24 



When gelatinoids are present, as may be the case with soups, stews, 

 and meat extracts, hot water may be used at once for solution or wash- 

 ing the original material, and this with the advantage of facilitating 

 the extraction of the less soluble simpler amids and amido-acids. 

 These are, as a rule, more easily dissolved in the presence of a little 

 free acid; hence acidification at an early stage of the treatment is 

 advantageous. In a case in which tyrosiu might be present, as in some 

 vegetable materials, and possibly among unabsorbed residua of food, 

 the use of hot water and the presence of free acid would greatly 

 increase the solubility of this substance. 



In food of vegetable origin where much starch is present it will be 

 better to avoid the use of hot water at first, so that the solution may 

 not be loaded with viscid material, rendering nitration difficult. 



In all cases in which the food material to be examined is already 

 fluid from the presence of water as, for instance, soup, milk, and the 

 like nitration will of course at once be resorted to, being almost always 

 much facilitated by the addition of sand or pulverized glass, and only 

 such further quantity of water will be used as is required for washing 

 the undissolved matter left upon the filter. 



In the presence of fat in large quantity, it may be well first to 

 remove this, or most of it, by extraction with etber. The simpler 

 amidic substances are, as a rule, insoluble in ether, but by way of pre- 

 caution the ethereal solution of fats might be shaken up two or three 

 times with acidified water, and the watery fluid evaporated and tested 

 for nitrogen. 



In regard to the method of reporting results, the most important 

 point is the separate statement of the amount of nitrogen present in 

 the form of proteids and their more closely related congeners and in 

 the form of the simpler amids and amido-acids. But in attempting to 

 calculate from the nitrogen found under these heads the actual amount 

 of the proximate nitrogenous coLStituents of the food material ex- 

 amined, the question arises, What factor should be used by which to 

 multiply the nitrogen found in each case? 



FACTORS FOR CALCULATION OF TOTAL NITROGEN. 



The error noticed by Professor Wiley 1 as involved in the multiplica- 

 tion of the total nitrogen of a sample of meat by 6.25, and the assump- 

 tion that the product represents the true quantity of nitrogenous matter, 

 is not restricted to the use of the same factor for the proteids and flesh 

 bases. While the multiplier should be a much smaller one for the lat- 

 ter, it also confounds under a single head these two classes of material, 

 unquestionably possessing very different nutritive values. 



It is evident that for each substance examined, or at any rate for 

 each class of generally similar food materials, there should be made a 

 qualitative investigation of the simpler amidic constituents present, 



Principles and Practice of Agricultural Analysis (1897), 3, 551. 



