224 ANNUAL REPOBT SMITHSONIAN INSTITUTION, 1923 



account for their low coefficients of digestion. These proteins ap- 

 peared to be less readily and completely disintegrated by the diges- 

 tive processes than the cereal proteins. This resistance to enzyme 

 hydrolysis was even more pronounced with cottonseed proteins. 



An interesting light has been thrown upon the digestibility of the 

 proteins of many of the legumes by recent investigations by Jones 

 and his associates in the Bureau of Chemistry (2). In these studies 

 it has been shown that the digestibility of the proteins from certain 

 legumes, including the white bean, the lima bean, and the Chinese 

 and Georgia velvet beans, is much improved by cooking. The value 

 of these proteins in growth experiments on rats in large part de- 

 pended upon whether or not the proteins were cooked or uncooked. 

 No explanation of this effect of cooking has been given, since it is 

 not predictable from any of the known properties of proteins. About 

 the same time Langworthy (3) and associates reported experiments 

 on men indicating that with most starches cooking is not essential 

 to complete digestibility, though from what is known of the solu- 

 bility of starch and its physical condition as deposited in plant 

 tissues, it has been widely taught that raw starch is very poorly 

 utilized in digestion. The improvement in the digestibility of raw 

 egg white by cooking is well known, though an unaccountable dis- 

 crepancy exists among the published reports of experiments con- 

 cerned with this question. 



While the wastage of protein in digestion thus seems to be largely 

 unrelated to its chemical structure and composition, the wastage of 

 protein in metabolism in covering the protein requirements of the 

 body is generally ascribed entirely to the chemical structure of the 

 food protein. From our knowledge of the amino acid make-up of 

 proteins, and of the marked differences in this particular among food 

 proteins, it is readily understandable that the chemical structure of 

 a protein may seriously limit its usefulness to the body. The value 

 of the digestible protein of a food in meeting the protein require- 

 ments of the body, allowance being made for this wastage of protein 

 in metabolism, is known as the biological value. 



Prominent among investigations of the biological values of pro- 

 teins, is the work of Thomas (4), reported 13 years ago, and still 

 the most complete study of its kind. Thomas' results are based 

 upon nitrogen balance studies upon himself, and his biological values 

 of the proteins tested express the number of parts of body protein 

 replacable by 100 parts of digestible food protein. This method, 

 therefore, measures the capacity of food proteins in replacing the 

 nitrogenous constituents of the tissues disintegrated and lost in the 

 so-called " wear and tear " processes of the body, or, in the terms of 



