Page images
PDF
EPUB

seasons are produced in excessive quantities and which may be kept in a durable form that will permit them to be used later. Whether the whole or only a part of the foodstuffs can be kept in a preservable form, is a question of secondary importance. The preparation of both products is dependent upon fermentative processes, that result from the activity of certain microorganisms. The silage, as well as the green cheese, affords certain important conditions conducive to the development of the microorganisms. These conditions are rich nourishment, moisture, and warmth. The microbiological conditions form a very important relationship between products that to casual observation are as diffierent as cheese and silage, and a study of the influence which the microflora of the ensilage can have on the microflora of the cheese, from a scientific standpoint, would have been justified and no doubt begun even without the urgent inducement arising from the bad results obtained in practice.

I. CONDITIONS NECESSARY FOR THE GROWTH OF MICROORGANISMS IN

SILOS.

If one considers that in order to have fermentation there is required not only special nourishment with certain favorable physical conditions, but also the presence of the typical fermenting bacteria, it seems that the conditions in the silo must be right, for, in general, vegetable matter, either green or half dried, is a carrier for countless microorganisms of all kinds. These are deposited on the plants by air currents or have grown on the plant. Hundreds of thousands of fermenting bacteria can be found in 1 gram of grass used for ensilage. Now to secure certain fermentations it is not so important that there be a great many organisms, but that the proper kinds be not lacking. Then too, in this respect, the situation in the silo is generally favorable for the growth of the lactic acid bacteria, which may be observed as the strongest natural factor in preserving organic substances and which are especially numerous in all fresh vegetable matter. These advantageous conditions are not common to all vegetable offal, because previous handling may free it from microorganisms; for example, the beet waste from sugar factories. Such material can be made useful by the inoculation of certain lactic acid bacteria.

In order to understand well the action of microorganisms in the silo, we must take into account the fact that the totality of the conditions controlling the packed mass influence the microorganisms in such a way that, as a rule, only a few varieties persist and through their action the final product, the silage, is given its peculiar character. In other words, the growth of the bacteria depends upon the nature of the vegetable matter used, its ripeness, its moistness, the fineness of its particles, the more or less thick layers, and other circumstances which this or that kind of organism finds advantageous to its growth. Certain conditions of growth control the occurrence and activity of a typical microflora and this flora regulates the special characteristics such as nutritive value, color, taste, and smell of the ensilage. Of course these characteristics are explained on the other hand by chemical and microbiological analyses of ensilage.

77612—24—19

II. THE RESULTS OF BACTERIOLOGICAL TESTS OF SAMPLES OF SWISS FOD

DERS.

Although the kind, condition, and placement of the silage is known to be decisive for the growth of the bacteria and thus for the final character of the silage, this relationship is not so comprehensive that one could definitely predict the microbiological nature of the product from the summation of all agencies active in it. It is almost impossible for one to describe the bearing of particular measures and conditions according to their mutual interaction taking place and effecting changes in the silage. A mixture of plants, for example, meadow grass, requires different and less-defined conditions for growth than does a singly constituted silage such as corn. But even with corn, a variation in the degree of ripeness that has been found best in practice, or unfavorable weather conditions during the growing period, could have a consequent effect due to the difference in the amount of water given off in the silo, or perhaps because the respiration has been diminished. Such conditions affect, fundamentally, the life of the bacteria, the moisture, oxygen content, and temperature, and it is easily seen that an alteration of these conditions is also an alteration of the correct bacterial development and therefore determines the resultant quality of the ensilage. From this it may be concluded that the biological nature of the final product of a certain silo may be predicted, judging from the nature of its contents, but reliable information can be obtained only by examining the ensilage in each case.

Concerning the processes that we have used during the last five years at our experiment station, we will discuss separately the three types: Sweet fodder, electrically treated fodder, and sour fodder. This order, which is not correct in regard to historical development of conserving fodder, was chosen because in the light of later developments the above-mentioned types succeed one another more logically in respect to scientific and practical experimentation. We will not discuss the chemistry of the food values, technology, or economic aspects of the silage question, but only the bacteriology of the following types:

Type A. Sweet fodder (sweet green fodder).—Some years ago some Swiss farmers recommended this scheme of preserving fodder, which consists in piling up layers upon layers of somewhat dried vegetable matter (originally and especially grass). This process causes a sudden and rather high temperature that kills most of the organisms which are sensitive to heat and thus, in a way, effects an automatic sterilization of the fodder, at least in the opinion of its advocates. The fundamentals of this conservation process had already been explained by an American named Miles, in 1883. After numerous samples had been taken from such silos the bacterial condition was characterized by three things: Presence of a very large number of the so-called hay and potato bacilli, total absence of lactic acid bacteria, and the presence of butyric acid bacilli ranging in quantity from copious to excessively large numbers. These bacilli were, as a rule, in the spore form, as were also the hay and potato bacilli.

