Fermentation.—When cooled, the wort is led to the vessels or tuns in which it is to undergo fermentation. The nature of the chemical changes known as fermentation, and the conditions under which they take place, have been described and indicated in a former article (see Alcohol, p. 194).
The sugar in the wort on its transformation into glucose takes up two equivalents of water ; therefore the combined weights of the carbonic acid and the alcohol resulting from the fermenta tion is greater than that of the sugar miginafly contained in the wort. It follows that there should be an increase in the specific gravity of the solution or wort during the time that the conversion of the cane sugar into grape sugar is taking place. This is actually the case, and it is a fact of which the brewer takes advantage. As the evolution of carbonic acid gas progresses, the specific gmvity of the liquid is observed to decrease ; and this gradual reduction of specific gravity is by brewers termed tho " attenuation." The attenuation of the wort is accompanied by a rise in tem perature, and it is by this increase of temperature, combined with the reduction of the specific gravity, as observed by the saccharometer, as, well as by the appearance of the head formed, that the brewer is enabled to judge how the process of fermentation is proceeding.
In the fermentation of a malt wort, the conversion of the saccharine constituents into alcohol and carbonic acid does not occur alone. The albumen and gluten become insoluble in the alcohol formed by the process of fermentation. Under the circumstances Of the English method of pro cedure, one portion of these insoluble substances is buoyed up by the ascending globules of carbonic acid forming the frothy head which collects on the surface. The remaining portion of the insoluble matters is deposited as bottom barm, which consists of gluten mixed with the denser impurities of the wort, and is a cruder material than the yeast floating on the surface. The proportion that the floating yeast will hear to the bottom barm will vary with the nature of the malt, the heat of the mashing process, and with the temperature at which the fermentation is carried on. In the ease of the Bavarian brewing process, in which the fermentation is allowed to proceed very slowly, a mere film is forrned on the surface of the wort, the insoluble matters being almost entirely deposited as a viscid sediment, termed the " unterhefe."
The initial temperature at which the wort is pitched or mixed with the yeast in the fermenting tuns exercises an important influence upon the energy of the fermentation, and it has to be regu lated according to the atmospheric temperature of the tun room and the strength of the wort. According to the English practice, the pitching temperature ranges from 11° (51° F.) to as high as 18° (64° F.), but in the Bavarian system it is kept as low as 7° to 10° (45° to 50° F.) In winter, the air, being at a low temperature, tends to check the energy of the fermentation ; whilst, in summer, as the air is frequently at a higher temperature than that at which the wort is pitched, the fermentation is more difficult to control. For these reasons, it is necessary that the pitching temperature should be lower in summer than in winter, unless means are provided for keeping the tun room cool. The smaller the vessels in which the fermentation is carried on, the greater will be the surface exposed by them in proportion to their contents, and the greater therefore will be the influence exerted by the atmospheric temperature. So long as the temperature of the air in the tun room is below that of the wort, the energy of the fermentation may be checked by dividing out tbe wort into a number of small vessels; but if the atmospheric temperature in the tun room is higher than that of the wort, the reverse effect would, of course, be produced by such a system of division. Care is generally taken to so construct and place the tun room that it may be kept at a moderate temperature even during the hottest weather, whilst in some instances special arrange ments for cooling the air are adopted. Another point to be considered with respect to the initial temperature is the character of the beer to be produced ; a light beer, intended for immediate consumption, may he pitched at a comparatively high temperature, but in the case of a strong stoek ale the initial temperature should not be higher than 12° or 13° (54° or 56° F.), and it should not be allowed to rise more than 10° (16° F.) during the process of fermentation. Pale ales, also, which usually receive a liberal allowance of yeast, should be pitched at a low temperature. The quantity of yeast to be added, like the pitching temperature, depends upon so many circumstances that it is impossible to give any general rule.