218 MILK AND DAIRY PRODUCTS PART 



mobilis liquefaciens and Granulobacillus saccharobutyricus mobilis 

 non liquefatiens, which have been studied by Schattenfroh and 

 Grassberger. 



1. Lactic Acid. 



Through the agency of the lactic acid bacteria (Bacterium 

 lactis acidi, Leichmann) 1 which are found almost everywhere 

 the milk sugar is converted into lactic acid, without formation 

 of secondary products, according to the equation 



This acid is the dextrorotary modification of lactic acid, 

 a-oxypropionic acid, and has the constitutional formula 

 CH 3 .CH(OH)COOH. 



When milk sours spontaneously a mixture of inactive and 

 dextrorotary lactic acid is obtained, because lactic acid bacteria 

 other than those mentioned above take part in the decomposi- 

 tion. Certain of these bacteria give rise to the Isevorotary 

 modification of lactic acid. 



Lactic acid is estimated quantitatively by titration with 

 standard alkali in the manner described for the determination 

 of the degree of acidity of milk (p. 31). The method is not 

 absolutely correct, but it is the one employed in the majority 

 of cases. After titration, the acidity of normal milk must 

 naturally be deducted. If a qualitative test only is needed for 

 lactic acid, then Uffelmann's reagent can be used with advan- 

 tage. It consists of a 1 per cent, carbolic acid solution, to 

 which 10-15 drops of ferric chloride have been added to each 

 100 c.c. In applying the test, a few drops of milk are added 

 to 10 c.c. of the reagent, and in the presence of lactic acid the 

 violet-blue colour changes to a greenish-yellow. This test was 

 originally worked out for the detection of lactic acid in the 

 contents of the stomach. 



To determine what modification of lactic acid is formed, the 

 zinc salt is generally prepared, and the following method gives 

 satisfactory results : 



One litre of well-mixed sour milk is filtered through a folded 

 filter, which is then repeatedly washed with cold water. The 

 filtrate is evaporated to dryness on the water-bath, the residue 



1 Chr. Barthel, " Contribution a la connaissance de la repartition des fer- 

 ments lactiques en dehors du lait," Revue Gen. du Laif, Vol. V, p. 223. 



