Outlines of Dairy Bacteriology Part 16

"Mechanical" holes due to lack of acid development; _P_, same cheese four days later, mechanical holes distended by development of gas.]

~Physical changes in ripening cheese.~ When a green cheese is taken from the press, the curd is tough, firm, but elastic. It has no value as a food product for immediate use, because it lacks a desirable flavor and is not readily digestible. It is nothing but precipitated casein and fat. In a short time, a deep-seated change occurs. Physically this change is demonstrated in the modification that the curd undergoes.

Gradually it breaks down and becomes plastic, the elastic, tough curd being changed into a softened ma.s.s. This change in texture of the cheese is also accompanied by a marked change in flavor. The green cheese has no distinctively cheese flavor, but in course of time, with the gradual change of texture, the peculiar flavor incident to ripe cheese is developed.

The characteristic texture and flavor are susceptible of considerable modification that is induced not only by variation in methods of manufacture, but by the conditions under which the cheese are cured. The amount of moisture incorporated with the curd materially affects the physical appearance of the cheese, and the rate of change in the same.

The ripening temperature, likewise the moisture content of the surrounding air, also exerts a marked influence on the physical properties of the cheese. To some extent the action of these forces is purely physical, as in the gradual loss by drying, but in other respects they are a.s.sociated with chemical transformations.

~Chemical changes in ripening cheese.~ Coincident with the physical breaking down of the curd comes a change in the chemical nature of the casein. The hitherto insoluble casein is gradually transformed into soluble nitrogenous substances (_caseone_ of Duclaux, or _caseogluten_ of Weigmann). This chemical phenomenon is a breaking-down process that is a.n.a.logous to the peptonization of proteids, although in addition to the peptones and alb.u.moses characteristic of peptic digestion, amido-acids and ammonia are to be found. The quant.i.ty of these lower products increases with the age of the cheese.

The chemical reaction of cheese is normally acid to phenolphthalein, although there is generally no free acid, as shown by Congo red, the lactic acid being converted into salts as fast as formed. In very old cheese, undergoing putrefactive changes, especially on the outside, an alkaline reaction may be present, due to the formation of free ammonia.

The changes that occur in a ripening cheese are for the most part confined to the proteids. According to most investigators the fat remains practically unchanged, although the researches of Weigmann and Backe[185] show that fatty acids are formed from the fat. In the green cheese considerable milk-sugar is present, but, as a result of the fermentation that occurs, this is rapidly converted into acid products.

~Bacterial flora of cheese.~ It might naturally be expected that the green cheese, fresh from the press, would contain practically the same kind of bacteria that are in the milk, but a study of cheese shows a peculiar change in the character of the flora. In the first place, fresh cottage cheese, made by the coagulation of the casein through the action of acid, has a more diversified flora than cheese made with rennet, for the reason, as given by Lafar,[186] that the fermentative process is farther advanced.

When different varieties of cheese are made from milk in the same locality, the germ content of even the ripened product has a marked similarity, as is ill.u.s.trated by Adametz's work[187] on Emmenthaler or Swiss hard cheese, and Schweitzer Hauskase, a soft variety. Of the nine species of bacilli and cocci found in mature Emmenthaler, eight of them were also present in ripened Hauskase.

Different investigators have studied the bacterial flora of various kinds of cheese, but as yet little comparative systematic work has been done. Freudenreich[188] has determined the character and number of bacteria in Emmenthaler cheese, and Russell[189] the same for cheddar cheese. The same general law has also been noted in Canadian[190] and English[191] cheese. At first a marked decrease in numbers is usually noted, lasting for a day or two. This is followed by an enormous increase, caused by the rapid growth of the lactic-acid type. The development may reach scores of millions and often over a hundred million organisms per gram. Synchronous with this increase, the peptonizing and gas-producing bacteria gradually disappear. This rapid development, which lasts only for a few weeks, is followed by a general decline.

In the ripening of cheese a question arises as to whether the process goes on throughout the entire ma.s.s of cheese, or whether it is more active at or near the surface. In the case of many of the soft cheese, such as Brie and limburger, bacterial and mold development is exceedingly active on the exterior, and the enzyms secreted by these organisms diffuse toward the interior. That such a condition occurs in the hard type of cheese made with rennet is extremely improbable. Most observers agree that in this type of cheese the ripening progresses throughout the entire ma.s.s, although Adametz opposes this view and considers that in Emmenthaler cheese the development of the specific aroma-producing organism occurs in the superficial layers. Jensen has shown, however, that the greatest amount of soluble nitrogenous products are to be found in the innermost part of the cheese, a condition that is not reconcilable with the view that the most active ripening is on the exterior.[192]

The course of development of bacteria in cheddar cheese is materially influenced by the ripening temperature. In cheese ripened at relatively low temperatures (50-55 F.),[193] a high germ content is maintained for a much longer period of time than at higher temperatures. Under these conditions the lactic-acid type continues in the ascendancy as usual. In cheese cured at high temperatures (80-86 F.) the number of organisms is greatly diminished, and they fail to persist in appreciable numbers for as long a time as in cheese cured at temperatures more frequently employed.

