96 



18 



thus permitting fermentation of the greatest quantity of sugar. The lactic acid can be 

 quantitatively determined When it has been liberated with sulphuric acid and afterwards 

 extracted With ether, but if it is only desired to accertain what quantity of the sugar 

 fermented is turned into acid, it is better not to add chalk to the cultures, as the quantity 

 of acid can then be simply determined by titration. In the case of the true lactic acid 

 bacteria, the quantity of volatile acids is as a rule small, and that of succinic acid generally 

 even less; we can therefore reckon all the acid formed as lactic acid, without risk of any 

 essential error. The calculation is thus very simple, when dealing with a hexose. It is 

 somewhat more complicated when, as in the case of milk, we have to start with a disaccha- 

 rid lactose, which may not only have become more or less hydrolysed by the bacteria, 

 but where also the two hydrolysis products, the grape sugar and the galactose, may have 

 been fermented in unlike degree. If the milk sugar be hydrolysed completely (by heating 

 to 115° for half an hour with 4% H^SO^,) then its power of reduction is increased some 

 40 — 43%^), and we can therefore, if such hydrolysis has been effected by the bacteria 

 (see Betacocciis bovis No. 34, Table III) even find an increase of sugar during fermentation. 

 It is consequently necessary to determine the degree of hydrolysis of the remaining sugar, 

 and correct accordingly. All the experiments noted in Table III were made with the same 

 milk, with 5.38% lactose, only in the last five experiments another milk was used, with 

 5.20% lactose (^12^/22^11^ ^zO) 



As will be seen, the remaining milk sugar is as a rule only slightly hydrolysed. In the 

 present experiments, only a few thermobacteria (apart from the betacoccus already 

 mentioned) were able to produce any considerable hydrolysis. As these formed over 1 14% 

 lactic acid, it follows that lactase can act even in liquids with a considerable degree of 

 acidity. In other experiments, where milk to which chalk had been added was used, it 

 was found that not only had the majority of the betacocci (also those of the species 

 Be. arabinosaceus) strongly hydrolysed the milk sugar, but generally also the thermobac- 

 teria, betal)acteria and strains of the species Streptococcus thermophilus, and Slreptobacleri- 

 um planlanim. Tlie power of hydrolysing milk sugar can hardly be employed as species 

 character, since it is met with, albeit not very frequently, in all species of lactic acid bac- 

 teria. The hydrolysis is only strong in old cultures, especially those in which the greater 

 part of the bacteria cells are dead, (in the culture of Betacoccus bovis No. 34, for instance, 

 all the cells were dead) which seems to suggest that the active lactase is in reality an 

 endocnzyme which is only given off when the cells are weakened, and that consequently, 

 hydrolysis of milk sugar outside the cell is not a normal process at all. Lactic acid bacteria 

 behave in exactly the same way towards saccharose and maltose, so that in all probability, 

 the disaccharides are taken in as such, and there is thus nothing to prevent their being 

 even better sources of carbon than the monosaccharides of which they are composed. 



As the differences shown in Table III between the quantity of sugar fermented and 

 the amount of acid formed lie as a rule within the limits of possible error (especially 

 considering that the quantity of sugar fermented cannot be corrected to entire accuracy) 

 we must take it that tru e lactic acid bacteria transform nearly all the sugar 

 to lactic acid. An exception, however, is Betacoccus bovis, and We shall shortly see that 



') hi milk cultures, we reckon with roughly speaking 40 "lo, as the more strongly reducing glucose 

 is often fermented in greater quantity than the less reducing galactose. 



