The starch of the potato is thought to furnish better material for the promotion of yeast growth than that of wheat flour; but whether the potato be first cooked, mashed, and then combined with the other ingredients, or grated raw and then cooked in boiling water, makes little difference so far as results are concerned, though the latter method may have the advantage of taking less time. If potatoes are used for this purpose, they should be perfectly mature. New ones will not answer. Sugar assists in promoting the growth of the yeast plant, and a small amount is usually employed in making yeast. Hops serve to prevent the yeast from souring, and an infusion of them is frequently used for this purpose. While it is essential that the water used should be boiling, it is also necessary that the mixture should cooled to a lukewarm temperature before the introduction of the original yeast, as intense heat will kill the yeast plant. Freezing cold will likewise produced the same result. While a cool temperature is one of the requisites for keeping yeast fresh, care must be taken, especially in winter, that it does not get chilled. When yeast is needed for bread, it is always the best plan to take a cup to the cellar or refrigerator for the desired quantity, and re-cover the jar as quickly as possible. A half hour in a hot kitchen would be quite likely to spoiled it. Always shake or stir the whole well before measuring out the yeast. In making yeast, used earthen bowls for mixing, porcelain-lined or granite-ware utensils for boiling, and silver or wooden spoons for stirring. BITTER YEAST.--It sometimes happens that an excessive use of hops in the making of yeast gives to it so bitter a flavor as to communicate a disagreeable taste to the bread. To correct this bitterness, mix with the yeast a considerable quantity of water, and let it stand for some hours, when the thickest portion will have settled at the bottom. The water, which will have extracted much of the bitterness, can then be turned off and thrown away. Yeast also sometimes becomes a bitter from long keeping. Freshly burnt charcoal thrown into the yeast is said to absorb the odors and offensive matter and render the yeast more sweet; however, we do not recommend the use of any yeast so stale as to need sweetening or purifying. Yeast that is new and fresh is always best; old and stale yeast, even though it may still possess the property of raising the dough, will give an unpleasant taste to the bread, and is much less wholesome. TESTS FOR YEAST.--Liquid yeast, when good, is light in color and looks foamy and effervescent; it has a pungent odor somewhat similar to weak ammonia, and if tasted will have a sharp, biting flavor. Yeast is poor when it looks dull and watery, and has a sour odor. Compressed yeast, if good, breaks off dry and looks white; if poor, it appears moist and stringy.