Science in the Kitchen. E. E. Kellogg
the investigations of modern chemistry have made clear the properties of yeast, that has come to be considered the best agent for setting up the process of alcoholic fermentation in bread. The use of leaven is still practiced to somewhat in some European countries. The bread produced with leaven, although light and spongy in texture, has an unpleasant, sour taste, and is much less wholesome than that produced with fresh yeast.
Yeast is a collection of living organisms or plants belonging to the family of fungi, which, like all other plants, require warmth, moisture, and food, in order to promote growth, and when properly supplied with these, they begin to grow and multiply rapidly. Fermentation will not take place at a temperature below 30°, it proceeds slowly at 45°, but from 70° to 90° it goes on rapidly. Fermentation may be arrested by the exhaustion of either the fermenting agent or the food supply, or by exposure to heat at the temperature of boiling water. This latter fact enables the housewife to arrest the process of fermentation, when the loaf has become sufficiently light, by baking it in a hot oven. Heat destroys most of the yeast cells; a few, however, remain in the loaf unchanged, and it is for this reason that yeast bread is considered less wholesome for dyspeptics than light unleavened bread. It is apparent, then, that the more thoroughly fermented bread is baked, the more wholesome it will be, from the more complete destruction of the yeast germs which it contains.
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.
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.
If there is any question as to the quality of yeast, it is always best to test it before use by adding a little flour to a small quantity and setting it in a warm place. If it begins to ferment in the course of fifteen or twenty minutes, it is good.
Starting the Bread.—Having secured good yeast, it is necessary in some way to diffuse it through the bread material so that it will set up an active fermentation, which, by the evolution of gas, will render the whole mass light and porous. As fermentation is more sure, more rapid, and requires less yeast to start it when set in action in a thin mixture than when introduced into stiff dough, the more common method of starting fermented bread is by "setting a sponge;" viz., preparing a batter of flour and liquid, to which potato is sometimes added, and into which the yeast is introduced. Some cooks, in making the batter, use the whole amount of liquid needed for the bread, and as the sponge rises, add flour in small quantities, beating it back, and allowing it to rise a second, third, or even fourth time, until sufficient flour has been added to knead; others use only half the liquid in preparing the sponge, and when it has well risen, prepare a second one by adding the remainder of the liquid and fresh flour, in which case the fermented batter acts as a double portion of yeast and raises the second sponge very quickly. The requisite amount of flour is then added, the dough kneaded, and the whole allowed to rise a third time in the loaf. Other cooks dispense altogether with the sponge, adding to the liquid at first the requisite amount of flour, kneading it thoroughly and allowing it to rise once in mass and again after molding into loaves. As to the superiority of one method over another, much depends upon their adaptability to the time and convenience of the user; light bread can be produced by either method. Less yeast but more time will be