CHAPTER X.

CHANGES OCCURRING AS THE RESULT OF OXIDATION.

THE formation of acetic acid by fermentation is a process differing from those changes already described, by not being hydrolytic, but produced by oxidation. This process is used on the large scale for the production of vinegar, and has been known for a long time. Acetic acid, the principal constituent, was one of the first acids known.

Before the time of Lavoisier, it was not known that oxygen was necessary for this conversion of alcohol into acetic acid, which takes place in wines when they are allowed to stand in the air; it was, however, generally supposed that air was necessary. Lavoisier showed that the of the air was absorbed in the process. oxygen In 1821 Davy discovered that alcohol could be converted into acetic acid when platinum black was moistened with alcohol; it was subsequently shown, in 1823, by Döbereiner, that oxygen was used up in this change, and that the alcohol was converted into acetic acid and water.

In this process a skin is formed on the surface of the liquid, which was named Mycoderma aceti, or, motherof-vinegar, by Persoon, in 1822. Kützing, in 1837,

re-investigated this scum, and concluded that it was of a living nature. This mycoderma was held by Liebig to be non-living, and porous like platinum black; its action depended on the pores in its substance, in which oxygen was mechanically contained; the formation of acetic acid was due to the movement set up by the decomposing matter, oxidation being thus caused.

The subject was carefully investigated by Pasteur, but he did not at once recognise the rôle played by the mycoderma, which was supposed to act like platinum black; he found that the change was only effected during the life of the organism, and that if the action were continued, carbon dioxide and water were produced from the acetic acid. In 1873 it was shown by von Knieriem and Ad. Mayer that these two processes of producing acetic acid were not comparable, since the mycoderma could only decompose weak alcoholic solutions at a temperature not above 40°; the platinum black could effect the transformation of strong alcohol, even at a high temperature. It is now generally supposed that the change is due to the direct action of living organisms, since no enzyme as yet capable of producing this change has been obtained from them.

The most recent and careful observations were carried out by A. J. Brown, in 1886, with two particular organisms which he had isolated. He has confirmed the older observation of Pasteur's, that if the process be continued beyond the complete oxidation of the alcohol to acetic acid, carbon dioxide and water are formed from the latter.

In this process Brown has shown that the alcohol

passes through two stages: aldehyde is formed first, and is subsequently converted into acetic acid, thus:

Brown has made several other investigations upon the oxidising action of these two micro-organisms; propyl alcohol was converted into propionic acid, but the oxidation of methyl, isoprimary butyl, and amyl alcohols could not be effected. Henneberg has also shown that propylalcohol can be oxidised to propionic acid by the acetic acid micro-organisms.

Glucose was oxidised to gluconic acid in the presence of calcium carbonate, added to neutralise the free acid formed,--a confirmation of Boutroux's result of 1880.

The following equation is given by Brown to represent this change

2CH2OH.(CHOH)4.CHO+O2=2CH2OH.(CHOH).COOH the oxidation thus affecting the aldehyde group.

Further, mannitol is oxidised to lævulose; since mannitol can be obtained from d-glucose by reduction with sodium amalgam, and mannitol can be converted by the Bacterium aceti into lævulose, a new method of converting d-glucose into lævulose has been discovered by Brown.

In the vinegar plant, a membrane is formed which contains cellulose, and Brown shows that this can be formed from dextrose, mannitol, and lævulose, but not from cane sugar, starch, or alcohol. He proposes the name of Bacterium xylinum for this microorganism.

The oxidation of sorbitol to sorbose by the sorbose

bacterium, which is probably identical with Bacterium xylinum, has been described by Bertrand, who has also shown that it can oxidise mannitol to lævulose, and glycerol to dihydroxyacetone. Here the carbon atoms in the molecules are a multiple of 3; but this is not a necessity, since the micro-organism can also oxidise erythritol, arabitol, and perseïtol; in each case two hydrogen atoms are removed. Bertrand states that only those alcohols seem to be oxidised which contain a CHOH chain where no hydrogen atom exists on the same side as the hydroxyl group, which is attacked in the oxidation. He has also shown that xylose can be oxidised to xylonic acid, aldose to a monobasic acid, arabinose to arabonic acid, glucose to gluconic acid, and galactose to galactonic acid.

Vincent and Délachanal obtained Bertrand's results with regard to the oxidation of mannitol to lævulose year earlier, and Emmerling has recently confirmed the oxidations of sorbitol and glycerol by the sorbose bacterium.

Boutroux has obtained a hydroxygluconic acid by the oxidation of gluconic acid in the presence of calcium carbonate, and he gives the formula

н онн

CH2OH-CO-Ċ- -C-C-COOH

ОН Н ОН

to this acid from the results which he obtained by oxidising it with dilute nitric acid.

G

CHAPTER XI.

CHANGES OCCURRING AS THE RESULT OF

OXIDATION.-Continued.

IN nature, both in the animal and the vegetable kingdom, changes are produced by enzymes, known collectively now as oxydases, which are the result of oxidation.

Yoshida, in 1883, noticed that the juice of the lac tree of South-Eastern Asia underwent the peculiar changes of turning black and setting hard, producing the material used in lacquer varnish. He thought that this was due to an enzyme, and he investigated the change from this point of view. The juice was found to consist of four substances urushic acid, to which he gave the formula C14H18O2, gum, water, and a nitrogenous body which contained an enzyme. The change caused by this enzyme is produced in the urushic acid, which is oxidised, forming oxyurushic acid, C14H1803. That oxidation was the cause of the change was shown by the transformation of urushic acid into oxyurushic acid by chromic acid. Some ten years later Bertrand reinvestigated this change, and confirmed Yoshida's result. He gave the name laccase to the enzyme, but he called urushic acid laccol, as it resembles in many of its properties the polyatomic phenols. This observer found that an alcoholic solution of laccol turned brown at once, and ultimately became black under the