This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
In fermenting, the first requisite is good yeast. The yeast of beer, or brewer's yeast, is most generally used, and its preference is founded on its fermentable power, and on the facility with which it may be procured in the market. Yeast is a frothy substance which is to be had of the brewers either in a liquid or solid state, that is to say, fresh or dry, or it may be obtained in cakes from the stores. Fresh yeast in a semi-fluid state is to be preferred, but it is very difficult to transport and preserve it; therefore dry yeast is most frequently used. The latter has been subjected to the action of a press, to deprive it of the beer and render it solid. In this state it is in the form of a uniform brittle paste, neither stringy nor sticky, of a yellowish-white, and having a slight aromatic odor of hops, without any mixture of an acrid or putrid taste. The fermentable power of yeast varies according to the quality of the beer from which it is derived. If it results from a strong beer, it is much more substantial, more certain, and is more apt to favor a healthy and sweet fermentation. If, on the other hand, it is derived from a small beer, it acts all at once with a sort of violence, and, after having excited in the wort (the unfermented liquid) a hasty bubbling and kind of effervescence, it loses all its energy, from which results a loss of a portion of the spirituous principle, and is frequently followed by acidity.
The facility with which yeast passes to a state of putrefaction renders it necessary to preserve it in the cellar, or some other cool place, for a slightly elevated temperature may readily alter or corrupt it. It may be preserved a sufficiently long time, especially as regards its freshness, when care is taken to cover it with water, which must be renewed every day. A means of preserving yeast at all seasons, and which has been employed with some success, consists in mixing this substance with very thick molasses, so as to form a hard paste. The ferment thus mixed with sugar or molasses will for years preserve its characteristic properties. A better result is obtained by spreading out a thin layer of fresh yeast, and allowing it to dry in the open air by exposure to the sun, or in a current of slightly heated air. The desiccation, or drying, is rendered more prompt by spreading the yeast, whipped to a smooth broth, on thick tables of plaster well dried, and thus rendered more absorbent. Another means is at least as efficacious. It consists in mixing the whipped yeast with very dry animal black in powder, or with starch strongly heated and cooled in a close vessel. The drying under these circumstances is easily finished in a current of air heated to 85° or 95° Fahrenheit. Whatever may be the method employed for preserving pure yeast, it is very certain that it will never possess either the strength or the energy of that which is newly prepared; therefore, it should never be used when fresh yeast can be obtained.
It is important to examine yeast with great care to be assured of its quality. That which is acid, or the result of a bad fermentation, should be rejected. The former is recognized as follows: A strip of litmus paper being dipped into the suspected yeast, if it is acid the blue will be changed to a permanent red; if, however, the yeast be good, fresh, and well preserved, the litmus paper will be slightly reddened, but if washed in fresh water the blue will be restored. As to that which results from a viscous fermentation, it is almost impossible to detect it, unless the decomposition is so far advanced in the altered leaven that the disagreeable odor which it exhales may be recognized. Frequently the dry yeast is adulterated. The fraud consists in the addition of rye or wheat flour, or, more likely, wheat or potato starch. This mixture is readily detected by dissolving a small quantity of the suspected yeast in a little boiling water, and pouring into it two or three drops of tincture of iodine. If it is pure, the liquid will not change color; if, however, it is adulterated, a decided blue color will be produced. These suggestions will be found of value to bottlers who care to go about their work in an intelligent manner.