Page images
PDF
EPUB

age parmesan (grana) est d'habitude très réduite, soit à cause de la température élevée de la cuisson, soit par l'antagonisme des ferments lactiques, si bien que, généralement, on ne doit à ce microorganisme un grand pourcentage d'écart.

L'on peut dire de même des ferments butyriques et des autres anaérobes sporogènes, qu'excepté des cas exceptionnels dûs au mauvais lait ou à une irrationnelle fabrication, j'ai toujours pu constater ou manquants ou présents, mais seulement en très petites quantités dans le fromage grana.

Très fréquemment se sont démontrées certaines fermentations tardives qui causent dans le fromage des trous irréguliers plus ou moins accentués, accompagnés parfois de feuilletage (sfogliatura ou fessurazione) sans cependant causer de variations notables à la saveur et à l'odeur du fromage.

En concluant, la flore microbique normale du fromage grana ne se distingue pas beaucoup de celle des autres fromages à pâte cuite et comme dans ceux-ci, aussi dans le grana, les bactéries qui prédominent et que pour plus longtemps maintiennent dans le fromage leur règne, sont les ferments lactiques et spécialement certaines brèves formes de bâtonnets; quelques-unes avec rapidité, d'autres avec une lente ou aucune coagulation du lait, avec légère ou sans production de gaz; et diverses coccus lesquels développent aussi une action favorable particulièrement en accélérant la maturation du fromage.

['Abstract.]

BACTERIAL CONTENT OF GRANA CHEESE WHILE RIPENING.

GnzO DALLA TORRE, Pli. 1., Experimental Institute of Cheese Making,

Lodi, Italy.

The normal bacteria of grana cheese do not differ very much from those of other cooked curd cheeses, and, as in the latter, the bacteria, which prevail and last longer in cheese are the lactic type, especially certain of the rod forms, some of which cause immediate coagulation of milk, and others slow coagulation or none at all, with only a very slight or no production of gas. Also, there are several cocci which develop a favorable action as they help to hasten the ripening of the cheese.

Concerning certain ferments injurious to cheese, the first ones to be mentioned are the bacteria of the coli-aerogenes group. Their action in grana, however, is greatly reduced, either by the high temperature of cooking or by the counteraction of lactic ferments, so that generally not a large percentage of waste should be attributed to these bacteria. The same may be said of butyric ferments and of other sporeforming anaerobes which, with the exception of rare cases due to bad milk or irrational treatment, are always absent, or, if present, they occur in very small quantities. Much more frequent are some belated fermentations which produce in the cheese very irregular eyes, which are more or less marked and frequently accompanied by crumbling. These, however, do not cause noticeable changes in the taste or odor of the cheese.

THE RIPENING OF CHEESE.

F. W. J. BOEKHOUT, Ph. D., director, bacteriological department, State Agricul

tural Experiment Station, Hoorn, Holland. It was formerly assumed that milk in the udder is sterile, on the ground of Pasteur's contention that all the juices of the body are sterile, and of Lister's researches. It has, however, been found that this view is not right. It is true that the milk is secreted in a sterile condition by the alveoli, but already in the spongy tissue, the milk ducts and the milk bosom, infection takes place with bacteria which are believed to find their way into the milk through the constrictor of the teat and to multiply in the milk so far as it is a favorable medium for them. At the moment the milk leaves the udder in milking it is in some measure infected, but this infection is slight in comparison with that which takes place in obtaining the milk in the way this is done on the modern farm. Bacteria from the skin of the cow, from the hands and the clothes of the milker, from the air, from feces that have got on the udder, from the milking utensils, and so on, cause the bacterial content to reach a very high figure. The bacteria which infect the milk that has just been drawn consist largely of different species of diplococci or monococci, some of which form colored colonies and several of which liquefy gelatin. If a number of these cocci are introduced into sterilized milk it is gradually curdled, a slight acidification taking place and pepton being formed. The fact that these species of bacteria curdle the milk at a low degree of acidity gives rise to the question whether they do not also secrete a rennet-like enzyme, since the increase of the acidity is not such as to be sufficient in itself to cause the curdling. That acid and rennet are formed by some bacteria is well-known; some investigators have observed this in the case of several bacteria.

In addition to these species of cocci, the freshly drawn milk is infected by lactic acid ferments, by which are understood those bacteria which form lactic acid out of milk sugar in such a way that few other decomposition products are formed. They ought therefore to be carefully distinguished from those species of bacteria which when introduced into milk produce, among other things, some lactic acid. There are many such bacteria.

