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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 
Bichet that the lactic acid never exceeds 1.6 per cent by. weight of 
the liquid undergoing fermentation, and according to recent observa- 
tions hv 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 
the amount of sugar decomposed and that the quantity of acid 
present at any one time during the time of the fermentation is not 
strictly proportional to the amount of sugar decomposed, for the 
reason that a part of the lactic acid resulting from the decomposi- 
tion of the sugar is in all probability decomposed into other sub- 
stances. According to this observer 1,000 lactic bacilli decomposed 
in one hour an amount of sugar varying according to conditions 
from 0.00001 to 0.008 milligram. 
The changes brought about in milk by micro-organisms are by no 
means confined to the production of lactic acid. In fact as we have 
already seen it is only by working with pure cultures of certain of 
the lactic acid organisms such as the Streptococcus lacticus, etc., 
under proper conditions, that lactic acid alone is produced, and in 
the souring of milk as this ordinarily takes place a great many sub- 
stances besides lactic acid are produced in larger or smaller amounts. 
Among these may be mentioned acetic, butyric, and succinic acids 
alcohol and gaseous substances such as hydrogen and carbon dioxide. 
In addition to these substances may be mentioned the production 
of small amounts of substances having characteristic ocfors usually 
of a disagreeable character. 
