There is still another method of adding bacteria to cream to insure a more advantageous ripening, which is frequently used, and, being simpler, is in many cases a decided advantage. This method is by the use of what is called a natural starter. A natural starter consists simply of a lot of cream which has been taken from the most favourable source possible—that is, from the cleanest and best dairy, or from the herd producing the best quality of cream—and allowing this cream to stand in a warm place for a couple of days until it becomes sour. The cream will by that time be filled with large numbers of bacteria, and this is then put as a starter into the vat of cream to be ripened. Of course, in the use of this method the butter maker has no control over the kinds of bacteria that will grow in the starter, but it is found, practically, that if the cream is taken from a good source the results are extremely favourable, and there is produced in this way almost always an improvement in the butter.
The use of pure cultures is still quite new, particularly in this country. In the European butter-making countries they have been used for a longer period and have become very much better known. What the future may develop along this line it is difficult to say; but it seems at least probable that as the difficulties in the details are mastered the time will come when starters will be used by our butter makers for their cream ripening, just as yeast is used by housewives for raising bread, or by brewers for fermenting malt. These starters will probably in time be furnished by bacteriologists. Bacteriology, in other words, is offering in the near future to our butter makers a method of controlling the ripening of the cream in such a way as to insure the obtaining of a high and uniform quality of butter, so far, at least, as concerns flavour and aroma.
BACTERIA IN CHEESE.
Cheese ripening.—The third great product of the dairy industry is cheese, and in connection with this product the dairyman is even more dependent upon bacteria than he is in the production of butter. In the manufacture of cheese the casein of the milk is separated from the other products by the use of rennet, and is collected in large masses and pressed, forming the fresh cheese. This cheese is then set aside for several weeks, and sometimes for months, to undergo a process that is known as ripening. During the ripening there are developed in the cheese the peculiar flavours which are characteristic of the completed product. The taste of freshly made cheese is extremely unlike that of the ripened product. While butter made from unripened cream has a pleasant flavour, and one which is in many places particularly enjoyed, there is nowhere a demand for unripened cheese, for the freshly made cheese has a taste that scarce any one regards as pleasant. Indeed, the whole value of the cheese is dependent upon the flavour of the product, and this flavour is developed during the ripening.
The cheese maker finds in the ripening of his cheese the most difficult part of his manufacture. It is indeed a process over which he has very little control. Even when all conditions seem to be correct, when cheese is made in the most careful manner, it not infrequently occurs that the ripening takes place in a manner that is entirely abnormal, and the resulting cheese becomes worthless. The cheese maker has been at an entire loss to understand these irregularities, nor has he possessed any means of removing them. The abnormal ripening that occurs takes on various types. Sometimes the cheese will become extraordinarily porous, filled with large holes which cause the cheese to swell out of proper shape and become worthless. At other times various spots of red or blue appear in the manufactured cheese; while again unpleasant tastes and flavours develop which render the product of no value. Sometimes a considerable portion of the product of the cheese factory undergoes such irregular ripening, and the product for a long time will thus be worthless. If some means could be discovered of removing these irregularities it would be a great boon to the cheese manufacturer; and very many attempts have been made in one way or another to furnish the cheese maker with some details in the manufacture which will enable him in a measure to control the ripening.
The ripening of the cheese has been subjected to a large amount of study on the part of bacteriologists who have been interested in dairy products. That the ripening of cheese is the result of bacterial growth therein appears to be probable from a priori grounds. Like the ripening of cream, it is a process that occurs somewhat slowly. It is a chemical change which is accompanied by the destruction of proteid matter; it takes place best at certain temperatures, and temperatures which we know are favourable to the growth of micro-organisms, all of which phenomena suggest to us the action of bacteria. Moreover, the flavours and the tastes that arise have a decided resemblance in many cases to the decomposition products of bacteria, strikingly so in Limburger cheese. When we come to study the matter of cheese ripening carefully we learn beyond question that this a priori conclusion is correct. The ripening of any cheese is dependent upon several different factors. The method of preparation, the amount of water left in the curd, the temperature of ripening, and other miscellaneous factors connected with the mechanical process of cheese manufacture, affect its character. But, in addition to all these factors, there is undoubtedly another one, and that is the number and the character of the bacteria that chance to be in the curd when the cheese is made. While it is found that cheeses which are treated by different processes will ripen in a different manner, it is also found that two cheeses which have been made under similar conditions and treated in identically the same way may also ripen in a different manner, so that the resulting flavour will vary. The variations between cheeses thus made may be slight or they may be considerable, but variations certainly do occur. Every one knows the great difference in flavours of different cheeses, and these flavours are due in considerable measure to factors other than the simple mechanical process of making the cheese. The general similarity of the whole process to a bacterial fermentation leads us to believe at the outset that some of the differences in character are due to different kinds of bacteria that multiply in the cheese and produce decomposition therein.