~Influence of temperature on curing.~ Temperature exerts a most potent influence on the quality of the cheese, as determined not only by the rate of ripening but the nature of the process itself. Much of the poor quality of cheese is attributable to the effect of improper curing conditions. Probably in the initial stage of this industry cheese were allowed to ripen without any sort of control, with the inevitable result that during the summer months the temperature generally fluctuated so much as to impair seriously the quality. The effect of high temperatures (70 F. and above) is to produce a rapid curing, and, therefore, a short lived cheese; also a sharp, strong flavor, and generally a more or less open texture. Unless the cheese is made from the best quality of milk, it is very apt to undergo abnormal fermentations, more especially those of a ga.s.sy character.

[Ill.u.s.tration: FIG. 31. Influence of curing temperature on texture of cheese. Upper row ripened eight months at 60 F.; lower row at 40 F.]

Where cheese is ripened at low temperatures, ranging from 50 F. down to nearly the freezing temperatures, it is found that the quality is greatly improved.[194] Such cheese are thoroughly broken down from a physical point of view even though they may not show such a high per cent of soluble nitrogenous products. They have an excellent texture, generally solid and firm, free from all tendency to openness; and, moreover, their flavor is clean and entirely devoid of the sharp, undesirable tang that so frequently appears in old cheese. The keeping quality of such cheese is much superior to the ordinary product. The introduction of this new system of cheese-curing promises much from a practical point of view, and undoubtedly a more complete study of the subject from a scientific point of view will aid materially in unraveling some of the problems as to flavor production.

~Theories of cheese curing.~ Within the last few years considerable study has been given the subject of cheese curing or ripening, in order to explain how this physical and chemical transformation is brought about.

Much of the misconception that has arisen relative to the cause of cheese ripening comes from a confusion of terms. In the ordinary use of the word, ripening or curing of cheese is intended to signify the sum total of all the changes that result in converting the green product as it comes from the press into the edible substance that is known as cured cheese. As previously shown, the most marked chemical transformation that occurs is that which has to do with the peptonization or breaking down of the casein. It is true that under ordinary conditions this decomposition process is also accompanied with the formation of certain flavor-producing substances, more or less aromatic in character; but it by no means follows that these two processes are necessarily due to the same cause. The majority of investigators have failed to consider these two questions of casein decomposition and flavor as independent, or at least as not necessarily related. They are undoubtedly closely bound together, but it will be shown later that the problems are quite different and possibly susceptible of more thorough understanding when considered separately.

In the earlier theories of cheese ripening it was thought to be purely a chemical change, but, with the growth of bacteriological science, evidence was forthcoming that seemed to indicate that the activity of organisms entered into the problem. Schaffer[195] showed that if milk was boiled and made into cheese, the casein failed to break down.

Adametz[196] added to green cheese various disinfectants, as creolin and thymol, and found that this practically stopped the curing process. From these experiments he drew the conclusion that bacteria must be the cause of the change, because these organisms were killed; but when it is considered that such treatment would also destroy the activity of enzyms as well as vital ferments, it is evident that these experiments were quite indecisive.

A determination of the nature of the by-products found in maturing cheese indicates that the general character of the ripening change is a peptonization or digestion of the casein.

Until recently the most widely accepted views relating to the cause of this change have been those which ascribed the transformation to the activity of micro-organisms, although concerning the nature of these organisms there has been no unanimity of opinion. The overwhelming development of bacteria in all cheeses naturally gave support to this view; and such experiments as detailed above strengthened the idea that the casein transformation could not occur where these ferment organisms were destroyed.

The very nature of the changes produced in the casein signified that to take part in this process any organism must possess the property of dissolving the proteid molecule, casein, and forming therefrom by-products that are most generally found in other digestive or peptonizing changes of this cla.s.s.

