Popular Science Monthly/Volume 40/April 1892/Bacteria in our Dairy Products
|BACTERIA IN OUR DAIRY PRODUCTS.|
By Prof. H. W. CONN.
THERE have been no discoveries in the last half-century more startling than those which are now accumulating upon the subject of bacteriology. Every one knows to-day that bacteria have a causal connection with certain diseases, and the whole civilized world has been recently agitated over the attempts that are being made to combat their effect in the human system. The study of the relation of these organisms to the animal body seems to be producing a revolution in the study of medicine, or rather perhaps is creating a science of medicine, for medicine of the past can hardly be called a science.
We have heard so much of the disease-germs and their evil effects that bacteria are usually looked upon as unmitigated nuisances. It is a doubtful chance if any knowledge of their beneficial effects has passed beyond the reach of the scientist's laboratory and lecture-room. But science has for a long time known that even the bacteria which are not connected with disease are of immense significance in the processes of Nature. The non-pathogenic germs were studied long before the pathogenic forms; but the great attraction offered by the study of disease has led the larger number of bacteriologists in this direction. To-day, however, we are beginning to recognize more than ever the great part played by the harmless bacteria, and to find out that their value in the world far outweighs the injury produced by their mischievous relatives. There is hardly a process in Nature which is not in some way connected with bacteria growth. Fermentation, the raising of bread, the formation of vinegar, the germination of seedlings, the growth of plants, the ripening of fertilizers, the decomposition of animal and vegetable bodies by means of which they are again incorporated into the soil, are all to a greater or less extent dependent on the growth of micro-organisms, either bacteria or yeasts. Without the agency of these organisms to prepare the soil, plants could not grow, and life would soon disappear.
There is no one who is not directly or indirectly connected with the dairy industry. The discoveries of the last twenty years, and more especially those of the last five years, have shown that dairy products are in a large measure connected with the growth of microscopic organisms—some dairy processes, indeed, being nothing more than gigantic breeding experiments. Each of the three chief products of the dairy—milk, butter, and cheese—has its own definite relations to bacteria growth and each must be considered separately.
Milk.—The souring of milk is such a universal phenomenon that it has until recently been considered a normal character of milk. The last twenty years have, however, demonstrated for us that it is universally caused by bacteria growth. The souring of milk is simply the formation in it of a certain amount of lactic acid, and the acid precipitates the casein of milk just as any other acid would do, and thus forms the curd. But it is bacteria which produce the lactic acid. The presence of micro-organisms in milk was first noticed fifty years ago by Fuchs, but it was not till twenty years later that Pasteur succeeded in showing that these organisms could really produce lactic acid and thus might be the cause of the souring of milk. Fifteen years more were required to show that they were the sole cause of the souring of milk, and to demonstrate the further important point that milk when drawn from the healthy cow contains no bacteria and has therefore no tendency to sour or undergo other unpleasant changes. Since this was first shown by Lister, in 1873, numerous observers have so successfully verified the conclusions of Pasteur and Lister that no possibility of doubt longer remains, and we now know that under normal conditions the milk while in the mammary gland of the healthy cow is free from bacteria, and we have abundant proof that such milk will never sour nor ferment if kept free from bacteria contamination.
Absolutely pure milk is, then, free from bacteria; but when we examine milk that has been standing for a few hours the number of bacteria found in it is almost incredible. By the time that it is five or six hours old milk will contain millions for each tumblerful, and by the time it has reached the city consumer it will frequently contain fifty millions to the quart. Now, if the milk while in the cow contains no bacteria, it follows that this numerous crop must have been planted in the milk during the milking or subsequently. At first thought it seems hardly possible to believe that this immense number of bacteria could have found their way into the milk since the milking. But when we learn that they are abundant in the air; that they are crowded in every particle of dust clinging to the hairs of the cow; that they are always present in the milk-duct for a short distance from its opening, living there in the remains of the milk left from the last milking; that the milk-pail in which the milk is drawn can not be washed clear of them by any ordinary methods; that the milk-cans will always contain them in cracks and chinks even after the most thorough cleansing; that they are always on the hands of the milker; and when, in addition to all this, we learn that bacteria multiply so fast that by actual experiment a single individual may in the course of six hours give rise to three thousand progeny—it no longer remains a marvel that their number is so great in milk of a few hours' standing.
