Popular Science Monthly/Volume 58/December 1900/Microbes in Cheese-Making

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By Professor H. W. CONN,


CHEMISTS tell us that cheese is one of the most nutritious and, at the same time, one of the cheapest of foods. Its nutritive value is greater than meat, while its cost is much less. But this chemical aspect of the matter does not express the real value of the cheese as a food. Cheese is eaten, not because of its nutritive value as expressed by the amount of proteids, fats and carbohydrates that it contains, but always because of its flavor. Now, physiologists do not find that flavor has any food value. They teach over and over again that our foodstuffs are proteids, fats and carbohydrates, and that as food flavor plays absolutely no part. But, at the same time, they tell us that the body would be unable to live upon these foodstuffs were it not for the flavors. If one were compelled to eat pure food without flavors, like the pure white of an egg, it is doubtful whether one could, for a week at a time, consume a sufficiency of food to supply his bodily needs. Flavor is as necessary as nutriment. It gives a zest to the food, and thus enables us to consume it properly, and, secondly, it stimulates the glands to secrete, so that the foods may be satisfactorily digested and assimilated. The whole art of cooking, the great development of flavoring products, the high prices paid for special foods like lobsters and oysters—these and numerous other factors connected with food supply and production are based solely upon this demand for flavor. Flavor is a necessity, but it is not particularly important what the flavor may be. This is shown by the fact that different peoples have such different tastes in this respect. The garlic of the Italian and the red pepper of the Mexican serve the same purpose as the vanilla which we put in our ice-cream; and all play the part of giving a relish to the food and stimulating the digestive organs to proper activity.

The primary value of cheeses is, then, in the flavors they possess. One can hardly realize the added pleasure they give to the life of hundreds of thousands of poor people whose food must be of the coarsest character. A bit of well-flavored cheese adds relish to the humblest meal and gives the highest delight. We must recognize, then, that the chief value of the cheese lies exactly in these flavors which the chemist does not include in his analysis of cheese and which the physiologist refuses to call food or to regard as having any nutritive value whatever. Incidentally, it is true that the cheese also furnishes a considerable amount of food material. Thus it nourishes as well as stimulates and delights; but, after all, we must recognize that its chief value is in its flavor rather than in its nutritive quality.

Hence it becomes a very significant question to inquire into the source of this flavor. We find, first, that the cheese as originally made possesses no flavor, or, at least, none of that peculiar flavor which we know as cheesy. Cheese is made from milk by causing the casein in the milk to be precipitated, i. e., causing the milk to curdle, commonly by the addition of rennet, or, in so-called Dutch cheeses, by simply allowing the milk to sour. The precipitated casein is then separated from the liquids of the milk, and the curd, when subsequently pressed and molded, becomes the cheese. But the freshly-made cheese possesses no flavor, nor does the flavor develop to any degree until after it has passed through a process known as 'ripening.' The ripening of cheese may take several days or several months, or, in some cases, one or two years; but the flavor always arises during this process. Moreover, the various cheeses with their varieties of flavors are mostly made from the same kind of milk, but are subjected to different modes of ripening, and the distinctive quality in the endless types of cheeses is due in large measure to differences in the method of bringing about this ripening. Clearly enough the flavor is a product of cheese ripening, and if we wish to find the source of these most valuable flavors we must seek it in the ripening process.

This cheese ripening proves to be a two-fold process. The first change in the cheese is a chemical one, which results in altering the chemical nature of the cheese in such a way as to render it more easy of digestion. This change appears to be due in part to a certain ferment which is found in milk. This material belongs to the class of chemical ferments or enzymes and is a normal constituent of milk, although its presence was not mistrusted until recently pointed out by two American investigators. With the chemical changes produced by this enzyme we are not here particularly concerned. It is certainly not the cause of all the flavors which develop in the cheeses, and, therefore, this character of the ripened cheese must be chiefly attributed to another factor. There is no doubt that this other factor is a living one. The flavors can generally be traced directly to the growth upon and within the cheese of a variety of plants; and the ripening is carried on in a fashion designed, at the same time, to stimulate the growth of some species of plants and to check the growth of others.