The infection of milk with lactic acid ferments, however, does not take place in such a degree as might be expected from the very general occurrence of these bacteria. Yet these bacteria, which are able to change not only lactose but also glucose, galactose, levulose. maltose, etc., find an excellent nutritive medium in the milk so that they soon outnumber the others.

Conn and Esten found that in milk received in sterile bottles, straight from the cow, there was, after six hours, an increase of the lactic acid ferments, and that after so short a time there were considerable numbers of the Bacterium acidi lactici. After 24 hours, their number often amounted to 90 per cent of all the bacteria in the milk, and sometimes more, two other species of lactic acid ferments having appeared besides. After 60 hours practically the only species left were three kinds of lactic acid bacteria, the other bacteria having died.

Now, if cheese is made from ordinary farm milk, the bacteria are for the greater part held fast by the curdled mass, part of them, about 20 per cent, being left in the whey, but most of them being inclosed in the curd. According as circumstances in regard to food, presence of oxygen, etc., are favorable to them or not, some species of bacteria will form colonies in the curd and others will not. Now the lactic acid bacteria are among those for which the curd is an excellent nutritive medium, and this is no wonder. A fresh cheese contains roughly 50 per cent of water, or 53.5 per cent of whey, if we reckon the solids not fat of the whey at 6.5 per cent; cheese, therefore, offers food in plenty to these ferments, in the form of milk sugar and proteins, to enable them to increase. As a result of this an intense lactic acid fermentation takes place in a cheese when it is under the press, in consequence of which, as in the case of milk, a large number of other species of bacteria die out. As their life is very short these latter species can hardly be said to have any vital influence.

The way in which the lactic acid ferments exercise their destructive influence on the other species of bacteria is not, however, the same in cheese as in milk. În milk the lactic acid bacteria, under ordinary circumstances, supplant the others by the action of the acid they secrete. In cheese other factors come into play. When a liquid containing the nutritive matter necessary for bacteria is inoculated with such microorganisms, these, if the other conditions required for their sustenance are not wanting, will naturally multiply. Their multiplication will not continue indefinitely, however, but will end either through the exhaustion of the medium, if too little nutritive matter is contained in it, or, in the opposite case, through the excretions of the bacteria themselves.

An instance of this is offered by sterile milk when inoculated with lactic acid ferments. In this case a lactic fermentation sets in, which converts the milk sugar into lactic acid. The lactic acid that is formed is unfavorable to the further growth of bacteria and puts a stop to it, in many cases when some 0.5 per cent has been formed. If the lactic acid is neutralized by an easily decomposable base of some metal which is not injurius to the microorganisms, a lactic acid salt is formed and multiplication will start again. Now milk contains different substances which are able in this way to combine with lactic acid, viz., calcium caseinate, calcium phosphate, and casein.

The influence, however, which these neutralizers have on the lactic fermentation, or, in other words, on the quantity of milk sugar which is converted into lactic acid in the milk, can never be very strong, because the quantity of lactose in milk is about 40 to 48 times as great as that of the Ca() that it contains in an insoluble condition. In the milk of Dutch cows the amount of insoluble CaO was found to be 0.1266, 0.1156, and 0.108 grams per 100 cubic centimeters, whereas the lactose amounted to 5 grams. Assuming, for the sake of simplicity, that all the calcium combines with lactic acid, which is not the case, these quantities can, according to the equations,

CH,204+H,O=4C,H,O, and

CaO+2C,H,O,=H,0+(C,H,O, Ca, neutralize only the lactic acid formed out of

0.1266

x0.171=0.3886, 0.353, and 0.330 grams

0.055 of milk sugar, respectively. These quantities amount to only onethirteenth to one-fifteenth of the total quantity of lactose contained in the milk.

If the influence exercised by the insoluble calcium salts in milk is slight, that which they exercise in cheese is of greater significance. The curdling of the milk on the addition of rennet naturally causes all insoluble substances to be precipitated, so that in the curd we find all those neutralizers of which mention was made. As fresh cheese is usually about half curd and half whey, the conditions in it differ from those in milk. While milk sugar predominates in milk, the reverse is the case in cheese. Here the insoluble substances predominate, and very strongly, too, as every kilogram of cheese contains 500 grams of curd against 500 grams of whey or 25 grams of milk sugar.