Fortunately, this immense number of bacteria in milk need not especially alarm us, for they are not disease-germs and are harmless to the healthy person. Nevertheless, they are undoubtedly a nuisance in the milk. They can not grow there without producing some effect upon it. Commonly the first change noticeable is the appearance of the well-known odor and taste of sour milk, followed shortly by its curdling. This souring is undoubtedly the result of bacteria growth, and it was at first supposed that there was a single definite species which alone had this power of producing lactic acid. So thought Pasteur and Lister at first, and such a species they described. The species of bacterium studied by them certainly had this power, and it was named Bacterium lactis by Lister. In later years the name Bacillus acidi lactici has been given to it. By the work of the last six years we have learned that more than one species has the power of souring milk by the formation of lactic acid. Lactic-acid formation is the characteristic of a class of bacteria comprising many species, and even in the ordinary souring of milk under normal conditions it is not always the same species of bacteria which produces the mischief.
While it is true that any one of a number of species of bacteria may produce lactic acid by their growth in milk and thus cause its souring, in other respects these different species do not have the same effect. The formation of lactic acid is not the only change that occurs in the souring of milk. Sour milk has a well-known odor, but this is not due to the lactic acid, since lactic acid is odorless. The formation of such an odor tells us, therefore, that there are other changes going on in the souring of milk. The fact is, that a decomposition of the albuminoids and other substances in the milk is going on, and it is these decomposition products that give the odor. Now, the different species of bacteria do not all produce the same sort of decomposition products. All who are familiar with milk will recall that the character of sour milk is by no means uniform. It differs in the hardness of the curd, in the amount of the whey, in odor, and even in taste. When different specimens of milk are examined just before or just after souring, it is found that the species of bacteria are by no means the same in the different specimens. Each will contain some of the acid forming class, but the particular species which happen to be present in the different specimens will vary with the different conditions. Different localities and different methods of handling the milk will affect the variety of bacteria that it contains. It will sour in all cases, since all have some of the members of the acid forming class; but the other accompanying phenomena may be different. Thus we have learned to attribute all the differences in the different specimens of sour milk to the fact that the souring has been produced under the influence of different species of bacteria. The souring of milk is therefore not a simple or a uniform phenomenon. While it is always the effect of bacteria growth, we recognize many varieties of souring corresponding to the variety of bacteria most abundant in the milk before souring. All this makes little difference to the consumer; in any case the milk is ruined for his purposes, and he is more concerned in preventing such troubles completely than in learning their variety. A remedy seems simple enough. When we have once learned that the whole trouble is caused by bacteria, we see that it is only necessary to keep these organisms out in order to preserve the milk pure and sweet.
From the standpoint of public health also the desirability of freeing milk from these organisms is becoming every day more apparent. It is true that the vast majority of the bacteria in milk are perfectly harmless to the healthy person, even when swallowed in such numbers as above indicated. But, at the same time, it not infrequently happens that disease-germs get into the milk and, finding there a suitable medium for growth, multiply rapidly. They are then served out to all the patrons supplied with the milk. Typhoid fever is certainly disseminated by means of the milk-supply, and there is a growing conviction that the fatal tuberculosis owes much of its prevalence to milk from diseased cows. Other epidemics have also been traced to the same source.
Even if no definite disease-germ chances to be present in the milk, the vast number of harmless forms may render the milk dangerous to all having weak or sensitive digestive organs; for they produce considerable lactic acid, and every one knows that acid is injurious in the food of infants and invalids. The presence of lactic acid is probably a less serious matter than the presence of certain decomposition products which are formed by the same bacteria. These are directly poisonous, and, although they are present in such small quantities that they have no effect on the healthy person, they may be injurious to one whose digestive organs are in a sensitive condition. For a long time doctors have recognized that boiled milk is a safer food for invalids than raw milk, supposing, however, the explanation to be that the cooking renders it more easily digested, just as it does other foods. Recent experiments have shown us that this is not true. On the contrary, boiled milk is less easily digested and absorbed by the system than raw milk. The real reason for the greater safety in drinking boiled milk lies in the fact that it is thus deprived of the disturbing action of the millions of bacteria ordinarily present.
