FERMENTATION 261 



normal cells are motile. In order to develop vigorously, 

 Bact. aceti not only requires a plentiful supply of oxygen, 

 but also a fairly high temperature. 



Vinegar has been defined to be ' the product of the 

 alcoholic and acetous fermentation of a vegetable juice or 

 infusion.' This definition includes all kinds of brewed 

 vinegar, but excludes wood-vinegar. Brewed vinegar of 

 whatever source will naturally be distinguished from wood- 

 vinegar (acetic acid and water), by containing extractive 

 matters which will remain when the sample is evaporated. 



In the case of malt-vinegar, by which we understand 

 vinegar brewed either entirely from malted barley or from 

 a mixture of not less than one-third malt and two-thirds 

 barley, we find the extractive matter to range about 2* 5 per 

 cent. 



The process of vinegar-making is as follows : The malt 

 or malt and barley (the latter finely ground) are ' mashed,' 

 or soaked in successive quantities of hot water till all that 

 is soluble is extracted. The clear liquor is then run off 

 into another vessel, and yeast added. Fermentation then 

 takes place, with evolution of carbonic acid. The ' wort,' 

 or ' wash,' is then pumped over piles of birch-twigs placed 

 in high vats, to which a regulated supply of air is supplied. 

 The twigs become coated with Mycoderma aceti, 'vinegar 

 plant,' and the alcohol produced by the fermentation is 

 then converted into acetic acid. 



Small quantities of other bodies — as acetic ether, alde- 

 hyde, etc.^ — -are formed, which give malt-vinegar its pleasant 

 taste and smell. 



In good working all the alcohol is not converted into 

 vinegar, as a little alcohol improves the flavour and assists 

 the ' keeping ' of the finished product, which is generally 

 kept for a year in order that the flavour may fully develop. 



Nitrification of Ammonia. — The well-known and important 



