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Quite recently Heinemann (10) has made an exhaustive study of 
the kinds of lactic acid produced in milk by the lactic acid bacteria. 
The following are his conclusions : 
1. Milk naturally soured at room temperature contains chiefly d-acid. Milk 
soured at 37° C. contains chiefly r-acid with 1-acid in excess if allowed to stand 
several days. 
2. Streptococcus lacticus and Streptococcus pyogenes produce the same kind 
of lactic acid, i. e., d-acid. B. aerogenes from milk (B. acidi lactici) and the 
ordinary laboratory strain of B. (lactis) aerogenes (Escherich) produce the 
same kind of lactic acid. i. e., 1-acid. 
3. The lactic acid produced in naturally soured milk varies : 
( a ) According to the relative numbers of Streptococcus lacticus and B. 
aerogenes present, the higher the number of B. aerogenes the more 1-acid is 
produced. 
( & ) According to the temperature at which the fermentation takes place, other 
conditions being equal, at 37° C. relatively more 1-acid is formed than at room 
temperature. 
(c) According to the length of time the fermentation has lasted, the longer 
the time the more 1-acid is formed. 
4. In “ certified ” milk, d-acid only was present at room temperature for nine 
days, while both d-acid and 1-acid were present in milk of poorer quality after 
one to four days. At 37° C. 1-acid was apparent after six days in “ certified ” 
milk and on the second day in other milk. It seems as if the purer the milk 
the longer the excess of d-acid persists. 
5. Racemic lactic acid is the result of the formation of pure d-acid and pure 
1-acid by at least 2 different species of micro-organisms. Racemic lactic acid is 
not known to be the product of one species only. 
6. Since it is known that B. aerogenes forms other acids besides lactic acid, 
often in appreciable amounts, while Streptococcus lacticus produces almost pure 
d-acid, the presence of d-acid may be taken as indicating desirable conditions 
for dairy work, because this shows the absence of the fermentation products of 
B. aerogenes, i. e., volatile acids, gas, and ethyl alcohol. 
According to Clafflin (17) in the lactic acid fermentation as carried 
out in the manufacture of the acid, in which process the acid pro- 
duced is neutralized by chalk practically as fast as formed (or at 
least never allowed to exceed 0.02 to 0.5 per cent by weight of the 
solution), 98 per cent of the sugar is converted into lactic acid in 
three to six days through the action of a pure culture of Bacillus 
acidi lactici. On the other hand, in the ordinary souring of milk, in 
which case of course no pains are taken to neutralize the acid as it 
accumulates in the liquid, smaller amounts of lactic acid are formed 
and much less than the total quantity of lactose present is changed. 
Thus we have seen from the earlier observations of Boutroux and 
Richet that the lactic acid never exceeds 1.6 per cent by weight of 
the liquid undergoing fermentation, and according to recent observa- 
tions by Blumenthal and Wolff (18), milk which has been kept four 
years may still contain 50 per cent of its original lactose unchanged. 
It has also been found by Haacke (19) that the amount of lactic 
acid produced in the lactic fermentation never exceeds one-third of 