To keep bacteria out of milk is a practical impossibility. Their presence in such quantities in all places renders their access to it a certainty, and it has only been by exercising the most extraordinary precautions that scientists have in a few cases succeeded in obtaining milk directly from the cow in such a way as to avoid its becoming contaminated during the milking. At the same time much may be accomplished by cleanliness in the barn and the dairy. The presence of disease-germs in the milk is always to be traced to filth or to carelessness in handling the milk, or to diseased cows. Typhoid-fever germs, for instance, can only get into the milk from some unusual source, and tuberculosis germs only from diseased cows. If it were possible to enforce cleanliness in the barns and dairies, and to obtain sufficient care in the handling of milk, we should have no more epidemics spread through the milk-supply. But, in the present state of public ignorance and carelessness as to health, such an enforcement is an impossibility. In our cities and large towns, therefore, the milk supply must be looked upon as one of the fertile sources for the spread of disease, and it behooves every one to look carefully to the condition of the milk he drinks during times of epidemics, especially of those affecting the digestive organs.
But even with the most extreme care it is impossible for our milkmen to avoid the presence of the more common forms of micro-organisms which will sour the milk. Recognizing, then, that bacteria are sure to get into the milk, we may next ask if there is no way of destroying them after they get in. If we can kill these bacteria, we can of course preserve the milk longer and render it more healthful. It is easy enough to kill the bacteria though every method is open to certain objections. Various chemicals have been suggested for poisoning the bacteria, or at least for delaying their growth, but they are one and all to be condemned, as likely to do more harm than good.
A safer and more effective method for preserving milk is by the use of heat. All bacteria may be killed by heat, and then, if the milk be kept in tightly closed bottles, it will keep sweet indefinitely. For this purpose many sterilizing machines have been invented in the last few years, all based upon the same principle, but differing much in detail. In all cases the milk is subjected to a high heat. Most of them simply heat the milk to a boiling temperature by means of steam or boiling water, but a few, by boiling under pressure, contrive to raise the temperature considerably above boiling water. Although many forms of apparatus have been devised for simplifying the matter, no apparatus is really needed for sterilization. All that is necessary is to put the milk into bottles and boil it for ten minutes with the mouth of the bottle open, then close the mouth and steam it for ten minutes more. This method of sterilization will not kill all of the bacteria in the milk, but it will kill all the disease-germs which are likely to be in it, and it will so decrease the numbers of the other bacteria that the milk will keep sweet for a long time.
All methods of sterilization that are in use in this country have the disadvantage of giving to the milk the taste which is peculiar to boiled milk, and also of rendering it less easily absorbed by the body. In France and Germany a method has been adopted which accomplishes the purpose without injuring the taste of the milk. Machines are in use in Paris and some other cities which will heat great quantities of milk to a temperature of about 155° Fahr. for a few minutes, and then cool it rapidly to a low temperature. The method has been called the pasteurization of milk. It does not kill all the bacteria, but it does destroy so many of them that it greatly increases the keeping properties of the milk. Moreover, it almost entirely destroys the danger from disease-germs in milk, since nearly all forms likely to occur in milk are killed by this temperature. The advantage of this method is that the temperature of 155° Fahr. does not give to the milk the taste of boiled milk, which most people find unpleasant, and does not render the milk difficult of digestion. These pasteurizing machines have not yet been introduced into this country, and the opportunity exists for some one to develop a thriving business by furnishing pasteurized milk in our large cities. A little experience with its superior keeping properties, and a little knowledge of its greater wholesomeness, would soon create a demand for it in America as it has already done in the larger cities of France and Germany.
