YEAST.--Next to good flour, the most important requisite in the manufacture of fermented bread is good yeast. The best of flour used in conjunction with poor yeast will not produce good bread. The most convenient and reliable kind of marketable yeast, when fresh, is the compressed yeast. The dry though they are always ready for use, the quality of the bread they produce is generally inferior to that made with either compressed yeast or good liquid yeast. If this sort of yeast must be depended upon, the cakes known as "Yeast Foam" are the best of any with which we are acquainted. Of homemade yeasts there are almost as many varieties as there are cooks. Their comparative value depends mainly upon the length of time they will keep good, or the facility with which they can be prepared. Essentially the same principles are involved in the making of them all; viz., the introduction of a small quantity of fresh, lively yeast into a mixture of some form of starch (obtained from flour, potato, or a combination of both) and water, with or without the addition of such other substances as will promote fermentation, or aid in preventing the yeast from souring. Under proper conditions of warmth, the small amount of original yeast begins to supply itself with food at once by converting the starch into dextrine, and then into grape sugar, and multiplies itself with great rapidity, and will continue to do so as long as there is material to supply it with the means of growth. While its growth is rapid, its decay is equally so; and unless some means of preservation be employed, the yeast will die, and the mixture become sour and foul. Ordinarily it can be kept good for several days, and under the best conditions, even three or four weeks. After it has been kept from four to six hours, it should be placed in some receptacle as nearly air-tight as possible and set in the cellar or refrigerator, where it can be kept at a temperature not conducive to fermentation. Thus the little yeast organisms will remain in a quiescent state, but yet alive and capable of multiplying themselves when again surrounded with favorable conditions. The yeast should be kept in glass or glazed earthen ware. The vessel containing it should be washed and scalded with scrupulous care before new yeast is put in, since the smallest particle of sour or spoiled yeast will ruin the fresh supply in a very short time. It is generally conceded that yeast will keep longer if the material of which it is made be mixed with liquid of a boiling temperature, or cooked for a few minutes at boiling heat before adding the yeast. The reason for this undoubtedly lies in the fact that the boiling kills foreign germs, and thus prevents early souring or putrefaction. The yeast must not be added, however, until the liquid has cooled to a little more than blood heat, as too great heat will kill the yeast cells.