Cheeses are of two kinds, hard and soft. As implied in the name, there is a difference in the consistency of the cheese. But this is not all; for on account of the methods of manufacturing, the ripening is produced by different classes of plants in the two classes of cheeses. In the soft cheese, the plants contributing most to the ripening and to the formation of the flavor are what are commonly called molds, at least in some cheeses, while in the hard cheeses the molds play probably no part, and bacteria are the most active agents in producing the flavors developed during the ripening.

In making the soft cheeses—little known in this country—the general mode of procedure is as follows: The milk, sometimes whole milk, sometimes partly skimmed, is caused to curdle by the action of rennet. The curd is either cut to pieces by knives designed for the purpose, thus allowing the whey to drain off more readily, or it is simply ladled out of the vessel in which it curdled and placed at once into forms. As the whey is drawn off from the forms, through holes in the sides or through a false straw bottom, the curd soon assumes the shape of the forms. It is at first very soft, since it is subjected to no pressure whatever. At short intervals this soft mass is turned, so as to rest upon a new surface, and this turning is continued for two or three days. By this time the curd has become dry and consistent enough to handle, and it is then carried off to the cheese cellar for ripening. The details of this process differ considerably. In quite a number of cheeses particular methods are adopted to favor and hasten the growth of molds. Sometimes it is laid upon special straw mats or wrapped in straw, which, having been used over and over again in the dairy, has become thoroughly impregnated with mold spores. The cheese is then placed in a cool, damp atmosphere, which causes the spores to germinate and grow upon the cheese, already slightly acid, and in a condition favorable to the growth of molds. They grow rapidly over the whole surface of the cheese, and this step in the process is not ended until a good covering of molds has developed. Sometimes, indeed, special methods are adopted to insure their proper development. In making the Roquefort cheese specially prepared bread is allowed to mold, and after it becomes thoroughly impregnated with the mold it is finely grated to a powder and mixed with the curd as it is placed in the form for shaping. Fine holes are pierced in the cheese by a special machine to let in the air which is necessary for the luxuriant growth of the molds. Such treatment insures, of course, a very rapid growth of these plants, inside as well as outside. Most commonly, however, the cheese-maker depends upon his straw mats for the molds, and expects them to grow chiefly on the surface. The molds which develop in the cheese are not all of the same species. The common blue mold is most usual, but most cheeses are not properly ripened until several species of molds grow together within them.

The development of molds, however, is by no means the end of the ripening, but rather its beginning. Indeed, in some of the soft cheeses their growth is entirely prevented by a thorough salting and washing of the surface. In such cheeses the mold may grow within the mass, but not on the surface. Whichever method is used, however, the cheese is presently removed to the so-called 'cheese cellar' for its proper ripening. These cellars may be cool, damp rooms, or caves, and the flavor of some kinds of cheeses is largely due to the nature of the caves in which the subsequent ripening is carried on. In these cellars there is a constant but not very high temperature, and the atmosphere is generally damp. Since the temperature and the moisture are kept as constant as possible during the whole year, the cheese ripening can continue slowly and indefinitely. To a considerable extent differences in the ripening of soft cheeses are due to the different temperatures of the cheese cellars, and this determines the kind of plant life that shall flourish in this soft, nutritious food.

After the removal to the ripening chambers, a new series of changes begins in the cheese, due to new kinds of plant life. But as yet neither the cheese-maker nor the bacteriologist, who has studied the matter most carefully, can tell us much of the nature of the actual changes which occur during this ripening. When the cheese is placed in the ripening chamber it is certain that the growth of the molds is largely stopped, and it is also certain that here begins a growth of a new class of plants which we call bacteria. This moldy cheese, rendered alkaline by the growth of the molds, furnishes a favorable medium for the growth of different species of bacteria. At high temperatures they would speedily decompose the mass, even to extreme putrefaction, but at the low temperatures of the cheese cellars a complete putrefaction does not occur. Bacteria growth takes place probably in all soft cheeses, and as a result the nature of the cheese is profoundly modified. Numerous new chemical products make their appearance, either as byproducts of decomposition or as actual secretions from the growing bacteria and molds. These new products have strong tastes and odors which, as they slowly develop, gradually produce the characteristic flavor of the ripened cheese. If the ripening continue long enough the decomposition grows too advanced even for the strongest palate. But when the proper ripening has been acquired and the tastes and flavors are of the desired character, the cheese is sent to market, highly flavored by the joint action of the bacteria and molds. It is still soft and moist, and the ripening process continues, so that the cheese will not keep good for a very long time. But while it is in the proper condition it furnishes the educated palate with a flavoring product of great intensity, and most highly relished by the numerous lovers of soft cheeses.

