182 THE ESTIMATION OF PROTEINS. 



N 

 titrating back the alkali with acid, using litmus paper as 



indicator, and subtracting the volatile acidity from the total 

 acidity to litmus ; the non-volatile acidity is taken as lactic acid. 



Determination of the Aldehyde Figure. Steinegger has devised 

 a method which adds another datum to those usually obtained 

 in the analysis of milk, and which serves as an indirect estimation 

 of proteins. It may be combined with the acidity estimation. 

 The method depends on the fact that when an amino-acid, which 

 has been neutralised is treated with an excess of formaldehyde, 

 it becomes acid, and requires the addition of a further quantity 

 of alkali to neutralise it. -^ 



Steinegger titrates the milk with -j- caustic soda solution 



till neutral to phenol- phthalein, adds 6 per cent, of the total 

 volume of 40 per cent, formaldehyde solution, and again titrates 

 till neutral ; the amount of alkali used, less the acidity of the 

 formaldehyde solution added, is the aldehyde figure which he 

 expressed in Soxhlet-Henkel degrees. -^ 



The author and Miller prefer the use of strontia solution 



for titrating, and take 10 c.c. or 11 c.c. of milk, neutralise to 

 phenol-phthalein, add 2 c.c. of 40 per cent, formaldehyde solu- 

 tion, and titrate again till neutral, and subtract the acidity, 

 previously determined, of 2 c.c. of formaldehyde solution. The 

 acidity developed by the addition of formaldehyde calculated 

 as degrees gives the aldehyde figure. The strontia aldehyde 



N 

 figure is about 1-1 times larger than that given with a soda 



solution. This figure varies in normal milks from 18-1 to 22-6, 

 and averages 19-9 ; in fresh milks it is practically equal to the 

 acidity. 



The aldehyde figure obtained with strontia solution multiplied 

 by 0-170 will give a close approximation to the percentage of 

 proteins ; it is not an absolutely exact measure of the proteins, 

 as casein and albumin do not give the same aldehyde figure, 

 and the relative proportion of these is liable to slight variations. 



De Graaf and Schaap also recommend strongly the aldehyde 



N 

 titration of milk, but use j soda, and give the factor to convert 



degrees to percentage of protein as 0-0777 (which equals when 

 multiplied by 2-5 to bring it to the equivalent of c.c. N per 

 litre, 0-1942), and the ratio between strontia and soda is thus 

 found to be 1-142. 



N 



Walker uses ^ soda and gives the factor for casein as 0-163, 

 y 