When the matter comes to be studied by bacteriology, the demonstration of this position becomes easy. That the ripening of cheese is due to growth of bacteria is very easily proved by manufacturing cheeses from milk which is deprived of bacteria. For instance, cheeses have been made from milk that has been either sterilized or pasteurized—which processes destroy most of the bacteria therein—and, treated otherwise in a normal manner, are set aside to ripen. These cheeses do NOT ripen, but remain for months with practically the same taste that they had originally. In other experiments the cheese has been treated with a small amount of disinfective, which is sufficient to prevent bacteria from growing, and again ripening is found to be absolutely prevented. Furthermore, if the cheese under ordinary conditions is studied during the ripening process, it is found that bacteria are growing during the whole time. These facts all taken together plainly prove that the ripening of cheese is a fermentation due to bacteria. It will be noticed, however, that the conditions in the cheese are not favourable for very rapid bacterial growth. It is true that there is plenty of food in the cheese for bacterial life, but the cheese is not very moist; it is extremely dense, being subjected in all cases to more or less pressure. The penetration of oxygen into the centre of the mass must be extremely slight. The density, the lack of a great amount of moisture, and the lack of oxygen furnish conditions in which bacteria will not grow very rapidly. The conditions are far less favourable than those of ripening cream, and the bacteria do not grow with anything like the rapidity that they grow in cream. Indeed, the growth of these organisms during the ripening is extremely slow compared to the possibilities of bacterial growth that we have already noticed. Nevertheless, the bacteria do multiply in the cheese, and as the ripening goes on they become more and more abundant, although the number fluctuates, rising and falling under different conditions.
When the attempt is made to determine the relation of the different kinds of ripening to different kinds of bacteria, it has thus far met with extremely little success. That different flavours are due to the ripening produced by different kinds of bacteria would appear to be almost certain when we remember, as we have already noticed, the different kinds of decomposition produced by different species of bacteria. It would seem, moreover, that it ought not to be very difficult to separate from the ripened cheese the bacteria which are present, and thus obtain the kind of bacteria necessary to produce the desired ripening. But for some reason this does not prove to be so easy in practice as it seems to be in theory. Many different species of bacteria have been separated from cheeses. One bacteriologist, studying several cheeses, separated about eighty different species therefrom, and others have found perhaps as many more from different sources. Moreover, experiments have been made with a considerable number of these different kinds of bacteria to determine whether they are capable of producing normal ripening. These experiments consist of making cheese out of milk that has been deprived of its bacteria, and which has been inoculated with large quantities of the species in question. Hitherto these experiments have not been very satisfactory. In some cases the cheese appears to ripen scarcely at all; in other cases the ripening occurs, but the resulting cheese is of a peculiar character, entirely unlike the cheese that it is desired to imitate. There have been one or two experiments in recent times that give a little more promise of success than the earlier ones, for a few species of bacteria have been used in ripening with what the authors have thought to be promising success. The cheese made from the milk artificially inoculated with these species ripens in a satisfactory manner and gives some of the character desired, though up to the present time in no case has the typical normal ripening been produced in any of these experiments.
But these experiments have demonstrated beyond question that the abnormal ripening which is common in cheese factories is due to the presence of undesirable species of bacteria in the milk. Many of the experiments in making cheeses by means of artificial cultures of bacteria have resulted in decidedly abnormal cheeses. Many of the cheeses thus manufactured have shown imperfections in ripening which are identical with those actually occurring in the cheese factory. Several different species of bacteria have been found which, when artificially used thus for ripening cheese, will give rise to the porosity and the abnormal swelling of the cheese already referred to (Fig. 24). Others produced bad tastes and flavours, and enough has been done in this line to demonstrate beyond peradventure that the abnormal ripening of cheese is due primarily to the growth of improper species therein. Quite a long list of species of bacteria which produce abnormal ripening have been isolated from cheeses, and have been studied and experimented with by bacteriologists. As a result of this study of abnormal ripening, there has been suggested a method of partially controlling these—remedying them. The method consists simply in testing the fermenting qualities of the milk used. A small sample of milk from different dairies is allowed to stand in the cheese factory by itself until it undergoes its normal souring. If the fermentation or souring that thus occurs is of a normal character, the milk is regarded as proper for cheese making. But if the fermentation that occurs in any particular sample of milk is unusual; if an extraordinary amount of gas bubbles are produced, or if unpleasant smells and tastes arise, the sample is regarded as unfavourable for cheese making, and as likely to produce abnormal ripening in the cheeses. Milk from this source would therefore be excluded from the milk that is to be used in cheese making. This, of course, is a tentative and an unsatisfactory method of controlling the ripening, and yet it is one of some practical value to cheese makers. It is the only method that has yet been suggested of controlling the ripening.
Our bacteriologists, of course, are quite confident that in the future more practical results will be obtained along this line than in the past. If it is true that cheeses are ripened by bacteria; if it is true that different qualities in the cheese are due to the growth of different species of bacteria during the ripening, it would seem to be possible to obtain the proper kind of bacteria and to furnish them to the cheese maker for artificially inoculating his cheese, just as it has been possible to furnish artificially cultivated yeasts to the brewer, and as it has become possible to furnish artificially cultivated bacteria to the butter maker. We must, however, recognise this to be a matter for the future. Up to the present time no practical results along the lines of bacteria have been obtained which our cheese manufacturers can make use of in the way of controlling with any accuracy this process of cheese ripening.