All ferments possess the following three properties:-- 1. They are nitrogenous bodies. 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.