The best yeast is considered to be that obtained from pale gyles towards the completion of alcoholic fermentation, this yea,st being denser than that thrown off during the earlier stages, and free from admixture with old yeast that has been added to the wort to induce fermentation. Yeast, if collected from the stillions and placed in reservoirs or tubs, is apt to work and lose strength ; it is better to let it remain in the stillions with a portion of the drawings, until required, when the draw ings should be strained off. If the yeast has to be stored, it is best covered with water, and kept in the coolest pla.ce available. The water should be quite cool, and should be occasionally renewed.
The heavier, or in other words the stronger, the wort, the greater will be the proportion of yeast necessary, and this proportion will also be affected by the degree of attenuation to be produce& The quantity of yeast required depends also upon the temperature at which the gyle is pitched ; the higher this temperature, the less the quantity of yeast necessary. Black states that if the worts are got together in the gyle tun at a temperature under 15° (60° F.), about 1 lb. of yeast a barrel for every 10 lb. of gravity, as indicated by Long's instrument, will be found to produce a loss of 1 lb. in attenuation for every degree of heat gained, and he considers this to be a good working rule. It should be remembered in all cases that a deficiency of yeast is better than an excess, since it is possible, if the fermentation is sluggish. to add more yeast ; whereas, if the latter be in excess, a too violent fermentation may be set up, which it may be impossible to control. Yeast added after the fermentation should be first mixed with a portion of the wort, and then well stirred in ; but this practice is not r,o be recommended, since it is likely to impart a rank flavour of yeast to the beer. In all cases where the weight of the yeast per gallon is not accurately known, the yeast should be apportioned out by weight, and not by measurement. Before the yeast is placed in the tun, it is mixed with a small quantity of wort, and left in a warm place until fermentation commences, when the mixture, termed "lobb," may be added to the gyle in the tun. Some brewers add the full quantity of yeast at once, whilst others reserve a certain proportion to be added subsequently to stimulate the fermentation. So long, however, as the quantity of yeast required is accurately known, the former system appears t,o be preferable.
In a thoroughly healthy fermentation, the rise of temperature which ensues as the process goes on should be steadily accompanied by a decrease in the gravity of the wort, or, for every degree of heat gained, a pound of saccharine matter per barrel should be transformed into carbonic acid and alcohol, an effect which will be shown by the sac,charometer. Of course, this correspondence between
the increase of heat and the attenuation attained is to some extent liable to modification by extremes of heat or cold. The stages of a healthy fermentation are as follows :—Some six or eight hours after the yeast has been added, minute bubbles of cexbonic acid gas begin to rise, and a thin creamy froth is formed, first round the edges of the tun, but gradually extending over the whole surface of the liquid. As the temperature rises, and the decomposition of the saccharine matter becomes rapid, the evolution of the carbonic acid gas takes place more freely, and, as a consequence, the froth rises, forming what ia termed the " cauliflower head." At this stage, the aroma becomes very perceptible. The cauliflower head should rise two or three feet above the surface of the gyle, and it should be of a brownish-white colour ; a bluish-white colour at some parts indicates un soundness. The next change consists in the breaking up of the cauliflower head into what is termed the " rocky head." The rocky head is produced by the bursting of the globules of froth, the yeast at flrst thrown off not being sufficiently viscid to retain the accumulation of carbonic acid. At this stage, the head should fall some three or four inches, and the aroma should be very pungent and vinous. In the next and last stage, the head again rises, forming what is known as the "close yeasty head," this having the appearance of yeast all over the surface. If the fermentation be a healthy one, the head will at this stage be covered with small bubbles at the top, these constantly bursting, discharging their gas, and being replaced by new ones. This goes on until the beer is considered to be ready for cleansing or skimming, a process which consists in removing the yeast from the surface. Different methods of conducting the cleansing will be described later. The object of the operation is to prevent the imparting of a bitterness or yeast-bitter flavour to the beer, which might be the case if the beer were allowed to remain in contact, at a temperature approaching 21° (70° F.), with yeast that had to some extent entered into putrefactive fermentation.