Imágenes de páginas
PDF
EPUB

of numerous varieties of yeasts ; and he also pointed out that there were special organisms for special beers. By using pure yeasts a scientific accuracy is obtained, and nothing is left to chance. The results of Hansen's researches have completely revolutionised the brewing industry, and pure growths of yeast are now sent out from his laboratory to all parts of the world, different yeasts being employed for different beers.

It will be noticed that alcoholic fermentation is caused by yeast and not by bacteria proper. But although the latter are not so directly concerned, they are important in so far as they cause diseases of beers and wines.

Acetous Fermentation. If we take some beer or weak solution of wine, and allow it to stand for a time in contact with air, it gradually turns sour owing to the conversion of alcohol into acetic acid. The change is essentially an oxidation of alcohol.

C,H,0 + 0, = C,H,02 + H,O. This oxidation can also be brought about by purely chemical means.

Thus if alcohol be passed over spongy platinum, acetic acid results as before. But this method is impracticable on a large scale.

To produce acetic acid for commercial purposes, weakened alcohol is poured through a tall cylinder filled with wood shavings, the latter having been first inoculated with some warm vinegar. After a number of hours the resulting fluid is charged with acetic acid.

It has been stated that the process is a purely chemical one, and that it takes place without the intervention of bacteria. But this can hardly be so in this method, for the addition of vinegar is really the addition of living microbes-the mycoderma aceti. These grow upon the shavings, and oxidise the alcohol into acetic acid.

The organisms concerned in the process are of various species. They usually form chains of short or long threads, and each appears to act best under different conditions.

Lactic Acid Fermentation. It is this fermen ation which is commonly seen to occur in fresh milk, and

any sample of sour milk may be relied upon to contain an abundance of lactic acid organisms. They curdle milk and produce lactic acid from milk sugar.

C2H,2062C,H,0g. Fresh milk possesses a rich bacterial flora, but lactic bacteria find conditions in milk so favourable to growth, and they so completely outstrip the other organisms, that at the end of twenty-four hours they outnumber and even check the growth of all other forms. They are abundant in the atmosphere of cowsheds and dairies, and are, therefore, a considerable source of annoyance both to the salesman and the consumer. Fortunately lactic organisms never form spores, and thus can easily be destroyed by a moderate heat.

Lactic fermentation plays an important part in the preparation of fodder, in the manufacture of butter, and in the tanning of leather. The souring of wines and beers is likewise due to the activity of the lactic bacteria.

Butyric Fermentation is important historically, having been first studied by Pasteur, and having led to the discovery of the anærobes. It is caused by the Bacillus butyricus and the Bacillus amylobacter—two organisms possessing this characteristic, viz., that they can only grow in the absence of oxygen. They occur plentifully in air, dust, and milk, and are also commonly met with in putrefactive processes.

In ordinary milk they are prevented from growing by

the lactic acid bacteria and the presence of air. But if milk be boiled they are uninjured (owing to spore formation) and produce butyric fermentation, especially if milk be left without access of air. They find their way into butter, and are the chief cause of the development of rancidity.

Indigo Fermentation.-Indigo is obtained from certain species of the Leguminosæ. The colouring matter does not exist ready formed in the plant, but is developed therefrom by the fermentation of a glucoside constituent, known as indican. The plants are cut down shortly before flowering time, immersed in water, and then allowed to ferment. The indican is first changed into indigo-white which remains in solution, but this is soon oxidised into indigo-blue and precipitated as an insoluble body. Of the intimate nature of this process, we are as yet

But there can be no doubt that it is a true fermentation induced by a special bacterium, which occurs on the leaves. When the leaves are sterilised, no fermentation follows, but the characteristic blue is at once seen if the specific organisms be added to the mass.

Commercial indigo contains, in addition to the blue, red, brown and other colouring matters in different proportions, so that the shade of colour is variable in different samples of indigo. To control this fermentation and thereby to produce at will any desired variety of pigment, is a problem well worth the study of modern bacteriologists.

Tobacco Fermentation.— Tobacco leaves after being dried are piled up in big heaps where they undergo a kind of fermentation. Within a short time the temperature rises, and when it reaches 130° F. the piles are taken down and rearranged. This is repeated six or seven times, and, at the

unaware.

end, the tobacco is in a proper condition for the market. The changes produced in the process are a decrease in nicotine, disappearance of sugar, and the production of flavour.

It cannot be claimed that all these changes are due to bacterial activity, as physico-chemical processes must also play an important part. But as various flavours are developed during the fermentation process, it is very probable that they are due to the different types of fermentation which the tobacco undergoes.

Now, if the flavours are due to bacterial agency, it should be possible to modify them by altering the fermentative process.

Experiments have recently been made with cultures of bacteria found in Havanna tobaccos, in artificially inoculating tobaccos grown elsewhere, in order to develop desired flavours; and the results are certainly encouraging.

Summary - The foregoing illustrations suffice to indicate the usefulness of bacterial agency. . It is obvious that without their beneficent aid we could have neither “ bread and butter” nor wine and cigar”.

But it must not be supposed that bacteria invariably produce these desirable fermentations. On the other hand, they are frequently the cause of great trouble. Undesirable bacteria often gain access during the fermentation process, produce unpleasant flavours, and greatly lower the value of the product. This is constantly seen in the various industrial processes cited above.

The problem, then, is : How to control the fermentative process so that it may be possible to produce the desired product at will ? An answer to this question has been partially furnished by Hansen, whose method of pure cultures of yeasts has already revolutionised the brewing

industry. The brewer now enjoys complete command over the process, there is less risk of“ diseases,” and more profit. The results in dairies are no less striking. Conn has recently isolated a bacillus, pure cultures of which give the desired flavour to butter. It is not improbable that the use of pure cultures in tobacco and indigo fermentations will lead to better results in the future.

The principle that in order to obtain certain desired species of plants its seeds should be sown free from the seeds of all other plants is well known to the students of horticulture ; and the use of pure cultures in fermentative industries is but a further application of this principle. Pure cultures, however, cannot alter the chemical composition of the substance, for this depends on soil and climatic conditions. But even with this limitation they have an immense range of usefulness in the scientific working of fermentative industries.

« AnteriorContinuar »