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 class of cheese. This organism, called by him Bacillus nobilis, 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 Babcock 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 deg. 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 albumose 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 deg. 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 Babcock, Russell and Vivian,[206] and simultaneously, although independently, by Jensen[207] in Emmenthaler cheese. In this digestive action, only albumoses 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 quantity of digestive enzyms excreted by them in their growth in milk is capable of continuing its action until a cumulative 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 mass 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 Hauskaese. 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 Babcock 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 sparse 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 glaucum, 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 deg. to 44 deg. F., so that the growth of the fungus is retarded considerably. The spread of the mold throughout the ripening mass is also assisted in a mechanical way. The partially-matured cheese are run through a machine that pricks them full of small holes. These slender canals allow the mold organism to penetrate the whole mass 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 quantities, 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 illustration 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 mass is reduced to the proper point by the development of other organisms, principally 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 butter 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.

Our knowledge regarding the exact nature of these indefinite faults is as yet too inadequate to enable many of these undesirable conditions to be traced to their proper source; but in many cases the taints observed in a factory are due to the abnormal development of certain bacteria, capable of evolving unpleasant or even putrid odors. Most of them are seeded in the milk before it comes to the factory and are due to careless manipulation of the milk while it is still on the farm. Others gain access to the milk in the factory, owing to unclean conditions of one sort or another. Sometimes the cheese-maker is able to overcome these taints by vigorous treatment, but often they pass on into the cheese, only to detract from the market value of the product. Most frequently these "off" flavors appear in cheese that are cured at too high temperatures, say above 65 deg. F.

"Gassy" fermentations in cheese. One of the worst and at the same time most common troubles in cheese-making is where the cheese undergoes a fermentation marked by the evolution of gas. The presence of gas is recognized by the appearance either of spherical or lens-shaped holes of various sizes in the green cheese; often they appear in the curd before it is put to press. Usually in this condition the curds look as if they had been punctured with a pin, and are known as "pin holey" curds. Where the gas holes are larger, they are known as "Swiss holes" from their resemblance to the normal holes in the Swiss product. If the development of gas is abundant, these holes are restricted in size. Often the formation of gas may be so intense as to cause the curds to float on the surface of the whey before they are removed. Such curds are known as "floaters" or "bloaters."

If "gassy" curds are put to press, the abnormal fermentation may continue. The further production of gas causes the green cheese to "huff" or swell, until it may be considerably distorted as in Fig. 33. In such cases the texture of the cheese is greatly injured, and the flavor is generally impaired.



Such abnormal changes may occur at any season of the year, but the trouble is most common in summer, especially in the latter part.

This defect is less likely to occur in cheese that is well cheddared than in sweet curd cheese. When acidity is produced, these gassy fermentations are checked, and in good cheddar the body is so close and firm as not readily to permit of gaseous changes.

In Swiss cheese, which is essentially a sweet curd cheese, these fermentations are very troublesome. Where large holes are formed in abundance (blaehen), the trouble reaches its maximum. If the gas holes are very numerous and therefore small it is called a "nissler." Sometimes the normal "eyes" are even wanting when it is said to be "blind" or a "glaesler."



One method of procedure which is likely to cause trouble in Swiss factories is often produced by the use of sour, fermented whey in which to soak the natural rennets. Freudenreich and Steinegger[209] have shown that a much more uniform quality of cheese can be made with rennet extract if it is prepared with a starter made from a pure lactic ferment.

The cause of the difficulty has long been charged to various sources, such as a lack of aeration, improper feeding, retention of animal gases, etc., but in all these cases it was nothing more than a surmise. Very often the milk does not betray any visible symptom of fermentation when received, and the trouble is not to be recognized until the process of cheese-making is well advanced.

Studies from a biological standpoint have, however, thrown much light on this troublesome problem; and it is now known that the formation of gas, either in the curd or after it has been put to press, is due entirely to the breaking down of certain elements, such as the sugar of milk, due to the influence of various living germs. This trouble is, then, a type fermentation, and is, therefore, much more widely distributed than it would be if it was caused by a single specific organism. These gas-producing organisms are to be found, sparingly at least, in almost all milks, but are normally held in check by the ordinary lactic species. Among them are a large number of the bacteria, although yeasts and allied germs are often present and are likewise able to set up fermentative changes of this sort. In these cases the milk-sugar is decomposed in such a way as to give off CO_{2} and H, and in some cases, alcohol. Russell and Hastings[210] found a lactose-splitting yeast in a severe outbreak of gassy cheese in a Swiss factory. In this case the gas did not develop until the cheese were a few weeks old. In severe cases the cheese actually cracked to pieces.

According to Guillebeau, a close relation exists between those germs that are able to produce an infectious inflammation (mastitis) in the udder of the cow and some forms capable of gas evolution.

