452 MICROBIOLOGY OF SPECIAL INDUSTRIES. 



FERMENTATION. If the raw material to be used is starchy or sugary, 

 it must be first changed into an alcoholic liquid containing from 6 to 1 2 

 per cent of alcohol by volume. This is accomplished by one of the methods 

 discussed in the preceding chapter. This alcoholic fermentation must 

 be kept rigidly distinct from the acetification and- is best carried out in a 

 separate building. The yeast must finish its work before the bacteria 

 commence theirs. The reason for this is that yeasts are very sensitive 

 to acetic acid and a small quantity may paralyze their activity and prevent 

 the change of all the sugar into alcohol, with a consequent loss of strength 

 and quality in the final product. 



The quality of the vinegar will depend on the quality of the raw 

 material from which it is made. Wine or cider spoiled by bacterial 

 fermentation, moldy casks, etc., will make inferior vinegar. An exception 

 to this may be made of so-called "pricked" wines, which are simply 

 wines in which acetic fermentation has started spontaneously. The 

 wine or other alcoholic liquid should be perfectly clear and clean tasting 

 and, if necessary, should be fined, filtered or pasteurized immediately 

 before use. It should contain no antiseptic which would interfere with 

 the development of the acetic bacteria. Sulphurous acid is particularly 

 troublesome in this respect, and should be removed or oxidized by 

 thorough aeration. 



Commerical alcohols made from corn, potatoes, beets, molasses and 

 other products can be used. The special flavors of these alcohols, due 

 to their origin, disappear almost completely in the vinegar. This, however, 

 is not true of denatured alcohol or that containing methyl alcohol or 

 acetone. 



The alcohol must be diluted to from 10 to 12 per cent by volume, and 

 then made suitable for the growth of acetic bacteria by the addition of 

 nutritive substances containing nitrogen and phosphates. This is ac- 

 complished usually by adding 10 per cent of wine, beer, malt-extract, yeast 

 decoction, or similar material to the diluted alcohol. The waste liquids 

 from a brandy distillery may be used instead of water for dilution. After 

 resting a few days, the mixture is filtered and is then ready for acetification. 



Before starting the acetic fermentation, it is a usual and good practice 

 to add about 10 per cent of good vinegar to the liquid, which is thus 

 rendered acid and therefore less liable to alteration by injurious bacteria 

 and other microorganisms. 



All the processes of vinegar-making depend on the same principle. 



