Benefits Derived From The Products Of Bacterial Life
( Originally Published 1897 )
While bacteria thus play a part in our industries simply from their power of producing de-composition, it is primarily because of the products of their action that they are of value. Wherever bacteria seize hold of organic matter and feed upon it, there are certain to be developed new chemical compounds, resulting largely from decomposition, but partly also from constructive processes. These new compounds are of great variety. Different species of bacteria do not by any means produce the same compounds even when growing in and decomposing the same food material. Moreover, the same species of bacteria may give rise to different products when growing in different food materials. Some of the compounds produced by such processes are poisonous, others are harmless. Some are gaseous, others are liquids. Some have peculiar odours, as may be recognised from the smell arising from a bit of decaying meat. Others have peculiar tastes, as may be realized in the gamy taste of meat which is in the incipient stages of putrefaction. By purely empirical means mankind has learned methods of encouraging the development of some of these products, and is to-day making practical use of this power, possessed by bacteria, of furnishing desired chemical compounds. Industries involving the investment of hundreds of millions of dollars are founded upon the products of bacterial life, and they have a far more important relation to our everyday life than is commonly imagined. In many cases the artisan who is dependent upon this action of microscopic life is unaware of the fact. His processes are those which experience has taught produce desired results, but,, nevertheless, his dependence upon bacteria is none the less fundamental.
BACTERIA IN THE FERMENTATIVE INDUSTRIES
We may notice, first, several miscellaneous in-stances of the application of bacteria to various fermentative industries where their aid is of more or less value to man. In some of the examples to be mentioned the influence of bacteria is pro-found and fundamental, while in others it is only incidental. The fermentative industries of civilization are gigantic in extent, and have come to be an important factor in modern civilized life. The large part of the fermentation is based upon the growth of a class of microscopic plants which we call yeasts. Bacteria and yeasts are both microscopic plants, and perhaps somewhat closely related to each other. The botanist finds a difference between them, based upon their method of multiplication, and therefore places them in different classes (Fig. 2, page 19). In their general power of producing chemical changes in their food products, yeasts agree closely with bacteria, though the kinds of chemical changes are different. The whole of the great fermentative industries, in which are invested hundreds of millions of dollars, is based upon chemical decompositions produced by microscopic plants. In the great part of commercial fermentations alcohol is the product desired, and alcohol, though it is some-times produced by bacteria, is in commercial quantities produced only by yeasts. Hence it is that, although the fermentations produced by bacteria are more common in Nature than those produced by yeasts and give rise to a much larger number of decomposition products, still their commercial aspect is decidedly less important than that of yeasts. Nevertheless, bacteria are not without their importance in the ordinary fermentative processes. Although they are of no importance as aids in the common fermentative processes, they are not infrequently the cause of much trouble. In the fermentation of malt to produce beer, or grape juice to produce wine, it is the desire of the brewer and vintner to have this fermentation produced by pure yeasts, unmixed with bacteria. If the yeast is pure the fermentation is uniform and successful. But the brewer and vintner have long known that the fermentation is frequently interfered with by irregularities. The troubles which arise have long been known, but the bacteriologist has finally discovered their cause, and in general their remedy. The cause of the chief troubles which arise in the fermentation is the presence of contaminating bacteria among the yeasts. These bacteria have been more or less carefully studied by bacteriologists, and their effect upon the beer or wine determined. Some of them produce acid and render the products sour ; others make them bitter; others, again, produce a slimy material which makes the wine or beer "ropy." Some-thing like a score of bacteria species have been found liable to occur in the fermenting material and destroy the value of the product of both the wine maker and the beer brewer. The species of bacteria which infect and injure wine are different from those which infect and injure beer. They are ever present as possibilities in the great alcoholic fermentations. They are dangers which must be guarded against. In former years the troubles from these sources were much greater than they are at present. Since it has been demonstrated that the different imperfections in the fermentative process are due to bacterial impurities, commonly in the yeasts which are used to produce the fermentation, methods of avoiding them are readily devised. Today the vintner has ready command of processes for avoiding the troubles which arise from bacteria, and the brewer is always provided with a microscope to show him the presence or absence of the contaminating bacteria. While, then, the alcoholic fermentations are not dependent upon bacteria, the proper management of these fermentations requires a knowledge of their habits and characters.
