VINEGAR FERMENTATION. 179 



little of the starch into sugar, which escapes decomposition into 

 alcohol and carbon dioxide, and remains in the bread, rendering 

 it somewhat sweeter to the taste than aerated bread, unless a little 

 sugar is purposely added to the latter during manufacture, as is 

 sometimes done. 



Expt. 193. To prepare Vinegar. It is worth noticing, that 

 whereas the formation of alcohol from sugar by splitting up 

 requires the presence of no free oxygen for its occurrence, and can 

 therefore go on in a corked bottle or closed cask, the subsequent 

 process of souring wine and beer so as to produce vinegar (from 

 the French vin aigre = sour wine), will not take place under such 

 conditions, access of air being indispensable as well as the growth 

 of a peculiar organism or vinegar ferment, analogous to yeast, but 

 not identical therewith. Half-empty casks of ale and bottles of 

 light wines are peculiarly apt to undergo the " acetic fermentation," 

 and turn sour, because the remaining liquid is necessarily in con- 

 tact with the air, which supplies the oxygen requisite to enable 

 the change to take place ; strong wines containing a large propor- 

 tion of alcohol are not so easily affected as lighter ones. Some- 

 times vinegar is prepared from injured batches of wine, damaged 

 cider, and such like materials, but preferably from properly fer- 

 mented malt worts, by filling a wooden cylinder with shavings and 

 allowing air to pass freely into it, whilst the liquor to be soured 

 into vinegar is allowed to trickle slowly over the shavings, and thus 

 to be exposed in thin layers to the air. The shavings soon become 

 coated with the vinegar plant, so that the souring action goes on 

 rapidly. Another class of vinegar, preferred by some, is made by 

 simply diluting pure acetic acid (Expt. 173) to taste, and some- 

 times colouring with a little burnt sugar. 



Opinions differ widely as to the relative goodness of vinegar 

 prepared in these two ways ; some prefer that made by a fermen- 

 tation process, as it has a somewhat different flavour ; thus old- 

 fashioned housewives sometimes prepare their own vinegar by 

 fermenting to sourness a mixture of sugar, &c., and certain 

 flowers and plants, e.g., primroses. Others consider that pure 

 acetic acid diluted with 25 or 30 or even more times its weight of 

 water, is preferable to all fermented products. 



Expt. 194. To detect Sulphuric Acid in Vinegar. Sulphuric 

 acid is sometimes used to adulterate vinegar, more especially 

 the coloured varieties; formerly this addition was permitted by 

 law to the extent of 1 part in 1000, but now it is forbidden 

 in this country. Add to some of the clear vinegar a solution 

 of nitrate of barium or chloride of barium; if sulphuric acid 

 be present, a cloudiness will be produced, and after standing 



