It is evident that the different ferments delineated in Plate I. are worthy of thorough study, in consequence of the fermentations to which they may give rise. Care must be taken to isolate the action of each of them in fermentations which we may call pure—a condition of some difficulty, but one that may be carried out by an adoption of the methods explained in this work.
All these diseased ferments have a common origin. Their germs, infinitesimal and hardly perceptible as they are, even with the aid of the microscope, form a part of the dust conveyed through the air. This dust the air is continually taking from or depositing upon all objects in nature, so that the dust that clings to the ingredients from which our beer is manufactured, may teem with the germs of diseased ferments.
During the process of fermentation, the occult power of diseased ferments, although it may escape the observation of the brewer, is manifested in a high degree.
During the last thirty years, or so, the art of brewing has undergone a radical change, at least in Europe. This change has been effected by a partial abandonment of the process of fermentation formerly used. Thirty years ago only one kind of beer was known; there are nowadays two distinct kinds—beer fermented at a high temperature, and beer fermented at a low temperature. Each of these is subdivided into many varieties, to which different names are given, according to their strength or colour. This is the case in England, where we find porter, ale, pale ale, stout, bitter beer, and other varieties of beer, although, as a matter of fact, the English have but one kind of beer, all the English beers being fermented at a high temperature.
Let us briefly examine the differences existing between the two kinds of beer.
Formerly all beer was fermented at a high temperature. The wort, after having been cooled in the backs, was run into a large vat, at a temperature of about 20° C. (68° F.). Yeast was then added to it, and when the fermentation began to show itself, in the formation of a light, white froth, upon the surface of the liquid, the wort was run into casks, having a capacity of from 50 to 100 litres (11 to 22 gallons)—75 litres being the commonest size. These casks were placed in cellars, having a temperature of from 18° C. to 20° C. (64° F. to 68° F.). The activity of fermentation soon produced a froth that grew thicker and more and more viscous in proportion to the quantity of yeast it contained. The yeast worked out of the bung-holes and dropped into a vessel placed under the casks; there it was gathered for subsequent operations. It always exceeded the quantity used in the first instance, the ferment increasing greatly during the process of fermentation. The increase in its weight varied with the weight of yeast used and the composition of the wort. Under the ordinary conditions of brewing, where the weight of the pitching yeast was about one thousandth part of the weight of the wort, the increase is said to have been from five to seven times the weight of the yeast; but such increase must naturally have been determined by the quality of the wort, the quantity of hops used, the action of oxygen, and the proportion of barm employed. The process of fermentation lasted from three to four days. By that time the beer was finished, and had become limpid, the fermentation having been completed. The bungs could then be placed in the casks, and the beer be delivered to the customer.[5] A certain amount of yeast still remained in the casks, and caused the beer to become thick, in transit; but a few days’ rest sufficed to restore its brilliancy, and render it fit for drinking or bottling.