106 MICRO-ORGANISMS AND FERMENTATION. 



2. Lactic Acid Bacteria. 



If the micro-organisms of milk are subjected to spon- 

 taneous development at a temperature of 30-35 C., the 

 lactic acid organisms soon begin to ferment the lactose present 

 (about 4 per cent.), and the acid produced protects the milk 

 from putrefaction. After a certain quantity of acid has been 

 formed it checks the activity of these bacteria, and the milk 

 mould (Oidium lactis) develops. This oxidises a portion of 

 the lactic acid, and thus enables the bacteria to restart their 

 action. The same effect is produced if the acid is neutralised, 

 for instance with calcium carbonate, and thus the complete 

 fermentation of the milk sugar may be carried out. Simul- 

 taneously with the formation of lactic acid, casein, which 

 forms the most important part of the albuminoid constituents 

 of milk, separates out. Before souring, the casein occurs as 

 a calcium salt (100 casein to 1-55 CaO), and is present in the 

 colloidal form. When the milk is soured, the lime combines 

 with lactic acid, liberating the casein, which is precipitated 

 in a fine flocculent condition, causing the curdling of milk. 

 In addition to this a greyish-yellow serum gradually separates 

 out containing calcium lactate, lactose, albumen, etc. 



Other varieties of lactic acid may be developed by exposing 

 a malt or other mash to a given temperature. If the mash is 

 maintained at 40 C. a pediococcus form develops vigorously, 

 if at about 50 C. a short rod form. If a fraction of the liquor 

 is transferred to another mash at the same temperature, the 

 respective forms each receive an impetus, and after a few 

 inoculations only the two forms can be discovered in the 

 respective mashes by an ordinary microscopical examination. 

 It will be noticed that by the process described there can be 

 no guarantee that a pure culture has been obtained, for in 

 each case other bacteria survive, even if in an extremely weak 

 condition, and, on the other hand, there is a possibility that 

 more than one species, or variety, of lactic acid bacterium may 

 develop at each temperature. Similarly in beer-wort and other 

 liquids spontaneous lactic acid fermentation may occur. This 

 is also the case in the souring of " sauerkraut," the preparation 

 of leaven, ensilage, etc., and the bacteria which develop in 