There are certain other fermentative processes of more or less importance in their commercial aspects, which are directly dependent upon bacterial action. Some of them we should unhesitatingly look upon as fermentations, while others would hardly be thought of as belonging to the fermentation industries.
The commercial importance of the manufacture of vinegar, though large, does not, of course, compare in extent with that of the alcoholic fermentations. Vinegar is a weak solution of acetic acid, together with various other ingredients which have come from the materials furnishing the acid. In the manufacture of vinegar, alcohol is always used as the source of the acetic acid. The production of acetic acid from alcohol is a simple oxidation. The equation C2H60 + 02 = C2H402 + H2O shows the chemical change that occurs. This oxidation can be brought about by purely chemical means. While alcohol will not readily unite with oxygen under common conditions, if the alcohol is allowed to pass over a bit of platinum sponge the union readily occurs and acetic acid results. This method of acetic-acid production is possible experimentally, but is impracticable on any large scale. In the ordinary manufacture of vinegar the oxidation is a true fermentation, and brought about by the growth of bacteria.
In the commercial manufacture of vinegar several different weak alcoholic solutions are used. The most common of these are fermented malt, weak wine, cider, and sometimes a weak solution of spirit to which is added sugar and malt. If these solutions are allowed to stand for a time in contact with air, they slowly turn sour by the gradual conversion of the alcohol into acetic acid. At the close of the process practically all of the alcohol has disappeared. Ordinarily, however, not all of it has been converted into acetic acid, for the oxidation does not all stop at this step. As the oxidation goes on, some of the acid is oxidized into carbonic dioxide, which is, of course, dissipated at once into the air, and if the process is allowed to continue unchecked for a long enough period much of the acetic acid will be lost in this way.
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 bac teria 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 them-selves through the string, and then alcohol be al-lowed 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.
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 improve-ment 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 lac-tic 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 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.
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.
Of the nature of the process we as yet know very little. That it is a fermentation is certain, and it has been proved that it is produced by a definite species of bacterium which occurs on the indigo leaves. If the sterilized leaves are placed in sterile water no fermentation occurs and no indigo is formed. If, however, some of the specific bacteria are added to the mass the fermentation soon begins and the blue colour of the indigo makes its appearance. It is plain, therefore, that indigo is a product of bacterial fermentation, and commonly due to a single definite species of bacterium. Of the details of the formation, however, we as yet know little, and no practical application of the facts have yet been made.
BACTERIA IN TOBACCO CURING
A fermentative process of quite a different nature, but of immense commercial value, is found in the preparation of tobacco. The process by which tobacco is prepared is a long and some-what complicated one, consisting of a number of different stages. The tobacco, after being first dried in a careful manner, is subsequently allowed to absorb moisture from the atmosphere, and is then placed in large heaps to undergo a further change. This process appears to be a fermentation, for the temperature of the mass rises rapidly, and every indication of a fermentative action is seen. The tobacco in these heaps is changed occasionally, the heap being thrown down and built up again in such a way that the portion which was first at the bottom comes to the top, and in this way all parts of the heap may be-come equally affected by the process. After this process the tobacco is sent to the different manufacturers, who finish the process of curing. The further treatment it receives varies widely ac-cording to the desired product, whether for smoking or for snuff, etc. In all cases, however, fermentations play a prominent part. Some-times the leaves are directly inoculated with fermenting material. In the preparation of snuff the details of the process are more complicated than in the preparation of smoking tobacco. The tobacco, after being ground and mixed with certain ingredients, is allowed to undergo a fermentation which lasts for weeks, and indeed for months. In the different methods of preparing snuff the fermentations take place in different ways, and sometimes the tobacco is subjected to two or three different fermentative actions. The result of the whole is the slow preparation of the commercial product. It is during the final fermentative processes that the peculiar colour and flavour of the snuff are developed, and it is during the fermentation of the leaves of the smoking tobacco—either the original fermentation or the subsequent ones—that the special flavours and aromas of tobacco are produced.
