136 ANALYSIS OF MILK. 



the salts of milk and carbonic acid are not appreciably acid to 

 litmus paper. 



The following figures obtained by the author on sour milks 

 will show the enormous difference in the two results ; the figures 

 have in both cases been calculated to lactic acid : — 



i I. II. III. IV. V. 



Acidity (to phenolphthalein), 1-24 1-89 1-82 1-52 1-32 



„ (to litmus paper), . 0-66 1-14 1-28 0-86 0-56 



There is no good method for the quantitative determination 



of lactic acid ; the results of the titration with litmus give 



approximate results. An approximation nearer to the truth 



N 

 may be made by distilling some of the milk into a little ^j: alkali, 



titrating back the alkali with rj-r- 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. 



The cui-ve on p. 135 (Fig. 14) shows the average rate of develop- 

 ment of acidity in milk on standing at a temperature of 20° C. 



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-aoid, 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 again neutralise it. -nt 



Steinegger titrates the milk with --r- caustic soda solution 



till neutral to phenolphthalein, 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. -vr 



The author and Miller prefer the use of -pp strontia solution 



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

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

 tion, and again titrate 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 



figure is about 11 times larger than that given with a — 



