2 THE VEGETABLE PROTEINS 



Rouelle (441) separated this glutinous substance from the juice of 

 hemlock by heating to a moderate temperature and filtering out the 

 coagulum, which had a bright green colour. He also obtained by 

 fractional coagulation a part which contained nearly all of the colouring 

 matter and, at a higher temperature, a part which was nearly free from 

 colouring matter. The colouring matter could also be extracted by 

 digesting the coagulum with alcohol. The protein nature of this sub- 

 stance was proved by the products of destructive distillation. Rouelle 

 was, therefore, the first to obtain evidence of the wide distribution of 

 protein substances in the different parts of plants. 



During the same year Parmentier (369) described his extensive 

 study of various vegetable substances used for food, and among other 

 questions to which he devoted his attention was the gluten of wheat. 

 He found that this substance, while insoluble in mineral acids, was 

 soluble in vinegar, and that by neutralising its solution with sodium 

 carbonate it was precipitated with apparently unchanged properties. 

 Its solution in vinegar when evaporated to dryness left a horny and 

 yellow residue which was not hygroscopic. When the gluten was 

 extracted with spirits of wine some was dissolved, and the yellow solu- 

 tion when evaporated left a transparent residue which burned with a 

 strong odour of burning animal matter. Nevertheless he appears to 

 have regarded this residue as a resin. When the gluten was boiled 

 with water it lost its tenacity and coherence and had evidently suffered 

 a decided physical change. When the gluten was exposed to dry air 

 at a low temperature it left a residue which, when treated with water, 

 regained its original moist weight ; from this he concluded that it oc- 

 curred in the seed in the dry form and on contact with water became 

 hydrated, hence the necessity for kneading dough. 



Parmentier (370) in another paper stated that on drying, gluten lost 

 two-thirds of its weight, and the dried product thus obtained, when 

 triturated with water, was restored to its original glutinous and elastic 

 condition. He also found that the quantity of gluten obtained from 

 different flours was proportional to their colour, the darkest flour yield- 

 ing nearly twice as much as the whitest. 



In 1776 Parmentier (371) further stated that wheat flour must be 

 very much altered in order to give no gluten, and that this substance 

 completely disappears only when the seed has germinated. 



At about this time Berthollet (32) reported that when gluten was 

 treated with nitric acid nitrogen was evolved and the residue became 

 yellow. 



In 1789 Fourcroy (122) gave an extensive account of the occurrence 