~Digestive bacterial theory.~ The first theory propounded was that of Duclaux,[197] who in 1887 advanced the idea that this change was due to that type of bacteria which is able to liquefy gelatin, peptonize milk, and cause a hydrolytic change in proteids. To this widely-spread group that he found in cheese, he gave the generic name _Tyrothrix_ (cheese hairs). According to him, these organisms do not function directly as ripening agents, but they secrete an enzym or unorganized ferment to which he applies the name _casease_. This ferment acts upon the casein of milk, converting it into a soluble product known as _caseone_. These organisms are found in normal milk, and if they function as casein transformers, one would naturally expect them to be present, at least frequently, if not predominating in the ripening cheese; but such is not the case. In typical cheddar or Swiss cheese, they rapidly disappear (p.

168), although in the moister, softer varieties, they persist for considerable periods of time. According to Freudenreich, even where these organisms are added in large numbers to the curd, they soon perish, an observation that is not regarded as correct by the later adherents to the digestive bacterial theory, as Adametz and Winkler.

Duclaux's experiments were made with liquid media for isolation purposes, and his work, therefore, cannot be regarded as satisfactory as that carried out with more modern technical methods. Recently this theory has been revived by Adametz,[198] who claims to have found in Emmenthaler cheese a digesting species, one of the Tyrothrix type, which is capable of peptonizing the casein and at the same time producing the characteristic flavor of this cla.s.s of cheese. This organism, called by him _Bacillus n.o.bilis_, the Edelpilz of Emmenthaler cheese, has been subjected to comparative experiments, and in the cheese made with pure cultures of this germ better results are claimed to have been secured.

Sufficient experiments have not as yet been reported by other investigators to warrant the acceptance of the claims made relative to the effect of this organism.

~Lactic-acid bacterial theory.~ It has already been shown that the lactic-acid bacteria seems to find in the green cheese the optimum conditions of development; that they increase enormously in numbers for a short period, and then finally decline. This marked development, coincident with the breaking down of the casein, has led to the view which has been so ably expounded by Freudenreich[199] that this type of bacterial action is concerned in the ripening of cheese. This group of bacteria is, under ordinary conditions, unable to liquefy gelatin, or digest milk, or, in fact, to exert, under ordinary conditions, any proteolytic or peptonizing properties. This has been the stumbling-block to the acceptance of this hypothesis, as an explanation of the breaking down of the casein. Freudenreich has recently carried on experiments which he believes solve the problem. By growing cultures of these organisms in milk, to which sterile, freshly precipitated chalk had been added, he was able to prolong the development of bacteria for a considerable period of time, and as a result finds that an appreciable part of the casein is digested; but this action is so slow compared with what normally occurs in a cheese, that exception may well be taken to this type of experiment alone. Weigmann[200] inclines to the view that the lactic-acid bacteria are not the true cause of the peptonizing process, but that their development prepares the soil, as it were, for those forms that are more directly concerned in the peptonizing process.

This they do by developing an acid substratum that renders possible the more luxuriant growth of the aroma-producing species. According to Gorini,[201] certain of the Tyrothrix forms function at high temperatures as lactic acid producing bacteria, while at lower temperatures they act as peptonizers. On this basis he seeks to reconcile the discrepancies that appear in the experiments of other investigators.

~Digestive milk enzym theory.~ In 1897 Babc.o.c.k and the writer[202] showed that milk underwent digestive changes spontaneously when bacterial activity was suspended by the addition of such anaesthetics as ether, chloroform and benzol. The chemical nature of the by-products produced by this auto-digestion of milk resembles quite closely those found in ripened cheese, except that ammonia is not produced as is the case in old cheese. The cause of the decomposition of the casein, they found to be due to the action of a milk enzym which is inherent to the milk itself. This digestive ferment may be separated from fresh milk by concentrating centrifuge slime extracts by the usual physiological reagents. This ferment, called by them _galactase_, on account of its origin in milk, is a proteolytic enzym of the tryptic type. Its activity is destroyed by strong chemicals such as formaldehyde, corrosive sublimate, also when heated to 175 F. or above. When such extracts are added to boiled milk, the digestive process is started anew, and the by-products produced are very similar to those noted in a normal cheese.

Jensen[203] has also shown that the addition of pancreatic extracts to cheese accelerated the formation of soluble nitrogenous products.

The action of galactase in milk and cheese has been confirmed by Freudenreich[204] and Jensen,[205] as well as by American investigators, and this enzym is now generally accepted as one of the factors concerned in the decomposition of the casein. Freudenreich believes it is able to change casein into alb.u.mose and peptones, but that the lactic-acid bacteria are chiefly responsible for the further decomposition of the nitrogen to amid form.

Failure before to recognize the presence of galactase in milk is attributable to the fact that all attempts to secure sterile milk had been made by heating the same, in which case galactase was necessarily destroyed. A brief exposure at 176 F. is sufficient to destroy its activity, and even an exposure at lower temperatures weakens its action considerably, especially if the reaction of the medium is acid. This undoubtedly explains the contradictory results obtained in the ripening of cheese from pasteurized milk, such cheese occasionally breaking down in an abnormal manner.

