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THE majority of the chemical changes which are the result of fermentation occur in two large classes of compounds—the carbohydrates and the albumins. These are the materials used by young plants and animals as foodstuffs; as such, however, they cannot be assimilated, but must first undergo the changes which will be described in the following pages, in order that they may be made assimilable and really serviceable as food-stuffs.

The changes, so far as they are known, generally consist in a splitting up of a complex molecule into simpler ones, but, at present, only in a few cases can the stages in the transformation be followed. The reverse changes, or synthetic processes, no doubt, also take place both in plants and animals, and they consist in the building up of a complex molecule from simpler ones; at the present time, however, we have no knowledge as to how they are produced, but during the last few years experimental


synthetic fermentation processes with the ferments maltase (Hill, Emmerling), lipase (Kastle and Loevenhart, Hanriot), lactase, and emulsin (Fischer and Armstrong), have been carried out in the laboratory, and it is very possible that processes of this kind take place in nature on similar lines.

All these changes are brought about either directly or indirectly by means of living organisms. In the latter case the change is caused by a soluble ferment, or enzyme, which is secreted by the organism, and which can be employed in the laboratory as a reagent; similar changes to those which occur in nature can thus be produced, and the different products which result at the various stages of the process can be obtained. In the former case, so far as is at present known, these changes are produced by the living organism as the result of its metabolism, though, in all probability, an enzyme is secreted, not excreted, which produces these metabolic changes in the interior of their constituting cells. The researches of recent times upon this subject have made this supposition very probable; for example, a soluble ferment, or enzyme, has been extracted by Buchner from yeast, which can cause the alcoholic fermentation of saccharine liquids—a change which until then had been regarded solely as the result of the life of the yeast-cells.

The chemical changes which occur as results of fermentation are generally brought about by hydrolysis; but oxidation, as well as reduction, also takes place. All enzymes, as pointed out by Schönbein, are capable of decomposing hydrogen peroxide with the evolution of oxygen, and of producing a blue colouration with guaiacum tincture,

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