The Story of Germ Life

by H. W. Conn

The oxidation of the alcohol in all commercial production of vinegar is brought about by the growth of bacteria in the liquid. When the vinegar production is going on properly, there is formed on the top of the liquid a dense felted mass known as the "mother of vinegar." This mass proves to be made of bacteria which have the power of absorbing oxygen from the air, or, at all events, of causing the alcohol to unite with oxygen. It was at first thought that a single species of bacterium was thus the cause of the oxidation of alcohol, and this was named Mycoderma aceti. But further study has shown that several have the power, and that even in the commercial manufacture of vinegar several species play a part (Fig. 18), although the different species are not yet very thoroughly studied. Each appears to act best under different conditions. Some of them act slowly, and others rapidly, the slow- growing species appearing to produce the larger amount of acid in the end. After the amount of acetic acid reaches a certain percentage, the bacteria are unable to produce more, even though there be alcohol still left unoxidized. A percentage as high as fourteen per cent, commonly destroys all their power of growth. The production of the acid is wholly dependent upon the growth of the bacteria, and the secret of the successful vinegar manufacture is the skilful manipulation of these bacteria so as to keep them in the purest condition and to give them the best opportunity for growth.

One method of vinegar manufacture which is quite rapid is carried on in a slightly different manner. A tall cylindrical chamber is filled with wood shavings, and a weak solution of alcohol is allowed to trickle slowly through it. The liquid after passing over the shavings comes out after a number of hours well charged with acetic acid. This process at first sight appears to be a purely chemical one, and reminds us of the oxidation which occurs when alcohol is allowed to pass over a platinum sponge. It has been claimed, indeed, that this is a chemical oxidation in which bacteria play no part. But this appears to be an error. It is always found necessary in this method to start the process by pouring upon the shavings some warm vinegar. Unless in this way the shavings become charged with the vinegar-holding bacteria the alcohol will not undergo oxidation during its passage over them, and after the bacteria thus introduced have grown enough to coat the shavings thoroughly the acetic-acid production is much more rapid than at first. If vinegar is allowed to trickle slowly down a suspended string, so that its bacteria may distribute themselves through the string, and then alcohol be allowed to trickle over it in the same way, the oxidation takes place and acetic acid is formed. From the accumulation of such facts it has come to be recognised that all processes for the commercial manufacture of vinegar depend upon the action of bacteria. While the oxidation of alcohol into acetic acid may take place by purely chemical means, these processes are not practical on a large scale, and vinegar manufacturers everywhere depend upon bacteria as their agents in producing the oxidation. These bacteria, several species in all, feed upon the nitrogenous matter in the fermenting mass and produce the desired change in the alcohol.

This vinegar fermentation is subject to certain irregularities, and the vinegar manufacturers can not always depend upon its occurring in a satisfactory manner. Just as in brewing, so here, contaminating bacteria sometimes find their way into the fermenting mass and interfere with its normal course. In particular, the flavour of the vinegar is liable to suffer from such causes. As yet our vinegar manufacturers have not applied to acetic fermentation the same principle which has been so successful in brewing—namely, the use, as a starter of the fermentation, of a pure culture of the proper species of bacteria. This has been done experimentally and proves to be feasible. In practice, however, vinegar makers find that simpler methods of obtaining a starter—by means of which they procure a culture nearly though not absolutely pure—are perfectly satisfactory. It is uncertain whether really pure cultures will ever be used in this industry.

LACTIC ACID.

The manufacture of lactic acid is an industry of less extent than that of acetic acid, and yet it is one which has some considerable commercial importance. Lactic acid is used in no large quantity, although it is of some value as a medicine and in the arts. For its production we are wholly dependent upon bacteria. It is this acid which, as we shall see, is produced in the ordinary souring of milk, and a large number of species of bacteria are capable of producing the acid from milk sugar. Any sample of sour milk may therefore always be depended upon to contain plenty of lactic organisms. In its manufacture for commercial purposes milk is sometimes used as a source, but more commonly other substances. Sometimes a mixture of cane sugar and tartaric acid is used. To start the fermentation the mixture is inoculated with a mass of sour milk or decaying cheese, or both, such a mixture always containing lactic organisms. To be sure, it also contains many other bacteria which have different effects, but the acid producers are always so abundant and grow so vigorously that the lactic fermentation occurs in spite of all other bacteria. Here also there is a possibility of an improvement in the process by the use of pure cultures of lactic organisms. Up to the present, however, there has been no application of such methods. The commercial aspects of the industry are not upon a sufficiently large scale to call for much in this direction.

At the present time the only method we have for the manufacture of lactic acid is dependent upon bacteria. Chemical processes for its manufacture are known, but not employed commercially. There are several different kinds of lactic acid. They differ from each other in the relations of the atoms within their molecule, and in their relation to polarized light, some forms rotating the plane of polarized light to the right, others to the left, while others are inactive in this respect. All the types are produced by fermentation processes, different species of bacteria having powers of producing the different types.

BUTYRIC ACID.

Butyric acid is another acid for which we are chiefly dependent upon bacteria. This acid is of no very great importance, and its manufacture can hardly be called an industry; still it is to a certain extent made, and is an article of commerce. It is an acid that can be manufactured by chemical means, but, as in the case of the last two acids, its commercial manufacture is based upon bacterial action. Quite a number of species of bacteria can produce butyric acid, and they produce it from a variety of different sources. Butyric acid is a common ingredient in old milk and in butter, and its formation by bacteria was historically one of the first bacterial fermentations to be clearly understood. It can be produced also in various sugar and starchy solutions. Glycerine may also undergo a butyric fermentation. The presence of this acid is occasionally troublesome, since it is one of the factors in the rancidity of butter and other similar materials.

INDIGO PREPARATION.

The preparation of indigo from the indigo plant is a fermentative process brought about by a specific bacterium. The leaves of the plant are immersed in water in a large vat, and a rapid fermentation arises. As a result of the fermentation the part of the plant which is the basis of the indigo is separated from the leaves and dissolved in the water; and as a second feature of the fermentation the soluble material is changed in its chemical nature into indigo proper. As this change occurs the characteristic blue colour is developed, and the material is rendered insoluble in water. It therefore makes its appearance as a blue mass separated from the water, and is then removed as indigo.