Butter.—If bacteria are the enemies of the milkman, they are the allies of the butter and cheese maker. The last few years have shown us that butter owes at least its flavor to bacteria growth in the cream. Butter is made by allowing the cream to separate from the milk by means of its less specific gravity, and then by shaking the cream vigorously until the butter collects in lumps. Now, it has been for a long time recognized that it is a difficult matter to churn sweet cream. It may be shaken for a long time without the separation of the butter, and a smaller amount of butter can be obtained from it than from cream that has been allowed to sour or "ripen" for a time before churning. This, at all events, is true of cream which is separated from the milk by the ordinary method of setting, though it seems less true of cream separated by means of a centrifugal machine. It has also been generally recognized that the butter made from sweet cream lacks the delicate flavor or aroma which is such an important factor in a first-class butter. Sweet-cream butter has a flat, creamy taste, which is not generally desired.
For these reasons butter-makers have learned not to churn cream when fresh, but to allow it to stand awhile and sour, or "ripen" The cream in a creamery is placed in large vats, and then kept at a constant warm temperature for about twenty-four hours. The cream is stirred frequently during this time, and at the end of the ripening it is seen to have changed its character. It is somewhat acid in taste, is slightly thickened, and has a pleasantly sour odor, though one quite different from that of sour milk. The cream is now churned, and the butter is found to separate readily, the quantity is at its maximum, and the butter obtained has the proper butter aroma.
Bacteriological study of the last few years has shown that this "ripening" is nothing more than a breeding of bacteria on a large scale. There were many bacteria in the cream at the beginning, and the ripening has been conducted at just the temperature at which bacteria grow rapidly. The result is, that their multiplication is marvelously rapid, and the number of bacteria present in ripened cream is beyond comprehension and almost beyond calculation. Five millions in a drop would not be too high an estimate for some specimens.
Now, what are the bacteria doing in the cream during their twenty-four hours' growth? They can not multiply so rapidly without producing profound changes in the cream. So far as the butter-maker is concerned their action is twofold: 1. There is produced in the cream a considerable amount of lactic acid, together with small quantities of other acids. 2. Various decomposition processes are going on which fill the cream with decomposition products, and these give rise to the odor and taste of ripened cream.
To understand the effect that this ripening has upon the butter-making, we must first ask what happens to the cream during the churning. If we look at a drop of milk under the microscope, we find that the butter-fat is in the form of the most minute drops. So small are they that they can not be readily separated from the liquid part of the milk. In cream we simply have the great mass of these drops together, but still not at all fused, like a lot of snow-balls floating in water. In the churn, however, the cream is agitated until the drops are shaken together and made to fuse with each other. They now form masses of fat large enough to be removed from the liquid in which they float, and these masses form the butter. But, looking at the cream more closely, we find a mechanical difficulty in the way of their ready fusion. The fat-drops are not free to move at will, for they are bound together in groups by a sort of slimy substance. As we watch the cream with our microscope we see the fat-globules are not easily shaken together, for the slimy matter prevents their direct contact. This slimy substance must be broken down and the drops shaken into each other before the butter can form into the large masses necessary for their separation from the liquid. It requires a deal of shaking to accomplish it when the slime is intact, and sweet cream may sometimes be churned for hours without producing the butter. But the ripening prepares the way for the churning. The acid formed by the bacteria gradually dissolves this slime, which is of an albuminous nature, and after it is thus dissolved the difficulty of the fusion is gone and a short shaking in the churn finishes the process. It is plain, too, that a larger amount of butter will be obtained from the cream, for in churning sweet cream much of the fat will be left behind in the form of small drops not to be separated from the slime even after the most vigorous churning.