While such is the general method of manufacture of the soft cheeses, it must be recognized that the details of the manufacture differ widely. Differences in the kind of milk used, whether whole milk, skim milk, sheep's milk, goat's milk, etc., differences in the handling of the soft curd, differences in the amount of salting and drying, differences in the temperature and moisture of the 'cheese cellar' all result in the growth of different kinds of molds and bacteria, producing variously flavored products. It is evident, too, that the character of the product will depend upon the abundance and varieties of the plants which furnish the flavor. Unless a dairy is supplied with the proper species of molds and bacteria, it is hopeless to expect the desired results. Here lies the work which the scientist must perform for the further development of the cheese industry.

The second type of cheeses, with which we are more familiar in this country, is the type of hard cheeses. These are not only of denser consistency, but they have commonly a less pronounced taste and odor and are not so suggestive of decomposition. They are, also, commonly made in much larger form, their denser nature making it possible for them to be made in very large sizes. They keep longer and are, therefore, much more generally exported into different countries.

The difference between the hard and soft cheeses, great as it is in the perfected article, is due to quite slight variations in the method of manufacture. The hard cheeses are made from curdled milk, curdled in just the same way as in the making of soft cheeses. But, after the curdling and the cutting up the curd to allow the whey to separate, the curd is broken up into small bits and placed in forms, where it is subjected to heavy pressure. Sometimes, immediately after the cutting of the curd, it is subjected to a moderate heat. For example, the Swiss cheeses are heated to about 110° Fahr. for a short time after cutting up the curd. This heating changes the nature of the curd somewhat and gives it a tougher and more elastic texture. In all the hard cheeses the curd is finally placed in wooden forms and then subjected to pressure, moderate at first, but soon increased until the pressure is quite high. This pressure converts the curd into a very dense, compact mass, and one in which microscopic plants cannot so readily grow.

But the hard cheeses require a ripening to develop the flavor as well as the soft cheeses, and the ripening is a longer and slower process. The pressed cheese is placed in rooms, or caves, or other locality where the temperature is not very variable or where it can, perhaps, be artificially controlled. Here it remains for weeks and frequently for months, during which time it slowly changes its chemical nature as a result of the action of the chemical or organic ferments, and simultaneously acquires the flavors which characterize the perfected product.

It is generally believed that the flavors here, as well as in the soft cheeses, are due to the growth of microscopic plants; but the subject has proved a very difficult one to investigate. Molds play little or no part in ripening the hard cheeses. Indeed, their growth is prevented by salting, oiling and rubbing the surface. But bacteria appears to have, if not the chief share, certainly a large share in the production of the flavors. Experiment has shown that bacteria grow abundantly in the cheese during the ripening; that some species of bacteria can produce in milk flavors similar to those found in the ripened cheese; that treatment which prevents the growth of bacteria prevents also the development of the flavors in the cheese. Further, in the manufacture of the famous Holland cheese (Edam cheese), the cheese-makers have learned that by planting certain species of bacteria in the milk out of which the cheese is to be made, the ripening may be hastened and made more uniform. In Holland about one third of the cheese is made by thus inoculating the milk with 'slimy whey,' which is simply a mass of whey containing in great numbers certain species of bacteria. These facts indicate strongly that the bacteria are agents in this flavor production. But, at the same time, the subject has proved so difficult of investigation that our bacteriologists are as yet by no means satisfied with the results. Indeed, they differ very decidedly in their conclusions. Some believe that the ripening is chiefly due to the same class of bacteria which produce the souring of milk; others think it due to bacteria which produce an alkaline rather than acid reaction; some believe it to be a combination of the two, while others, again, decide that cheese ripening is a long process, involving the action of many species of bacteria and perhaps of molds as well. The difficulty lies in the fact that, since the ripening is a long process, many species of bacteria are found in the cheese at different times. This makes it almost impossible to determine what is the cause of the ripening and what is only incidental.

