2. They are unstable, i.e. they are destroyed by heat, chemicals, etc.
3. A relatively small quantity of the ferment is capable of producing great changes in the substances upon which it acts, especially if the products of the change can be removed as they are formed.
The general character of fermentation will be best understood by a closer study of the yeast cell, which has already been described ([p. 12]), and its life-history briefly sketched. It has been shown that it is a growing plant of a very simple type, belonging to the fungi. These are devoid of the green colouring matter which enables the higher plants to utilise the energy of sunlight to assimilate the carbonic acid of the atmosphere, exhaling its oxygen, and employing its carbon for the building up of tissue; and they must therefore, like animals, have their nutriment ready formed, and capable of supplying energy by its oxidation. For yeast, as has been stated, the appropriate nourishment is glucose, or “grape-sugar.” This is broken down, in the main, into the simpler compounds, alcohol and carbonic acid, while a small portion is utilised for the building up of the cell and the formation of secondary products. The main reaction is represented by the following equation:
| C6H12O6 | = | 2C2H6O | + | 2CO2 |
| Glucose | Alcohol | Carbon dioxide |
Yeast cannot directly ferment ordinary cane-sugar (C12H22O11), but secretes a substance called invertase, which so acts on the sugar as to break it up, with absorption of one molecule of water, into two molecules of fermentable glucose (dextrose and levulose) which serve as nourishment for the yeast.[4] This invertase is the type of the series of bodies which are known as “unorganised ferments,” enzymes, or zymases, differing from the organised ferments in being simply chemical products without life or power of reproduction, but capable of breaking up an unlimited quantity of the bodies on which they act, without themselves suffering change. The way in which this is done is not clearly understood, but some parallel may be found to it in the action of sulphuric acid on alcohol, of which it will convert an unlimited quantity into ether, without itself suffering any permanent change. The action of enzymes is limited to breaking down complex bodies into simpler forms, often with absorption of water, as in the case of sugar, while some of the products of living ferments are often complex, a part of their nutriment being broken down into simple products such as carbonic acid, marsh gas and ammonia, to supply the necessary energy to elaborate the remainder.
[4] Compare O’Sullivan and Thompson, Jour. Chem. Soc., 1890, p. 834; 1891, p. 46.
Very many different unorganised ferments are known to exist, as they are not only produced by yeasts and bacteria, but are formed by the cells of higher plants and animals; thus the digestive principles, pepsin, trypsin, ptyalin, are of this character—ptyalin, like diastase, converting starch into sugar; and such bodies fulfil many functions both in animal and vegetable economy. In fermentation, as in disease, it is often difficult to distinguish what is due to the direct action of bacteria, and what to the unorganised ferments which they produce, and the question is further complicated by the fact that in most natural fermentations more than one ferment-organism is present. Sometimes the action of the unorganised ferments may be distinguished by the fact that the addition of chloroform has little effect on their activity while it paralyses that of the living organism. By exposure to high temperature both are destroyed, the bacteria, yeasts and moulds being killed and the unorganised ferments coagulated like white of egg, and so rendered inoperative. Many antiseptics also destroy the activity of both organisms and enzymes; but others, like chloroform, have no action on the latter. In some cases, as in that of invertase, the actual zymase can be precipitated by alcohol from its aqueous solution, filtered off, and restored to activity by transference into water. Since both classes of ferments are destroyed by high temperatures, all fermentation-processes are completely and permanently arrested by exposure to sufficient heat, and subsequent preservation in vessels so closed that no new ferment-germs can gain access. A familiar instance is that of tinned meats. All fully developed bacteria are destroyed by a very short exposure to a boiling temperature, and most by 60° to 70° C., but many species produce spores which are extremely difficult to destroy. The thermophilic bacteria discovered by Globig and further investigated by Rabinowitsch,[5] thrive at a temperature of 60° C. About eight species are known, and they take part in the heating of hay and similar fermentations where high temperatures are involved, and are therefore presumably present in spent tan.
[5] Centr. Blatt für Bakt., II. Abth. vol. i. p. 585.
For absolute sterilisation it is therefore necessary either to boil under pressure so as to raise the temperature to, say 110° C., or to heat repeatedly for a short time to temperatures of 80°-100° C. at successive intervals of 24 hours, in order to allow the spores to develop. This process is frequently performed for bacteriological observation in flasks or test-tubes merely stopped with a plug of sterilised cotton-wool, which has been found to efficiently filter the germs from the air which enters through it (see L.I.L.B., p. 270).
The ferment-organisms cannot thrive and multiply unless they have proper nourishment and conditions of growth, the amount of moisture and the temperature being two of the most important of the latter. Use is made of this in the preservation of many articles of food, etc., since by ensuring that at least one of the conditions necessary for growth shall be absent, these substances are prevented from decomposing. For instance, hides are preserved by drying them; the absence of sufficient moisture hindering the growth of any organisms in them so long as they are dry, but as soon as they become somewhat damp, putrefaction commences at once.