BACTERIA 31 



"pseudo" may be taken as corresponding with the concepts of 

 useful and harmful lactic acid bacteria. 



Lactic acid was first studied systematically by the Swedish 

 chemist Scheele in 1782 1 . It is a syrupy liquid, entirely soluble in 

 water, alcohol and ether. The most convenient method of 

 preparing it pure is to evaporate sour whey to a syrup, and to 

 extract this with ether. If chalk is stirred into the fermenting 

 liquid from time to time the acid will be neutralised, and thus 

 prevented from weakening the bacteria, which will then be able to 

 ferment the whole of the sugar. Lactic acid may be prepared on 



FIG. 34. Thermobacterium bulgaricum. Grown in milk pasteurised by heat- 

 ing previously to 80 C. for half an hour. Stained with methylene blue. 

 The grains are long-drawn and red. Capsule clearly shown X 1,000. 



a large scale from a 10 to 20 per cent, solution of maltose, made 

 by saccharifying starch with extract of malt containing active 

 diastase. The solution is treated with chalk, sterilised and fer- 

 mented by a pure culture for a week or two at 50 C., the culture 

 being kept pure at this temperature. As calcium lactate is formed, 

 the solution gradually sets to a pasty mass of crystals, which is 

 pressed and decomposed with sulphuric acid ; after filtering off 

 the calcium sulphate the liquid is evaporated, preferably in vacuo, 

 till it contains 40 to 80 per cent, of lactic acid. The yield is about 

 75 per cent. Lactic acid and its salts are largely used as mordants 

 in the dyeing and tanning industries. Before the War, Germany 



1 " Kgl. Vetenskaps Academiens nya Handlingar," Bd. III., p. 120. 



