44 THE VEGETABLE PROTEINS 



the chief cause of the difficulty encountered in attempting to prepare 

 definite salts of the protein with metals. 



E. Denaturing by Heat. 



It appears to be commonly believed that all proteins having the 

 properties of globulin are completely coagulated by heating their slightly 

 acid solutions and that this property is also shared by nearly all of the 

 other native proteins. In this respect the seed proteins differ in a 

 marked degree from the animal proteins, for most of them are very 

 incompletely coagulated by heating their solutions, even to boiling, 

 and many of them are not coagulated at all under these conditions. 

 In this connection should be considered the part which acid plays in 

 the production of a heat coagulum, for it is well known that alkaline 

 solutions of proteins cannot be coagulated by heat and that neutral 

 solutions are usually coagulated with some difficulty. It is customary 

 in attempting to separate a protein from its solution by heat to add a 

 very small quantity of acetic acid. From what we have said in rela- 

 tion to the denaturing effect of acid on protein it is evident that acid 

 cannot be added to the solutions of many of the seed proteins without 

 of itself causing a change in the solubility of the protein, and it becomes 

 difficult in such cases to distinguish between the denaturing effect of 

 the acid and that of the heat. It has, therefore, been the practice in 

 determining the effect of heat on vegetable proteins to heat them in 

 solution without the addition of any more acid than that which has 

 combined with them during the process of isolation. Osborne and 

 Campbell (60 1) have shown that crystallised ovalbumin, which has an 

 acid reaction toward phenolphthalein similar to that of the preparations 

 of the seed proteins, is completely coagulated when its solutions are 

 sufficiently heated. If, however, alkali is first added in sufficient quan- 

 tity to exactly neutralise the acid combined with the ovalbumin, using 

 phenolphthalein as an indicator, its solution when heated yields no 

 coagulum, although the albumin has suffered a chemical change which 

 is made evident by cooling its solution and then adding a quantity of 

 acid corresponding to that originally present in the crystalline substance. 

 The behaviour of edestin solutions toward heat is similar to that 

 of a large number of other seed globulins, and in this connection the 

 experiments of Chittenden and Mendel (71) are of interest, for they 

 show that the acid combined with the crystallised edestin is insufficient 

 to effect its complete coagulation, as the part of the edestin which is 

 not coagulated remains unchanged even after long heating in a boiling 

 solution. Chittenden and Mendel employed a sodium chloride solution 