It can not be claimed for a moment that these changes by which the tobacco is cured and finally brought to a marketable condition are due wholly to bacteria. There is no question that chemical and physical phenomena play an important part in them. Nevertheless, from the moment when the tobacco is cut in the fields until the time it is ready for market the curing is very intimately associated with bacteria and fermentative organ-isms in general. Some of these processes are wholly brought about by bacterial life; in others the micro-organisms aid the process, though they perhaps can not be regarded as the sole agents.
At the outset the tobacco producer has to contend with a number of micro-organisms which may produce diseases in his tobacco. During the drying process, if the temperature or the amount of moisture or the access of air is not kept in a proper condition, various troubles arise and various diseases make their appearance, which either injure or ruin the value of the product. These appear to be produced by micro-organisms of different sorts. During the fermentation which follows the drying the' producer has to contend with micro-organisms that are troublesome to him ; for unless the phenomena are properly regulated the fermentation that occurs produces effects upon the tobacco which ruin its character. From the time the tobacco is cut until the final stage in the curing the persons engaged inpreparing it for market must be on a constant watch to prevent the growth within it of undesirable organisms. The preparation of tobacco is for this reason a delicate operation, and one that will be very likely to fail unless the greatest care is taken.
In the several fermentative processes which occur in the preparation there is no question that micro-organisms aid the tobacco producer and manufacturer. Bacteria produce the first fermentation that follows the drying, and it is these organisms too, in large measure, that give rise to all the subsequent fermentations, although seemingly in some cases purely chemical processes materially aid. Now the special quality of the tobacco is in part dependent upon the peculiar type of fermentation which occurs in one or an-other of these fermenting actions. It is the fermentation that gives rise to the peculiar flavour and to the aroma of the different grades of tobacco. Inasmuch as the various flavours which characterize tobacco of different grades are developed, at least to a large extent, during the fermentation processes, it is a natural supposition that the different qualities of the tobacco, so far as concerns flavour, are due to the different types of fermentation, The number of species of bacteria which are found upon the tobacco leaves in the various stages of its preparation is quite large, and from what we have already learned it is inevitable that the different kinds of bacteria will produce different results in the fermenting process. It would seem natural, therefore, to assume that the different flavours of different grades may not unlikely be due to the fact that the tobacco in the different cases has been fermented under the influence of different kinds of bacteria.
Nor is this simply a matter of inference. To a certain extent experimental evidence has borne out the conclusion, and has given at least a slight indication of practical results in the future. Acting upon the suggestion that the difference between the high grades of tobacco and the poorer grades is due to the character of the bacteria that pro-duce the fermentation, certain bacteriologists have attempted to obtain from a high quality of tobacco the species of bacteria which are infesting it. These bacteria have then been cultivated by bacteriological methods and used in experiments for the fermentation of tobacco. If it is true that the flavour of high grade tobacco is in large measure, or even in part, due to the action of the peculiar microbes from the soil where it grows, it ought to be possible to produce similar flavours in the leaves of tobacco grown in other localities, if the fermentation of the leaves is carried on by means of the pure cultures of bacteria obtained from the high grade tobacco. Not very much has been done or is known in this connection as yet. Two bacteriologists have experimented independently in fermenting tobacco leaves by the action of pure cultures of bacteria obtained from such sources. Each of them reports successful experiments.: Each claims that they have been able to improve the quality of tobacco by inoculating the leaves with a pure culture of bacteria obtained from tobacco having high quality in flavour. In addition to this, several other bacteriologists have carried on experiments sufficient to indicate that the flavours of the tobacco and the character of the ripening may be decidedly changed by the use of different species of micro-organisms in the fermentations that go on during the curing processes.
In regard to the whole matter, however, we must recognise that as yet we have very little knowledge. The subject has been under investigation for only a short time; and, while considerable information has been derived, this information is not thoroughly understood, and our knowledge in regard to the matter is as yet in rather a chaotic condition. It seems certain, however, that the quality of tobacco is in large measure dependent upon the character of the fermentations that occur at different stages of the curing. It seems certain also that these fermentations are wholly or chiefly produced by micro-organisms, and that the character of the fermentation is in large measure dependent upon the species of micro-organisms that produce it. If these are facts, it would seem not improbable that a further study may produce practical results for this great industry. The study of yeasts and the methods of keeping yeast from contaminations has revolutionised the brewing industry. Perhaps in this other fermentative industry, which is of such great commercial extent, the use of pure cultures of bacteria may in the future pro-duce as great revolutions in methods as it has in the industry of the alcoholic fermentation.
