Elements of the theory and practice of cookery

mixture ? In what other ways may this gas be produced ? What effect has it when introduced into batter or dough? As it is heavier than air, this gas may, with care, be poured into a tumbler from the bowl in which bread is rising, and tested by pouring lime-water into ^ the tumbler. Fig. 14. — Apparatus for proving that growing yeast produces carbon dioxide. flask containing yeast mixture, vessel of warm water, bealver containing' lime-water. glass tube. 228. Story of the yeast plant ; what it needs in order to grow. — Like mushrooms and other fungi (singular fungus) which have no green coloring-matter (^chlo- rophyll)^ yeast needs no light ; and like these, it grows and multiplies fast. It is like green plants in tliat it grows only when kept warm and moist. It thrives best at 78° F. It may be forced to grow and bud unnaturally fast by a higher temperature, just as hot- house plants are ; but at about 130° F. it loses its live- liness, and by heat greater than this it is killed. Cold checks its growth, but even after being frozen it will, if thawed, revive and grow as well as fever. It needs water, and either some nitrogenous foodstuff, such as gluten, or some mineral matter containing nitrogen to feed upon. Dried yeast-cells floating in the air revive if they fall where conditions are right for their develop- 128 THEORY AND PRACTICE OF COOKERY ment, and grow just as seeds do that fall on good ground. 229. How yeast obtains oxygen. — A name meaning sugar-fungus has been given to yeast, because, while most vigorous when well supplied with air, it will always, when sugar is at hand, take from this a part of the oxygen it needs. To get oxygen out of sugar, the yeast-cell digests the latter by means of a fluid it secretes. It is not known whether this fluid acts inside or outside of the yeast-cell, but it is known that it splits the sugar into simpler compounds, the most im- portant of these being alcohol and carbon dioxide. 230. Fermentation ; ferments. — A chemical change of this kind, where one organic substance is decomposed by another, is called fermentation, and the substance causing the change is called a ferment. The action of yeast in bread-raising is an alcoholic fermentation of sugar. 231. The digestion of starch, both in animals and in plants, the digestion of proteids, the souring of milk, and the clotting of milk by rennet are other kinds of fermenta- tion. What is the ferment in each of these cases ? What are some of the substances produced? (§§ 129, 174, 175, 176, 325.) 232. Cultivated yeasts. — Liquid yeast is yeast culti- vated in a mixture of potatoes, sugar, water, and hops. No home-made or baker's yeast can be as uniformly good as the pure yeast cultivated by brewers and dis- tillers. Distiller's yeast, skimmed from fermented rye, BREAD 129 washed, mixed with starch and pressed, is most com- monly used for making bread. Each cake contains about fifty billion yeast-cells.