This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
Fermentation is that change of organic substances by which their starch, sugar, gluten, etc., under the influence of water, air and warmth are decomposed, usually with evolution of gas and heat, and their elements are re-combined into new compounds. This chemical process is classified, to wit: the saccharine fermentation changes starch 51 and gum into sugar; the vinous converts sugar into alcohol; the acetous changes alcohol and other substances into vinegar; the viscous (ropy) converts sugar into a mucilaginous or gummy substance; the putrefactive attends the decomposition of substances containing nitrogen.
Beverages dependent upon fermentation for their production include wine, lager beer, ale, porter, cider, and sometimes birch, root and similar small beers. In some of these drinks vinous or alcoholic fermentation plays an important role. As noted above this is the peculiar change by which sugar in solution is converted into carbonic acid, which is eliminated, and into alcohol, which remains in solution in the fermented liquor. The presence of a "ferment" is essential to excite the vinous fermentation, as a solution of absolutely pure sugar remains unaltered, even though exposed to the conditions most favorable to its accession.
Ferment is a substance undergoing decomposition or putrefaction, the particles of which are in continual motion. The yeast - the foundation of vinous fermentation - is a plant which must grow and feed upon some substance, like any other plant. It should therefore be treated in a similar manner, placed in vigorous substance, given the necessary warmth, that it may take root and grow, and also given the appropriate food, otherwise the desired effect and good results will surely fail to make its appearance. There are three things essential to all vegetable growth, whether it be the yeast plant or any other. First, the germ or seed; second, the proper temperature; and third, the proper food upon which the plant must feed, including, of course, the moisture. The fermentation or growth is carried on in two different ways; one working to the top, and called top fermentation; the other working towards the bottom, and called bottom fermentation; chemists therefore distinguish two kinds of yeast, viz.: the surface yeast and the sediment yeast. The former method of fermenting is the one most commonly used for the brewing of white beer, pale beer, and all light fermented beverages, also the English ales and beers, while the latter method is the one that is commonly applied everywhere by the brewers of lager beer, etc. This should be carefully considered by brewers of small beers, as the percentage of alcohol in the finished beer depends greatly upon the manner in which the fermentation is conducted, and the excise regulations which limit the amount of alcohol to two per cent. The fermentation induced by the yeast collecting on the surface is rapid and irregular, whilst that produced by the sediment yeast is slow and quiet, their chemical composition appearing to be identical. The surface yeast is formed at temperatures of 65 to 77° F., whilst the sediment yeast is produced at 32 to 45° F.
The essential condition of a ferment, to be able to excite the pure vinous fermentation, is to be sufficiently acidulous to act on blue test paper; and this acidity should arise from the presence of certain vegetable acids and salts, capable of conversion into carbonic acid and carbonates by their spontaneous decomposition. Those acids and salts which are found to pre-exist in fermentable fruits and liquors, as the tartaric, citric, malic, and lactic acids, and their salts, should be chosen for this purpose; preference being given to the bitartrate of potassa, on account of its presence in the grape. The addition of any substances to a saccharine solution renders its fermentation both more active and complete.