As mentioned above, the second advantage derived from ripening is the development of the aroma of a first-class butter. Sweet-cream butter is tasteless, and the cause of the butter aroma is to be found in the decomposition products of bacteria growth. While growing in the cream they are splitting up the sugars and albuminoids present and producing decomposition products. Among them are many volatile products which have a prominent odor and taste, and these, as we have seen, produce the odor and taste of ripened cream. Now, of course, the butter obtained from such cream will be affected by these compounds, and thus we see that the delicate aroma of first-class butter is produced by the decomposition products of bacteria growth in the cream. These are volatile, and eventually pass away from the butter in large measure. It is well known that the delicate butter aroma is found only in fresh butter. Old butter is strong enough in its taste, but the peculiar delicate aroma is gone. When first made, however, these volatile substances permeate the butter and explain its flavor. Of course, it is highly essential that only the proper decomposition products should be developed, and for this reason it is a matter of high importance that the ripening shall be stopped at just the right time. If it is not continued long enough, the proper decomposition will not take place; and, on the other hand, if it is continued too long, the volatile products will approach those of putrefaction and give a strong-tasting butter. At just the right moment they are present in sufficient amount to give the butter a pleasant flavor without being so abundant as to give a disagreeable one. The experience of the butter-maker guides him in determining when to stop the bacteria growth, and here is one of the points of skill in butter-making. When the cream is ripe enough he churns it, and this ends the process, so far as the bacteria are concerned, for they cease to grow when the butter is made.
But why should they cease to grow? Why do they not continue to cause the decomposition in the butter? What becomes of them after the churning? The answer to these questions is simple. Many of the bacteria go off in the buttermilk; many more are removed by the water used in washing, but many of them still remain in the butter. Here, however, their active life is nearly over, for the salt added to the butter checks their growth and their numbers begin to diminish. Butter is not a good medium for their development, and, after a few weeks, they practically disappear. Their growth in the butter is thus so slight that it is of no importance and ordinarily produces no noticeable result. To be sure, the butter may subsequently become rancid, and until recently it has been supposed that the rancidity of butter was due to bacteria growth. Some species of bacteria certainly produce butyric acid, and this is one of the most prominent characteristics of rancid butter. But it has been recently shown that butter may become rancid independently of bacteria growth, the direct oxidizing power of the air producing the effect. Bacteria, it is true, may hasten the process, but they are probably not a necessary cause. After the butter is made, then, the bacteria are of no further importance, and unless there should chance to be some disease-germs among them nothing further will result from their action.
The butter-maker thus forces the bacteria to give to his butter a flavor for which he gets a good price in the market. Unfortunately for him, however, there is more than one species of bacteria which may readily get into his cream and produce its ripening, and not all of them are equally serviceable to him. Many species of bacteria give a very unpleasant flavor to the butter if they are abundant in the ripening cream. While they cut the slime that holds the fat-globules and thus make the churning easy, the aroma produced by different species is by no means always satisfactory. It has been found that many of the species which commonly grow in ripening cream will produce very disagreeable butter if they are allowed to act alone. Others acting alone produce good butter, and the latter must, of course, outweigh the former, or the butter will be unsatisfactory.
The fact is, that during the ripening of the cream a great battle is going on among the different species of bacteria. Some of them find the conditions of the ripening cream favorable to their growth, while others find it less favorable. The favored species multiply rapidly, and may largely crowd out of existence those less favored. Some species may chance to get the start of others by being in greater numbers at the outset, while another species will make up for all drawbacks by having a more rapid rate of multiplication. The final result of the struggle will depend upon an infinite variety of conditions, which will be entirely beyond our knowledge. The condition of the cow, the manner of milking, the manner of setting the cream, the temperature, etc, will all be important factors favoring one form of bacteria and hindering others. If the battle results in favor of the beneficial species, a good-flavored butter will result, while, if the injurious species should get the upper hand, the butter will be bad. The results are at present beyond the control, of the butter-maker. By practice he has found the methods which will commonly result in a good product; but even with his greatest precautions he is occasionally unable to obtain the best butter. At certain seasons of the year failure to obtain good butter is about as common as success even in our best creameries.
