrogenous body termed 

 "diastase;" and according to Payen,* one part of diastase is capable 

 of converting into glucose 2,000 parts of starch. All the ferments 

 belong to the class of albumenoid substances. Many of these sub- 

 stances are themselves liable to catalytic transformation under the 

 influence of ferments. Such transformations are certainly the principal 

 acts in the digestion and assimilation of food ; and changes of a 

 similar kind are so general and so important throughout the body 

 that some physiologists are inclined to attribute to their influence 

 all the essential phenomena of nutrition and waste. Each ferment 

 operates with the greatest vigor under certain special conditions ; as, 

 for example, an acid, neutral or alkaline medium, the presence or 

 absence of a saline solution, or a slightly higher or lower tempera- 

 ture ; but they are all arrested by the strong acids or alkalies, by 

 concentrated saline solutions, by the absence of moisture, or by a boil- 

 ing or freezing temperature. The most favorable temperature is 

 usually about that of the living body. 



At a temperature of 300 C. or over, the albumenoid substances are 

 decomposed into gaseous products. But if subjected for a certain time 

 to a temperature of about 125 C., they undergo a change by which a 

 peculiarly agreeable flavor is developed, and by which many of them 

 become suitable for human food. It is this flavor which is produced 

 in the process of cooking, and which always depends upon the pres- 

 ence of a certain quantity of albumenoid matter in the substance 

 employed. If the temperature at which the cooking process is carried 

 on be too low, the characteristic flavors are not developed ; if it be too 

 high, they are destroyed and replaced by empyreumatic odors, from 

 the combustion or decomposition of the ingredients of the food. 



Lastly, the albumenoid substances are distinguished by the property 

 of putrefaction. This is a process in which dead animal substances, 

 when exposed to the atmosphere at a moderately warm temperature, 



* Substances Alimentaires, Paris, 1865, p. 5. 