The results mentioned on page 172, in which cheese failed to ripen when treated with disinfectants,--experiments which were supposed at that time to be the foundation of the bacterial theory of casein digestion--are now explicable on an entirely different basis. In these cases the casein was not peptonized, because these strong disinfectants destroyed the activity of the enzyms as well as the bacteria.

Another important factor in the breaking down of the casein is the _pepsin_ in the rennet extract. The digestive influence of this agent was first demonstrated for cheddar cheese by Babc.o.c.k, Russell and Vivian,[206] and simultaneously, although independently, by Jensen[207]

in Emmenthaler cheese. In this digestive action, only alb.u.moses and higher peptones are produced. The activity of pepsin does not become manifest until there is about 0.3 per cent. acid which is approximately the amount developed in the cheddar process. These two factors undoubtedly account for by far the larger proportion of the changes in the casein; and yet, the formation of ammonia in well ripened cheese is not accounted for by these factors. This by-product is the main end product of proteid digestion by the liquefying bacteria but their apparent infrequency in cheese makes it difficult to understand how they can function prominently in the change, unless the small quant.i.ty of digestive enzyms excreted by them in their growth in milk is capable of continuing its action until a c.u.mulative effect is obtained. Although much light has been thrown on this question by the researches of the last few years, the matter is far from being satisfactorily settled at the present time and the subject needs much more critical work. If liquefying bacteria abound in the milk, doubtless they exert some action, but the role of bacteria is doubtless much greater in the production of flavor than in the decomposition of the curd.

~Conditions determining quality.~ In determining the quality of cheese, several factors are to be taken into consideration. First and foremost is the flavor, which determines more than anything else the value of the product. This should be mild and pleasant, although with age the intensity of the same generally increases but at no time should it have any bitter, sour, or otherwise undesirable taste or aroma. Texture registers more accurately the physical nature of the ripening. The cheese should not be curdy and harsh, but should yield quite readily to pressure under the thumb, becoming on manipulation waxy and plastic instead of crumbly or mealy. Body refers to the openness or closeness of the curd particles, a close, compact ma.s.s being most desirable. The color of cheese should be even, not wavy, streaked or bleached.

For a cheese to possess all of these characteristics in an optimum degree is to be perfect in every respect--a condition that is rarely reached.

So many factors influence this condition that the problem of making a perfect cheese becomes exceedingly difficult. Not only must the quality of the milk--the raw material to be used in the manufacture--be perfectly satisfactory, but the factory management while the curds are in the vat demands great skill and careful attention; and finally, the long period of curing in which variation in temperature or moisture conditions may seriously affect the quality,--all of these stages, more or less critical, must be successfully gone through, before the product reaches its highest state of development.

It is of course true that many phases of this complex series of processes have no direct relation to bacteria, yet it frequently happens that the result attained is influenced at some preceding stage by the action of bacteria in one way or another. Thus the influence of the acidity developed in the curds is felt throughout the whole life of the cheese, an over-development of lactic-acid bacteria producing a sour condition that leaves its impress not only on flavor but texture. An insufficient development of acid fails to soften the curd-particles so as to permit of close matting, the consequence being that the body of the cheese remains loose and open, a condition favorable to the development of gas-generating organisms.

~Production of flavor.~ The importance of flavor as determining the quality of cheese makes it imperative that the nature of the substances that confer on cheese its peculiar aromatic qualities and taste be thoroughly understood. It is to be regretted that the results obtained so far are not more satisfactory, for improvement in technique is hardly to be expected until the reason for the process is thoroughly understood.

The view that is most generally accepted is that this most important phase of cheese curing is dependent upon bacterial activity, but the organisms that are concerned in this process have not as yet been satisfactorily determined. In a number of cases, different species of bacteria have been separated from milk and cheese that have the power of producing aromatic compounds that resemble, in some cases, the peculiar flavors and odors that characterize some of the foreign kinds of cheese; but an introduction of these into curd has not resulted in the production of the peculiar variety, even though the methods of manufacture and curing were closely followed. The similarity in germ content in different varieties of cheese made in the same locality has perhaps a bearing on this question of flavor as related to bacteria. Of the nine different species of bacteria found in Emmenthaler cheese by Adametz, eight of them were also present in ripened Hauskase. If specific flavors are solely the result of specific bacterial action, it might naturally be expected that the character of the flora would differ.

