This section is from the book "Elements Of The Theory And Practice Of Cookery", by Mary E. Williams. Also available from Amazon: Elements Of The Theory And Practice Of Cookery; A Textbook Of Domestic Science For Use In Schools.
Pare potatoes and keep in cold water. Mix flour, sugar, and salt in a large bowl, and grate the potato in as quickly as possible. Mix at once with a wooden spoon. Pour on boiling water directly from the tea-kettle, stirring constantly, until enough water has been added to make the mixture the consistency of thin starch. (If it does not thicken, heat it to the boiling-point.) Strain, and let it cool. When it is lukewarm, stir in the yeast. Beat well several times during the day. At the end of 24 hours, put it into earthen or glass jars, fasten the covers down tight, and put in a cool place. It will keep two weeks. Save the last cupful to start new yeast.
A name meaning sugar-fungus has been given to yeast, because, while most vigorous when well supplied with oxygen from the air, it will, 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 sugar by means of a juice it secretes, which splits sugar into simpler compounds. The most important of these compounds are alcohol and carbon dioxide. This juice commonly acts inside the yeast-cell, but it can be extracted from yeast, and then acts on sugar as living yeast does.
Fermentation is caused by substances called enzyms.
Other examples of fermentation of enzym action are the digestive processes in animals and plants, the souring of milk, the formation of vinegar. Enzyms come from living things, animals, plants, or minute forms of life such as yeasts and bacteria. Amylase, diastase, rennin, pepsin, and the milk-souring substance produced by lactic acid bacteria are all enzyms. (Pp. 70, 81, 86, 97, 99, 368.)
Housewives and bakers used to grow yeast in a mixture of potatoes, sugar, water, and hops.1 Such yeast cannot be as uniformly good as the pure yeast cultivated by brewers and distillers. Compressed yeast-cakes are made of fresh yeast skimmed from fermented distillery rye. It spoils quickly if not kept cold.
Dried yeast is made of fresh yeast mixed with starch and dried. It is used where fresh yeast cannot be obtained. It keeps for weeks, even months, but is best when fresh. Drying kills some of the yeast-plants; in time, all of them. 1 Hops are used to cheek the growth of bacteria.
Dried yeast works slowly. Soak it in warm water with a little sugar and always set a sponge when using it for bread.
Dough, after yeast is mixed with it, becomes a yeast-garden, which we must tend carefully in order to have a good crop of yeast and a plentiful yield of carbon dioxide. The water supplies moisture; the flour supplies sugar, which the yeast-plant, in its greed for oxygen, turns into alcohol and carbon dioxide. More oxygen is supplied by beating and kneading in air. The right temperature, 78° to 90° F., is insured by using lukewarm liquid and by keeping the sponge and dough warm until it is ready to be baked. What is the result? a dough expanded by bubbles of gas given off by the lively yeast-cells; a dough that has lost a little of its sweetness, but gained other pleasant flavors through various fermentative actions of the yeast on the flour.
After alcoholic fermentation has gone on for some time, another enzym begins to work on the alcohol, turning it into acetic acid (the acid found in vinegar). This is why dough sours if allowed to rise too long or at too high a temperature.
1 Do not try to neutralize sour dough with baking-soda. Soda forms with acetic acid an unwholesome compound, and besides, since there is no way of knowing exactly how much acid has been formed, you are likely to use too much soda. Bread "sweetened" with soda is more unwholesome than bread a little sour.