i 



EXTERNAL CONDITIONS OF LACTIC FERMENTATION 157 



tant organisms associated with lactic acid fermentation will be 

 found at a later stage in the present volume. Here we 

 may content ourselves with the general remark that as a rule 

 the lactic fermentation bacteria are short rods, do not liquefy 

 gelatine, nor do they form spores. They grow readily on gela- 

 tine at room temperature, forming as a rule small circular colonies, 

 white or grey in colour, with sometimes a tinge of yellow, and the 

 surface of the colony is smooth and glistening. The lactic acid 

 organisms produce appreciable amounts of lactic acid only at 

 somewhat elevated temperatures. If the amount of acid rises 

 much above 2 per cent, the growth of the lactic acid bacteria is 

 inhibited. Many other substances, as we have seen, are produced 

 in addition to lactic acid {e.g. acetic and ferric acids, alcohol, 

 methane, COoH.N., etc). Lactic acid organisms (as non-spore 

 bearers) are readily killed by pasteurisation. The economic 

 function of the lactic ferments concerns, of course, the manu- 

 facture of butter and cheese. 



External conditions affecting lactic fermentation in milk. — Time j 

 , and temperature are the two chief external conditions necessary 

 j for the development of fermentation, and the exact time of sour- 

 ing varies in relation to temperature. At 10' C. milk becomes sour 

 I after 120 hours, and curdles on or about the seventh day. At 

 I 15° C. it is sour in sixty-six hours, and curdles on or about the 

 seventh day. At 37" C. (blood-heat) milk is sour in sixteen or 

 eighteen hours, and curdles on the second day. At boiling point 

 there is a diminution of the total acidity in milk, partly due to the 

 j decomposition of calcium salts having an acid reaction, and partly 

 ' to the expulsion of carbonic acid. The general rule is t hat milk 

 turns sour most readily. at between 35' Cajid42°X^ and when 2 

 I per cent of lactic acid is reached the process of further production 

 I ceases. If the milk be then neutralised with carbonate of lime, 

 the process of fermentation will recommence until the acidity 

 again stands at 2 per cent Pepsin may also be added to re- 

 start the process. As will be seen elsewhere in the present 

 volume, there is abundant evidence to show that lactic acid fer- 

 mentation is most active in milk between twelve or twenty hours 

 and fifty or sixty hours after milking. After this period of time 



of the aerobic species produce gas, and are the lactic organisms which set up ^ 

 " gassy " changes in cheese making. Such organisms are present in milk in 

 from 2 to 5 per cent. Some lactic organisms liquefy gelatine, and probably a 

 few produce spores. The quantity of lactic acid produced by different species 