It must not, however, be inferred that the differences in grades of tobacco grown in different parts of the world are due solely to variations in the curing processes and to the types of fermentation. There are differences in the texture of the leaves, differences in the chemical composition of the tobaccoes, which are due undoubtedly to the soils and the climatic conditions in which they grow, and these, of course, will never he affected by changing the character of the fermentative processes. It is, however, probable that in so far as the flavours that distinguish the high and low grades of tobacco are due to the character of the fermentative processes, they may be in the future, at least to a large extent, controlled by the use of pure cultures in curing processes. Seemingly, then, there is as great a future in the development of this fermentative industry as there has been in the past in the development of the fermentative industry associated with brewing and vinting.
Opium for smoking purposes is commonly allowed to undergo a curing process which lasts several months. This appears to be somewhat similar to the curing of tobacco. Apparently it is a fermentation due to the growth of micro-organisms. The organisms in question are not, however, bacteria in this case, but a species of allied fungus. The plant is a mould, and it is claimed that inoculation of the opium with cultures of this mould hastens the curing.
Before leaving this branch of the subject it is necessary to notice some of the troublesome fermentations which are ever interfering with our industries, requiring special methods, or, indeed, sometimes developing special industries to meet them. As agents of decomposition, bacteria will of course be a trouble whenever they get into material which it is desired to preserve. Since they are abundant everywhere, it is necessary to count upon their attacking with certainty any fermentable substance which is exposed to air and water. Hence they are frequently the cause of much trouble. In the fermentative industries they occasionally cause an improper sort of fermentation to occur unless care is taken to prevent undesired species of bacteria from being present. In vinegar making, improper species of bacteria obtaining access to the solution give rise to undesirable flavours, greatly injuring the product. In tobacco curing it is very common for the wrong species of bacteria to gain access to the tobacco at some stage of the curing and by their growth give rise to various troubles. It is the ubiquitous presence of bacteria which makes it impossible to preserve fruits, meats, or vegetables for any length of time without special methods. This fact in itself has caused the development of one of our most important indus-tries. Canning meats or fruits consists in nothing more than bringing them into a condition in which they will be preserved from attack of these micro-organisms. The method is extremely simple in theory. It is nothing more than heating the material to be preserved to a high temperature and then sealing it hermetically while it is still hot. The heat kills all the bacteria which may chance to be lodged in it, and the hermetical sealing prevents other bacteria from obtaining access. Inasmuch as all organic decomposition is produced by bacterial growth, such sterilized and sealed material will be preserved indefinitely when the operation is performed carefully enough. The methods of accomplishing this with sufficient care are somewhat varied in different industries, but they are all fundamentally the same. It is an interesting fact that this method of preserving meats was devised in the last century, before the relation of micro-organisms to fermentation and putrefaction was really suspected. For a long time it had been in practical use while scientists were still disputing whether putrefaction could be avoided by preventing the access of bacteria. The industry has, however, developed wonderfully within the last few years, since the principles underlying it have been understood. This understanding has led to better methods of destroying bacterial life and to proper sealing, and these have of course led to greater success in the preservation, until to-day the canning industries are among those which involve capital reckoned in the millions.
Occasionally bacteria are of some value in food products. The gamy flavour of meats is nothing more than incipient decomposition. Sauer Kraut is a food mass intentionally allowed to ferment and sour. The value of bacteria in producing butter and cheese flavours is noticed elsewhere. But commonly our aim must be to prevent the growth of bacteria in foods. Foods must be dried or cooked or kept on ice, or some other means adopted for preventing bacterial growth in them. It is their presence that forces us to keep our ice box, thus founding the ice business, as well as that of the manufacture of refrigerators. It is their presence, again, that forces us to smoke hams, to salt mackerel, to dry fish or other meats, to keep pork in brine, and to introduce numerous other details in the methods of food preparation and preservation.