The method of making this silage explains its microbiological nature. After loosely packing the silage, the temperature rises quickly to about 50° C. and more, and consequently the ordinary lactic acid bacteria are destroyed, and those that have survived the higher temperatures, for other reasons, find it difficult to sustain themselves. Absence of lactic acid fermentation is, however, generally in mixtures of organic substances, one of the conditions of growth for different spore-forming organisms, especially the acid-sensitive hay and potato bacilli.

It should not be said that the butyric acid bacilli necessarily, or as a rule, develop in sweet ensilage. Their absence in certain parts of the silage, as well as the decrease in bacilli count in different silos, leads us to believe that the ineasure of their development in silage is dependent upon very distinct conditions that have not yet been discovered. The occurrence of butyric acid bacilli in so-called sweet green fodder gave basis for our warning Swiss farmers against its use. These organisms, as we shall later see, have proven themselves to be the worst evil in the manufacturing of Emmental cheese.

Type B. Electrically treated silage.-The recently developed idea of running an electric current through green silage to preserve it deserves some attention. Indeed, one should not expect on the grounds of earlier knowledge of the effect of electric currents on microorganisms, that the bacteria would be destroyed and the silage thus preserved, as was the idea of the inventors. But we must admit the possibility that heat generated by the alternating current in the ensilage, which is the resistance, can have a beneficial effect on those conditions which take place during the conversion of the fodder into ensilage. This effect induces, above all, the activity of the organisms which is retarded in cold, wet fodder for the reason that the warmth produced by the respiration of the plant cells is taken up by the relatively large amounts of water and so the stimulating effect on the organisms, and particularly the lactic acid bacteria, is lost. The inventors therefore advertise, as an important advantage of this method of conserving silage, that any vegetable matter of suitable nature and moisture content can be utilized in this way.

We were interested most in the questions bearing on the bacteriological nature of this type of conservation. From the very first tests of samples put at our disposal by the inventors the presence of millions of lactic acid bacteria has always been indicated, and it was hoped that the occurrence of a lactic acid fermentation would suppress the growth of butyric acid bacilli. The distillery affords an old familiar process whereby a strong lactic acid fermentation may be induced in the mash so that the butyric acid fermentation is suppressed. But we were disappointed in our expectations. In the electric silo erected at our experiment station, as well as in similar silos belonging to farmers, an ensilage was produced which, to be sure, contained a strong growth of lactic acid bacteria, but also a quantity of spores of the harmful butyric acid bacilli, which made the electrically treated silage appear little better than the sweet ensilage for use in producing milk for Emmental cheese.

There is no doubt that a lactic acid bacteria fermentation can not under all conditions suppress the growth of butyric acid bacilli, and this opinion is substantiated by various laboratory tests which can not be discussed in this paper. We also know that other experimenters have made more successful tests with electrically treated silage, and we might well believe that occasionally this process does produce a silage that from a microbiological standpoint could not be contested as to its use for the production of milk for cheese. We have also found exceptional cases of sweet silage, that showed a low count of butyric acid bacilli. But an ensilage, whose process of production can not be controlled so that perfect quality is the rule, proven by the exceptional failure, can not be considered for the production of milk used in the manufacture of a high-class export cheese like the Emmental.

