358 MILK 



contain the "fermentation lactic acid" (racemic or inactive acid), 

 but that frequently there was a preponderance of d-acid. They 

 furthermore determined that Bacterium lactis acidi (Streptococcus 

 lacticus) in pure culture always produced pure d-acid. Leich- 

 mann also found that Bacterium lactis acidi produced only d-acid 

 in pure culture, but when milk soured at high temperature racemic 

 acid was formed. The investigations of Thiele have shown that 

 when milk sours at room temperature mostly d-acid is formed 

 during the first few days, but after seven days there was chiefly 

 r-acid, with a shght excess of d-acid. When this author ex- 

 amined milk that had soured at 37° C. he also found d-acid at 

 first, but r-acid appeared after thirty-six hours, and finally 1-acid 

 was in excess of r-acid. Kozai's work is not entirely in accord 

 with that of Thiele, inasmuch as he found in mUk som^ed at room 

 temperature either pure d-acid or d-acid with a shght amount 

 of r-acid. In milk soured at 37° C. Kozai found r-acid and 1-acid. 

 Utz is of the opinion that in sour milk r-acid or r-acid with some 

 d-acid are present, whether the fermentation occmred at room 

 temperature or at 37° C. HolHng examined a number of samples 

 of milk and came to the conclusion that milk incubated either 

 at 22° C. or at 37° C. contained d-acid. Pure cultures of Strep- 

 tococcus lacticus and strains of streptococci isolated from infants' 

 stools and from the feces of horses, asses, cows, rabbits, and dogs 

 produced only d-acid in milk, while a culture of a non-motile strain 

 of Bacillus coli produced 1-acid. 



On the surface these results do not appear harmonious. How- 

 ever, MacKenzie argued that the disagreement of results obtained 

 by different investigators may in part at least be due to the 

 methods employed. The usual proceeding for determination of 

 the kind of lactic acid present in milk is to prepare the zinc salts 

 of lactic acid and then determine the rotatory power of the zinc 

 lactate, which is the reverse of that of the corresponding acid. 

 Sour, loppered milk is filtered and the clear filtrate evaporated 

 to a syrupy consistency on a water-bath. The residue is strongly 

 acidulated with phosphoric acid, and the liberated lactic acid 

 shaken out with ether. After the ether has been evaporated the 

 golden-yellow fluid is dissolved in water and the solution boiled 

 with zinc carbonate until effervescence ceases. The salt is then 

 crystallized from the solution. Addition of animal charcoal aids 

 in producing a colorless solution of the zinc lactate. 



According to MacKenzie, active zinc lactates may remain in 

 solution after the inactive salt has been crystallized, since the 

 active salts are soluble in 17.5 parts of water at 15° C, whileHhe 

 inactive salt requires at least 53 parts of water for solution. The 

 inactive salt crystallizes, therefore, more readily than the active 



