The yeast mash we will assume is composed of equal parts of barley malt and rye meal. Hot water at 166∞ F. is first put into the mash tub. The rake or stirrers are then rotated and the meal run in slowly. The stirring is continued for twenty minutes after the meal is all in, during which the mash has become saccharified. The mash is then allowed to stand for about twenty hours, and to grow sour by lactic fermentation. The lactic acid so produced protects the mother yeast from infection by suppressing wild yeast and bacteria. During this period great care is taken to prevent the temperature of the mash falling below 95∞ F. and consequent butyric and acetous fermentation following. After it has so stood the sour mash is cooled by circulating water in the coils and stirring until it is reduced to from 59∞ to 68∞ F. depending on whether the mash is thin or thick. Start yeast during the cooling of the mash when at above 86∞ F. is added and stirred in. For the next twelve hours the yeast ferments and when a temperature of 84∞ F. has been attained the mash is cooled to 65∞ F. at which temperature it is maintained until allowed to enter the fermenting tubs through the pipe leading thereto from the yeast tub. There are four principal kinds of fermentation: alcoholic, acetous, lactic and viscous. =Alcoholic Fermentation.= This may be briefly described as follows: The mash in the fermenting vat having been brought to the proper temperature, the ferment is thrown in, and the whole is well stirred together. This is known as pitching. [Illustration: FIG. 5.--Yeasting and Fermenting Apparatus. (_To face page 22_)] The proper pitching temperature varies with the method of fermentation adopted, the length of the fermenting period, the materials of the mash, its thickness or attenuation. It must always be remembered that there is a great increase in the temperature of the "beer" during fermentation and that the temperature at its highest should never under any circumstances, become greater than 86∞ F. and with thick mashes that even a less heat is desirable. Therefore the pitching temperature should be such that the inevitable rise due to fermentation shall not carry the temperature to or beyond the maximum point desired for the particular mash being treated. It is to accurately control the pitching temperature and the fermenting temperature that the fermenting tanks are provided with cooling appliances.