It will be readily understood that the problem of cheese ripening is one most eagerly studied by bacteriologists. The immense financial interests involved in the discovery of definite methods of handling the manufacture and the ripening of cheese would insure this, entirely independently of any scientific interest. A very large per cent, of cheeses are ruined by improper ripening, and the discovery of methods for preventing this loss would mean the saving of millions of dollars annually. Moreover, many favorite cheeses have hitherto been capable of manufacture only in certain localities, probably because these localities are filled with the peculiar species of micro-organisms needed for their ripening. If it were possible to cultivate the requisite organisms and use them for artificial inoculation, it might be possible to manufacture any type of cheese anywhere. Already it has been found that new cheese factories may sometimes be stocked with the proper microorganisms by rubbing the shelves and vessels with fresh cheeses imported from localities where the desired variety is nominally made. It is evident that immense financial interests may be involved in the proper scientific solution of the micro-organisms for cheese ripening, and the practical application of the facts to cheese making.

As the result of these facts, many bacteriologists are engaged in the study of the problems connected with cheese ripening. Many new discoveries have been made, and various practical suggestions in cheese making have resulted from these researches. But every bacteriologist has been studying a different problem. In Holland some valuable studies of the ripening of Edam cheese have been made, but naturally, the results differ decidedly from those obtained by Swiss bacteriologists in their study of the ripening of Swiss cheeses, inasmuch as the Holland cheese itself is such a different product from that made in Switzerland. The study of cheese ripening in our own country will probably show little agreement with the researches in Europe, since our cheeses differ so much in taste from most of the continental cheeses, although they are not so very unlike the English cheeses. In short, the problems to be solved are as numerous as the varieties of cheese, and each problem has shown itself to be so complex as, thus far, almost to baffle the most patient investigation. It is true that one or two bacteriologists have announced that they have discovered the species of bacteria and molds which produce the ripening of the particular type of cheese that they have been studying, and in some cases cultures of these bacteria have been placed on the market for use in cheese making. In one case, a scientist announces that he has made many thousands of pounds of cheese by means of his artificial cultures and has met with the highest success. But, in general, these cultures have been of problematical value, none of them having, as yet, resulted in the extension of the manufacture of special types of cheeses in localities where it had been hitherto impossible.

As stated before, this country is perhaps more interested in the successful issue of these investigations than any other. Hitherto, Swiss cheeses have been made in Switzerland, Holland cheeses in Holland and all other types of cheeses in their own rather limited localities. This includes hard cheeses as well as soft. If we desire any of these products we are obliged, in the main, to import them. Certain imitations have been produced in this country, it is true; but the imitations are more in shape than in quality. If it were possible, however, for our dairymen to learn a method of making, not inferior imitations of European cheeses, but products actually their equal in flavor and quality, it is certain that an immense market would be speedily opened to them. This condition is probably dependent upon the success of the scientist in solving the problem of regulating the growth of bacteria and molds in the ripening cheese. As fast as the bacteriologist succeeds in showing how the ripening process may be so controlled as to make it possible for our dairymen to produce cheeses similar in character and equal in grade to those of the European market, we may look for the expansion of the industry.

What the future may develop cannot be foretold. The problem is a large one, but the fruits of successful solution are great. Students of dairy bacteriology recognize the possibilities and have in recent years turned their attention quite largely to this subject. From continued experiments and investigations we may confidently expect some practical results, and it is not at all improbable that in a few years at all events, we may see an almost complete revolution in the manufacture of cheeses, especially in such a large country as this, where the possibilities for the development of cheese manufacture are almost unlimited, and where the demand must be as varied as the population.