640 MICROBIOLOGY OF ALCOHOLIC FERMENTATION 



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 12 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 im- 

 mediately before use. It should contain no antiseptic which would 

 interfere with the development of the acetic bacteria. Sulphurous 

 acid, if present in the free state, 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 1 2 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. 