If pure cultures of these gas-producing bacteria are added to perfectly sweet milk, it is possible to artificially produce the conditions in cheese that so frequently appear in practice.

Treatment of "pin-holey" curds. When this type of fermentation appears during the manufacture of the cheese, the maker can control it in part within certain limits. These methods of treatment are, as a rule, purely mechanical, as when the curds are piled and turned, and subsequently ground in a curd mill. After the gas has been forced out, the curds are then put to press and the whole mats into a compact mass.

Another method of treatment based upon bacteriological principles is the addition of a starter to induce the formation of acid. Where acid is developed as a result of the growth of the lactic-acid bacteria, the gas-producing species do not readily thrive. Another reason why acid aids in repressing the development of gas is that the curd particles are partially softened or digested by the action of the acid. This causes them to mat together more closely, and there is not left in the cheese the irregular mechanical openings in which the developing gas may find lodgment.

Another method that is also useful with these curds is to employ salt. This represses gaseous fermentations, and the use of more salt than usual in making the cheese will very often restrain the production of gas. Tendency to form gas in Edam cheese is controlled by the addition of a starter prepared from slimy whey (lange wei) which is caused by the development of an acid-forming organism.

Some have recommended the custom of washing the curds to remove the whey and the gas-producing bacteria contained therein. Care must be taken not to carry this too far, for the removal of the sugar permits taint-producing organisms to thrive.[211]

The temperature at which the cheese is cured also materially affects the development of gas. At high curing temperatures, gas-producing organisms develop rapidly; therefore more trouble is experienced in summer than at other seasons.

If milks which are prone to undergo "gassy" development are excluded from the general supply, it would be possible to eliminate the source of the entire trouble. To aid in the early recognition of such milks that are not apparently affected when brought to the factory, fermentation or curd tests (p. 76) are of great value. The use of this test in the hands of the factory operator often enables him to detect the exact source of the trouble, which may frequently be confined to the milk delivered by a single patron.

"Fruity" or "sweet" flavor. Not infrequently the product of a factory may acquire during the process of ripening what is known as a "sweet" or "fruity" flavor. This flavor resembles the odor of fermented fruit or the bouquet of certain kinds of wine. It has been noted in widely different sections of the country and its presence bears no relation to the other qualities of the cheese. The cause of this trouble has recently been traced[212] to the presence of various kinds of yeasts. Ordinarily yeasts are rarely present in good cheese, but in cheese affected with this trouble they abound. The addition of starters made from yeast cultures resulted in the production of the undesirable condition.

Mottled cheese. The color of cheese is sometimes cut to that extent that the cheese presents a wavy or mottled appearance. This condition is apt to appear if the ripening temperature is somewhat high, or larger quantities of rennet used than usual. The cause of the defect is obscure, but it has been demonstrated that the same is communicable if a starter is made by grating some of this mottled cheese into milk. The bacteriology of the trouble has not yet been worked out, but the defect is undoubtedly due to an organism that is able to grow in the ripening cheese. It has been claimed that the use of a pure lactic ferment as a starter enables one to overcome this defect.

Bitter cheese. Bitter flavors are sometimes developed in cheese especially where the ripening process is carried on at a low temperature in the presence of an excess of moisture for a considerable length of time.

Guillebeau[213] isolated several forms from Emmenthaler cheese which he connected with udder inflammation that were able to produce a bitter substance in cheese.

Von Freudenreich[214] has described a new form Micrococcus casei amari (micrococcus of bitter cheese) that was found in a sample of bitter cheese. This germ is closely related to Conn's micrococcus of bitter milk. It develops lactic acid rapidly, coagulating the milk and producing an intensely bitter taste in the course of one to three days. When milk infected with this organism is made into cheese, there is formed in a few days a decomposition product that imparts a marked bitter flavor to the cheese.

Harrison[215] has recently found a yeast that grows in the milk and also in the cheese which produces an undesirable bitter change.

It is peculiar that some of the organisms that are able to produce bitter products in milk do not retain this property when the milk is worked up into cheese.

Putrid or rotten cheese. Sometimes cheese undergoes a putrefactive decomposition in which the texture is profoundly modified and various foul smelling gases are evolved. These often begin on the exterior as small circumscribed spots that slowly extend into the cheese, changing the casein into a soft slimy mass. Then, again, the interior of the cheese undergoes this slimy decomposition. The soft varieties are more prone toward this fermentation than the hard, although the firm cheeses are by no means exempt from the trouble. The "Verlaufen" or "running" of limburger cheese is a fermentation allied to this. It is where the inside of the cheese breaks down into a soft semi-fluid mass. In severe cases, the rind may even be ruptured, in which case the whole interior of the cheese flows out as a thick slimy mass, having sometimes a putrid odor. The conditions favoring this putrid decomposition are usually associated with an excess of moisture, and an abnormally low ripening temperature.

