With regard to the chemical conditions, our author says that the most favorable medium is that which contains the most appropriate nutritive elements. And as yeast contains water, mineral salts, especially potassium, magnesium, and calcium phosphates, therefore water and the alkaline and alkaline-earthy phosphates will be necessary. We find, besides, a great proportion of nitrogenous substances, either albuminous or otherwise; and therefore the food of yeast must contain nitrogen. It is supposed, however, that the cells are not directly nourished by albuminoids in the juices of fruits, the wort of beer, or yeast water, but by analogous compounds contained in them, which have the property of passing by osmose through the membranes; for the albuminoids themselves, it is said, cannot pass through. Pasteur has shown by his experiments, that mineral salts are absolutely necessary to the development and nutrition of the yeast cell; and Mayer follows him with details as follows: Preparations of iron, in small quantities, seem to have no influence; in larger proportions, they are injurious. Potassium phosphate is indispensable, and the absence of lime has little effect. Magnesium, on the contrary, appeared to be very useful, if not indispensable. The combinations of sodium present no material effects.
Sugar is one of the most important elements in the nourishment of the yeast cells, and Pasteur has shown that, in alcoholic fermentation, a part of the sugar is fixed in the yeast, in the state of cellulose or some analogous body, for, when the fermentation is completed, it is found that more yeast is present than at the commencement. Water is necessary, and the yeast cell manifests its activity, develops and is nourished within the limits of 40 and 80 per cent. of water, though yeast, dried with precaution, may regain its power when moistened. And the fact that a solution containing over 35 per cent. of sugar will not ferment, is explained on the theory that such a solution takes from the cells by osmose a sufficient quantity of water to lower their hydration below 40 per cent. The cells of the Saccharomyces cerevisiæ, introduced into a liquid medium, absorb oxygen with great rapidity, and develop a corresponding quantity of carbon dioxide. This constitutes respiration, comparable to that of animals. By careful experiments it has been shown that yeast breathes when placed in contact with dissolved oxygen, and the respiration is more active than that of fishes, and it plays as important a part in the life of those minute vegetable cells as in the higher forms of vegetable and animal life. Oxygen is furnished by atmospheric air, and fermentation is more rapid when a large surface of the liquid is exposed, and then the budding is more active.
Action of various Chemical and Physical Agents.—“It has long been known that certain chemical compounds, especially those which coagulate albuminous substances, and disorganize the tissues, or which, by their presence in sufficient quantities, are incompatible with life, are opposed to fermentation; such are the acids and alkalies in suitable proportions, silver nitrate, chlorine, iodine, the soluble iron, copper, and lead salts, tannin, phenol, creosote, chloroform, essence of mustard, alcohol when its strength is above 20 per cent., hydrocyanic and oxalic acids, even in very small quantities.
“An excess of neutral alkaline salts or sugar acts in the same manner, by diminishing in the interior of the cell the minimum quantity of water, which is necessary to the manifestation of its vital activity.
“The red mercury oxide, calomel, manganese peroxide, the alkaline sulphites and sulphates, the essences of turpentine and of lemon, etc., also interfere with, and destroy alcoholic fermentation.
“Phosphoric and arsenious acids are, on the contrary, inactive.”
Experiments have shown that sparks of electricity passing through yeast do not modify its power of changing cane sugar into glucose, nor its activity as an alcoholic ferment. Fermentation is slower in the dark, and also in a vacuum. Flour of sulphur did not sensibly affect fermentation, but the carbonic acid evolved contained sulphuretted hydrogen. Sulphurous acid, however, arrests fermentation. Yeast is always acid, but an addition of an excess of different acids arrests the decomposition of sugar. If one hundred times the amount of acid contained in the yeast is added, fermentation does not take place.
M. Dumas has shown the action of various salts on yeast, but the subject has little if any interest for the wine maker.
Viscous or Mannitic Fermentation is also excited, according to Pasteur, by special ferment acting on glucose, transforming it into a kind of gum or dextrin, mannite, and carbon dioxide. This ferment is also formed of small globules united as in a necklace, whose diameter varies from .000047 to .000055 of an inch. These globules, sown in a saccharine liquid containing nutritive nitrogenous matter and mineral substances, always give rise to viscous fermentation. One hundred parts of cane sugar give: mannite, 51.09; gum, 45.48; and carbon dioxide, 6.18. The liquids which are most apt to produce viscous fermentation can also undergo lactic and butyric fermentation, but in this case the organized forms of life which are developed in the liquid are of a different nature. The conditions of action necessary to these gummy and mannitic ferments are the same as those which suit alcoholic ferment. The most favorable temperature is 30° C. (86° F.) This fermentation is what gives rise to the disease of wines, called by the French la graisse, or ropiness. White wine is more subject to it than red, and it is generally due to the want of tannin. (See [Ropiness].)
Lactic Fermentation is the transformation which certain sugars, as sugar of milk and grape sugar, undergo, and by which they are changed into lactic acid. This takes place in the souring of milk. The most favorable temperature for it seems to be about 95° F. This also depends on a special ferment. Sugar solutions are also capable of butyric fermentation and putrefaction, and we generally see viscous, lactic, and butyric fermentation appear in succession.