During the progress of fermentation, the temperature of the liquid rises, the maximum heat being attained when the fermentation is at its highest point. In some instancea, the rise of temperature is upwards of 14° (30° F.), but generally lower, and in the case of stock, and Scotch ales, it is as low as 10° (16° F,). If the heat is allowed to rise too high, the glutinous constituents of the beer are not perfectly removed in the yeast, and as the gyle does not cleanse perfectly, an after ferrunntation ensues, technically termed the " fret." On the other hand, too low a temperature causes sluggish fermentation, and, as a consequence, the beer is apt to gain a yeast-bitter flavour from being retained too long in contact with the yeast. To avoid these results, various contrivances are employed to keep the temperature at all times under control.
In oonnection with the subject of fermentation, it is advisable to discuss briefly the determination of what are called "original gravities," or the gravities of the worts from which any given samples of beer may have been made. According to Act of Parliament, 10th Victoria, cap. 5, a drawback of 5s. a barrel is granted on all beer exported, on condition that the worts before fermentation were not of less specific gravity than 1.081. A brewer knows the strength of the wort from which the beer has been made ; but it is necessary that the revenue officer also should have the means of obtaining independently from a sample of the beer the same information, and this necessity has led to the close investigation of changes which take place during fermentation. For each sample of beer there have to be determined the original gravity of the wort from which it was produced, the specific gravity of the beer itself, or, as it is sometimes called, the beer gravity ; the spirit indication; and the proportions of unfermented solid matter, or extractive matter, held in solution by the beer. The specific gravity of the beer can be determined by the hydrometer, while the extract gravity, or the specific gravity, of the beer without its spirit, may be obtained by par tially evaporating a given quantity of beer, to expel the alcohol, and making up the original bulk by the addition of water. By comparison of the specific gravity of the beer with the extract gravity, an indication may be obtained of the quantity of alcohol in the beer. This quantity may also be ascertained by distillation, by the refracting power of the beer on rays of light, or by observation of its boiling point, which lowers with increase of alcohol. It is possible, if the amount
of alcohol in the beer is known, to roughly determine the original gravity of tbe wort by increasing the extract gravity by the amount due to the quantity of starch sugar which would have to be decom posed during fermentation to produce the known quantity of alcohol. Original gravities thus deter mined, however, are useless for practical purposes, because the final or beer gravity is the result, not merely of the attenuation produced by the decomposition of the sac,charine matter, but also of the changes effected in other constituents of the wort during the process of fermentation.
In comparing the specific gravities of various solutions of sugar, malt, and other ingredients, Graham, Hofmann, and Redwood take as their standard of comparison the proportion of e,arbon that a given solution contains, and they have proved that the specific gravity of a solution, contaioing a given proportion of carbon will vary to some extent according to whether that carlson is present in the form of sugar, dextrine, or extractive matter. The annexed table shows the specific gravities of various solutions compared in this way :— It has been stated that, when fermentation occurs in a aolution of cane sugar, there is at first a slight increase of density, due to the transformation of the cane sugar into starch sugar, followed by attenuation, due to the formation of alcohol and tlae evolution of carbonic acid. In a wort con taining cane sugar, a transformation of this into grape or starch sugar precedes the vinous fermenta tion, and this change occasions an increase of gravity of nearly 3°, in a solution of which the original gravity is 1055. The rate at which the riso in gravity occurs varies according to the amount of yeast added to produce fermentation. A similar effect results when the transformation of the cane into starch sugar is effected hy the addition of acids instead of yeast. The comparative densities of solutions of cane and starch sugar are given in the preceding table, aud the fact that they differ is important, for the original gravity of a fermented liquor or beer must be different, according as it was derived from a wort of cane sugar or of starch sugar. Since, in a small wort, the saccharine matter is present in the form of starch sugar, there is no increase of density previous to fermentation.