Now, bacteriologists would not pretend that the bacteria content of the ripening cream is the sole reason of the variations in the quality of the butter product. Different conditions of the cattle, different food, etc., will all affect the butter, but beyond doubt bacteria have an important part to play. Now, uniformity in the product of the dairy is the great desideratum of the butter-maker. Usually he can make good butter, but sometimes he fails from unexplained causes. The complexity of the ripening process makes it impossible for him to be sure of uniformity in this respect, even though other conditions are constant. But what is to prevent the bacteriologist finding the right bacteria to produce a proper aroma to the butter and furnishing them in quantity to the butter-maker to use in time of trouble? They may then be planted in the cream, and thus a ripening always assured which shall be of the best character. It seems to be entirely possible thus to produce uniformity in this direction. Already in Germany and Denmark and in this country experiments have been started looking in this direction with much promise of success. It is not unlikely, therefore, that before long the butter-maker will have at his command a method of assuring success in the aroma of his butter if he only exercises ordinary skill in the process of its manufacture. If such an artificial ferment may be obtained, uniformity in the ripening of cream will be easy. Perhaps the result will be to bring the different creameries into greater likeness to each other, enabling those which now are unable to obtain a first-class product to improve its flavor by using the right species of bacteria for ripening in the place of the inferior species which are afforded by some localities. This would perhaps not improve the best qualities of butter, but would bring the inferior qualities to a higher standard.
Cheese.—If bacteria are an aid to the butter-maker, they are absolutely indispensable to the cheese manufacturer. Some people do enjoy the taste of sweet-cream butter, and there has been for some time an evident tendency toward a desire for less strongly tasting butter. But no one desires to eat fresh cheese. When first made, cheese is soft and tastes somewhat like milk curd. It has none of the palatable taste which we find in the cheese of our table. It is a long ripening which gives this taste to the cheese.
Here, again, the ripening process is one of bacteria growth. The millions of bacteria that were in the milk are stored away in the cheese, and instead of being killed here, as they are in the butter, they begin to multiply immediately. Here, too, there is a battle of bacteria, and now one species is in the ascendency and now another. If the wrong species gets the upper hand, the cheese becomes bad, and cheese-makers have their greatest trouble from this source. The bacteria do not grow so rapidly as they do in cream, for the conditions are less favorable, but the ripening is kept up for months, and during the whole time the bacteria are growing. Under their action the character of the cheese slowly changes. Here, again, the decomposition products are responsible for the taste and odor. In some cases, such as Limburger cheese, the action is allowed to continue to the verge of putrefaction. Ordinarily it is not continued so far, but in all cases the cheese-maker favors the growth of certain forms of bacteria by regulating the temperature at which the ripening is carried on. As the ripening continues, certain parts of the cheese are digested and decomposed by the bacteria growth, and, as the products of decomposition accumulate, the taste grows stronger. After a time it is considered fit for the market, but the longer the ripening continues the stronger the taste becomes.
Little is known yet as to the bacteriology of different kinds of cheeses. Whether the different tastes of Edam, Limburger, and other characteristic cheeses is largely due to the character of the bacteria ripening them can not yet be said. Cheese-makers do, however, have much trouble with various irregular forms of ripening, and a great drawback in this business is the lack of uniformity in this respect. Beyond doubt this is due largely, perhaps chiefly, to the variety and number of bacteria which succeed in gaining a foothold in the cheese and contribute to its ripening.
Along the line of cheese manufacture our bacteriologists are promising us help from their researches. Of course, the cheese maker has never paid any attention to the sort of bacteria which he plants in his cheeses, for he has never heard of them. Sometimes he has unwittingly planted species which produce violent poisons, as is shown by the many instances of death from eating poisonous cheese. Now, our bacteriologists are suggesting that the ripening of cheese may be easily controlled. Artificial cultures of the proper sort may be furnished the cheese-maker, and if these are planted in the cheese not only will the danger from poisonous cheese be prevented, but at the same time the desired taste of the cheese be assured. More than this, when we recognize the great variety of decomposition products which the different species of bacteria produce, we can see ahead a great development in the varieties of cheese. Who can tell what may be the numerous varieties of cheeses produced when our cheese-makers have learned to ripen their product with pure cultures of different species of bacteria, instead of depending as they do now upon "wild" species which get into the cheese by accident from the milk!