Some suggestive experiments were made by Babc.o.c.k and Russell on the question of flavor as related to bacterial growth, by changing the nature of the environment in cheese by washing the curds on the racks with warm water. In this way the sugar and most of the ash were removed.

Under such conditions the character of the bacterial flora was materially modified. While the liquefying type of bacteria was very spa.r.s.e in normal cheddar, they developed luxuriantly in the washed cheese. The flavor at the same time was markedly affected. The control cheddar was of good quality, while that made from the washed curds was decidedly off, and in the course of ripening became vile. It may be these two results are simply coincidences, but other data[208] bear out the view that the flavor was to some extent related to the nature of the bacteria developing in the cheese. This was strengthened materially by adding different sugars to washed curds, in which case it was found that the flavor was much improved, while the more normal lactic-acid type of bacteria again became predominant.

~Ripening of moldy cheese.~ In a number of foreign cheeses, the peculiar flavor obtained is in part due to the action of various fungi which grow in the cheese, and there produce certain by-products that flavor the cheese. Among the most important of these are the Roquefort cheese of France, Stilton of England, and Gorgonzola of Italy.

Roquefort cheese is made from goat's or cow's milk, and in order to introduce the desired mold, which is the ordinary bread-mold, _Penicillium glauc.u.m_, carefully-prepared moldy bread-crumbs are added to the curd.

At ordinary temperatures this organism develops too rapidly, so that the cheese to ripen properly must be kept at a low temperature. The town of Roquefort is situated in a limestone country, in a region full of caves, and it is in these natural caves that most of the ripening is done. These caverns are always very moist and have a temperature ranging from 35 to 44 F., so that the growth of the fungus is r.e.t.a.r.ded considerably. The spread of the mold throughout the ripening ma.s.s is also a.s.sisted in a mechanical way. The partially-matured cheese are run through a machine that p.r.i.c.ks them full of small holes. These slender ca.n.a.ls allow the mold organism to penetrate the whole ma.s.s more thoroughly, the moldy straw matting upon which the ripening cheese are placed helping to furnish an abundant seeding of the desired germ.

When new factories are constructed it is of advantage to introduce this necessary germ in quant.i.ties, and the practice is sometimes followed of rubbing the walls and cellars of the new location with material taken from the old established factory. In this custom, developed in purely an empirical manner, is to be seen a striking ill.u.s.tration of a bacteriological process crudely carried out.

In the Stilton cheese, one of the highly prized moldy cheeses of England, the desired mold fungus is introduced into the green cheese by exchanging plugs taken with a cheese trier from a ripe Stilton.

~Ripening of soft cheese.~ The type of ripening which takes place in the soft cheeses is materially different from that which occurs in the hard type. The peptonizing action does not go on uniformly throughout the cheese, but is hastened by the development of molds and bacteria on the outside that exert a solvent action on the casein. For this reason, soft cheeses are usually made up in small sizes, so that this action may be hastened. The organisms that take part in this process are those that are able to form enzyms (similar in their action to trypsin, galactase, etc.), and these soluble ferments gradually diffuse from the outside through the cheese.

Most of these peptonizing bacteria are hindered in their growth by the presence of lactic acid, so that in many cases the appearance of the digesting organisms on the surface is delayed until the acidity of the ma.s.s is reduced to the proper point by the development of other organisms, princ.i.p.ally molds, which prefer an acid substratum for their growth.

In Brie cheese a blue coating of mold develops on the surface. In the course of a few weeks, a white felting appears which later changes to red. This slimy coat below the mold layer is made up of diverse species of bacteria and fungi that are able to grow after the acid is reduced by the blue mold. The organisms in the red slimy coat act upon the casein, producing an alkaline reaction that is unfavorable to the growth of the blue mold. Two sets of organisms are, therefore essential in the ripening process, one preparing the soil for the ferment that later produces the requisite ripening changes. As ordinarily carried on, the process is an empirical one, and if the red coat does not develop as expected, the maker resorts to all kinds of devices to bring out the desired ferment. The appearance of the right form is dependent, however, upon the proper reaction of the cheese, and if this is not suitable, the wished-for growth will not appear.

INFLUENCE OF BACTERIA IN ABNORMAL CHEESE PROCESSES.

The reason why cheese is more subject to abnormal fermentation than b.u.t.ter is because its high nitrogen content favors the continued development of bacteria for some time after it is made. It must be borne in mind, in considering the more important of these changes, that not all defective conditions in cheese are attributable to the influence of living organisms. Troubles frequently arise from errors in manufacturing details, as too prolonged cooking of curds, too high heating, or the development of insufficient or too much acid. Then again, the production of undesirable flavors or impairment in texture may arise from imperfect curing conditions.