Type C. Sour silage.--The preservation of plant waste of various kinds (chiefly beet leaves) by packing and pressing in pits, or also in stone jars, is no doubt the oldest method of utilizing the preserving powers of the plants themselves or of the bacteria which they bear. In this process, to which type the preparation of sauerkraut belongs, various organic acids occur as a rule as the by-products of fermentation, among them certain volatile acids, which give the silage a peculiar taste and smell. It was wrong to call a preservative “sweet” because it lacked an acid smell; nor in the narrow sense of the word is it sweet, but in reality acids are formed and among others lactic acid, which is stable and odorless. The purpose of producing a green-silage preservative that would not smell of volatile acids coincides with the purpose of inducing a pure lactic acid fermentation in the vegetable matter. Therefore the fermentation must be produced by those kinds of lactic acid bacteria which form only lactic acid and no volatile acids, and all fermentative organisms, whose characteristic metabolic products are volatile acids such as acetic and butyric acids, must be excluded. It has been shown in our

, experiments that this exclusion is not easily accomplished, and that a strong growth of the ordinary Streptococcus lactis, for example, does not prevent the growth of butyric acid bacilli. Entirely different conditions must be obtained if harmful bacilli would be eliminated from the vegetable material which will later be converted into milk used in cheese making. We believed that this condition might be brought about by the use of green-corn silage, in view of the success of American farmers with this type. The silage tests carried out at Liebefeld, using in one case green corn and in another corn that had been allowed to dry in the field, have yielded a silage which, like the above-mentioned types, is quite unsuitable for the production of milk used in the manufacture of Emmental cheese, because of its butyric acid bacilli content. We achieved a better result in our tests with green corn ensilage, prepared after the American fashion, in which special importance is attached to the experience of the American farmer who found that corn cut when the kernels were beginning to ripen gave the best silage. Of all ensilages tested, this was the only one that showed a butyric acid bacilli count not higher than the

a ordinary green fodder of Swiss farms. It is most probable that Emmental cheese prepared from milk produced from this silage will not show signs of inflation or those other defects that are known to be traceable to the ensilage. Whether or not the taste is affected by the influence of such ensilage can only be determined by experimenting on large quantities with the necessary controls.

From a bacteriological standpoint, the question might be raised as to why in filling the silo with corn of the desirable ripeness, a low butyric acid bacilli count was obtained in this silage, while green corn, cold or artificially warmed by an electric current, in spite of the preponderance of lactic acid bacteria, gives an undesirable number of the noxious bacilli. The discussion of this question will not be undertaken here, except to mention that our corn ensilage (prepared in the American fashion) gave very decided rodshaped bacteria (so-called lactobacilli) in a natural, pure culture and many millions of these bacteria per gram. The corn plant at a certain stage of maturity seems to lend itself very well to the lactic acid fermentation and to the exclusion of other fermentations, and it is remarkable that in this case, as also in other previous cases, extensive use has shown and proven that which has been subsequently determined scientifically to be the most desirable.

III. EXPERIMENTS ON THE SUITABILITY OF MILK FROM SILO DAIRIES FOR

CHEESE MAKING.

Influenced by the widespread propaganda that was current in the early eighties, numerous investigations concerning the preservation of green fodder were carried on in Switzerland. Wherever Emmental cheese was made of the milk from cows fed chiefly on this ensilage, defective cheese was produced. These experiences ought to have been taken into consideration, but they had been made a long time ago and therefore had lost their discouraging effect; on the other hand the advocates of the old idea put into new terms claimed to possess now a better method, which was the sweet green-fodder method mentioned above.

From the very first use of silage in our experimental dairy, serious defects appeared in the cheese, namely, an excessive number of large holes, that is, inflation to the extent of splitting the rind, and also bad flavor and smell. The same defects appeared in the cheeses of other creameries in the country that had been using milk. from silage-fed cows. The bacteriological investigations of the defective cheese revealed a large number of the mobile butyric acid bacilli, and since the same organisms as a rule are found in large quantities in sweet green-fodder ensilage, there could be no doubt as to the relation to the bacteriological nature of the cheese. The beginning and ending of the inflation of the cheese coincided so decidedly with the beginning and ending of the feeding of the silage, and the effect of the butyric acid bacteria was so pronounced, that in the future the judgment of a new type of feed and particularly ensilage will depend upon its butyric acid bacilli spore count.

That we have thus discovered the right method was proven in our tests with electrified silage and with the different types of sour silage. Wherever milk was used that had been obtained from cows fed on this kind of feed the results were no better than where sweet silage was used, and the entire experimental evidence obtained during the last few years justified the attitude of the Swiss dairymen in turning against all kinds of fermented feeds. More will be said later regarding the exceptional case of American corn silage.

« PreviousContinue »