In the light of these figures it is quite clear that there are such - large quantities of insoluble calcium salts that the greater part of the lactic acid that can be formed out of the 25 grams of lactose can be neutralized by them. Since 22 liters of milk, on an average, are needed to make an Edam cheese, the insoluble calcium compounds contained in that quantity are found in the curd. As was said above. the amount of undissolved calcium in the milk of Dutch cows is 0.1266 grams CaO per 100 cubic centimeters. In this case the curd would therefore contain 220X0.1266 grams=27.85 grams CaO. Now, if we take the conditions least favorable to combination by supposing that this calcium occurs as bicalcium phosphate, Ca H,P,0g, so that only half of it can act as a neutralizer, then only about 14 grams of CaO would be present in this case to combine with the acid formed out of 42 grams of milk sugar. As a small Edam cheese weighs about 2 kilograms, about 21 grams of the 25 grams of milk sugar that the cheese contains per kilogram would, on conversion into lactic acid, be neutralized by the calcium salts alone.

If this is the case when the conditions are taken as unfavorable as possible for the salts to combine with lactic acid, and when the paracasein is left out of consideration, it will be seen that very little free lactic acid will be present in a normal Edam cheese and that the lactic fermentation may be allowed to proceed till all the milk sugar has been converted into lactic acid. T'he speed at which this takes place will, however, depend on the number and the virulence of the lactic acid ferments. It is true that 'these will grow into colonies. but as the nature of the nutritive medium-i. e., the curd-compels them to remain where the mother cell was, different centers will be formed, from which the lactic acid will spread and which the milk sugar will reach by diffusion. The exhaustion of the nutritive me dium with regard to the milk sugar therefore depends on the number and the virulence of the colonies, and consequently of the lactic acid bacteria in the milk.

The greater the number of these organisms the sooner will the exhaustion follow, and as it is desirable that it follow quickly, lactic acid ferments in the form of normal or ropy whey are added to the milk in cheese making. The reason that it is considered desirable from a practical point of view that the lactic fermentation should be rapid is not far to seek. The disappearance of the lactose, the influence of the monocalcium phosphate and the lactic acid, and the combining of the paracasein with lactic acid produce a sour medium, thereby rendering the existence of a very great number of bacteria impossible, owing, on the one hand, to the removal of the milk sugar, which supplies the carbon needed for their sustenance, and, on the other hand, to the production of a sour medium which is inimical to their growth.

While in milk the lactic acid that was formed played the principal rôle in the disappearance of a great number of bacteria, there are other factors at work in cheese, and here the main factor is the disappearance of the lactose. This, however, has consequences which

, are fatal to the real lactic ferments themselves. These also need milk sugar for the supply of carbon and the gradual decrease of this substance renders less favorable the conditions for the life of the lactic ferments until their growth is at length stopped by want of food. The death of these organisms does not result at once, however, but only a transition to a condition of latent life.

This may be shown in a simple manner. If cheeses of different ages are examined by means of whey gelatin—that is to say, in a medium that contains milk sugar-it appears, when the cheese is not too old, that only colonies of lactic ferments are formed. This proves that the other organisms which were to be found in the milk have died, but it does not prove that the lactic ferments are still active, because whey gelatin is a medium which supplies milk sugar to them and thus causes them to become active again and to grow into colonies. If we take a gelatin obtained by making an extract from cheese—that is to say, a nutritive medium which differs as little as possible from cheese—no real lactic ferments are found. Hence, they are not dead in the cheese, but their activity is suspended. On further examination by means of cheese gelatin or cheese bouillon we always find that the bacteria which afterwards appear in Edam cheese are of a kind which might be called facultative ferments. They are distinguished from the real lactic ferments in the cheese, which are diplococci or streptococci, by their rodlike shape and especially by their property of being able to form lactic acid in media which contain milk sugar without depending on lactose for their growth. These species can, therefore, go on developing after the milk sugar has disappeared from the cheese.

The number of colonies of these organisms in the cheese depends, as in the case of the real lactic ferments, on the number of bacteria present in the milk; but, on the whole, their number is very small in comparison with that of the real lactic ferments. As a result of their property of using milk sugar as a source of carbon, these rod-shaped bacteria are also able to form colonies on whey gelatin. That they are very seldom found on this nutritive medium in the case of young cheeses may be accounted for by their smaller number and slower growth having caused them to be supplanted by the real lactic ferments. In examining older cheeses, however, in which the lactic ferments have begun to die off, we find that the rod-shaped facultative lactic ferments, having no longer been hindered in their growth, are the more numerous in the colonies that appear, the older the cheese. Although these organisms also die off, the life of some is very long. In cheeses which were 15 months old they could still be shown to be present by inoculating with a large quantity.

To sum up, the bacteriological process in a cheese is as follows: There is first a preinfection, when bacteria get into the milk as it is drawn; next there is an intense lactic fermentation which stops.

« PreviousContinue »