Rusty spot. This name is applied to the development of small yellowish-red or orange spots that are formed sometimes throughout the whole mass of cheddar cheese. A close inspection shows the colored points to be located along the edges of the curd particles. According to Harding,[216] this trouble is most common in spring and fall. The cause of the difficulty has been traced by Connell[217] to the development of a chromogenic bacterium, Bacillus rudensis. The organism can be most readily isolated on a potato surface rather than with the usual isolating media, agar or gelatin.

Other pigment changes. Occasionally, with the hard type of cheese, but more frequently with the softer foreign varieties, various abnormal conditions arise that are marked by the production of different pigments in or on the cheese. More frequently these are merely superficial and affect only the outer layers of the cheese. Generally they are attributable to the development of certain chromogenic organisms (bacteria, molds and yeasts), although occasionally due to other causes, as in the case of a blue discoloration sometimes noted in foreign cheese made in copper kettles.[218]

De Vries[219] has described a blue condition that is found in Edam cheese. It appears first as a small blue spot on the inside, increasing rapidly in size until the whole mass is affected. This defect he was able to show was produced by a pigment-forming organism, B. cyaneo-fuscus. By the use of slimy whey (lange wei) this abnormal change was controlled.

Moldy cheese. With many varieties of cheese, especially some of the foreign types, the presence of mold on the exterior is not regarded as detrimental; in fact a limited development is much desired. In hard rennet cheese as cheddar or Swiss, the market demands a product free from mold, although it should be said that this condition is imposed by the desire to secure a good-looking cheese rather than any injury in flavor that the mold causes. Mold spores are so widely distributed that, if proper temperature and moisture conditions prevail, these spores will always develop. At temperatures in the neighborhood of 40 deg. F. and below, mold growth is exceedingly slow, and often fructification does not occur, the only evidence of the mold being the white, felt-like covering that is made up of the vegetating filaments. The use of paraffin has been suggested as a means of overcoming this growth, the cheese being dipped at an early stage into melted paraffin. Recent experiments have shown that "off" flavors sometimes develop where cheese are paraffined directly from the press. If paraffin is too hard, it has a tendency to crack and separate from the rind, thus allowing molds to develop beneath the paraffin coat, where the conditions are ideal as to moisture, for evaporation is excluded and the air consequently saturated. The use of formalin (2% solution) has been suggested as a wash for the outside of the cheese. This substance or sulfur is also applied in a gaseous form. Double bandaging is also resorted to as a means of making the cheese more presentable through the removal of the outer bandage.

The nature of these molds has not been thoroughly studied as yet. The ordinary blue-green bread mold, Penicillium glaucum, is most frequently found, but there are numerous other forms that appear, especially at low temperatures.

Poisonous cheese. Cases of acute poisoning arising from the ingestion of cheese are reported from time to time. Vaughan has succeeded in showing that this condition is due to the formation of a highly poisonous alkaloid which he has isolated, and which he calls tyrotoxicon.[220] This poisonous ptomaine has also been demonstrated in milk and other milk products, and is undoubtedly due to the development of various putrefactive bacteria that find their way into the milk. It seems quite probable that the development of these toxic organisms can also go on in the cheese after it is taken from the press.

Prevention or cheese defects. The defective conditions previously referred to can rarely be overcome in cheese so as to improve the affected product, for they only become manifest in most cases during the later stages of the curing process. The only remedy against future loss is to recognize the conditions that are apt to prevail during the occurrence of an outbreak and see that the cheese are handled in such a way as to prevent a recurrence of the difficulty.

Many abnormal and undesirable results are incident to the manufacture of the product, such as "sour" or "mealy" cheese, conditions due to the development of too much acid in the milk or too high a "cook." These are under the direct control of the maker and for them he alone is responsible. The development of taints due to the growth of unwelcome bacteria that have gained access to the milk while it is yet on the farm are generally beyond the control of the cheese maker, unless they are so pronounced as to appear during the handling of the curds. If this does occur he is sometimes able, through the intervention of a starter or by varying some detail in making, to handle the milk in such a way as to minimize the trouble, but rarely is he able to eliminate it entirely.

One of the most strenuous duties which the maker must perform at all times is to point out to his patrons the absolute necessity of their handling the milk in such a way as to prevent the introduction of organisms of a baleful type.

FOOTNOTES:

[178] Russell, 13 Rept. Wis. Expt. Stat., 1896, p. 112; Campbell, Trans. High. & Agr. Soc. Scotland, 5 ser., 1898, 10:181.

[179] Winkler, Milch Zeit. (Hildesheim), Nov. 24, 1900.

[180] Campbell, No. Brit., Agric., May 12, 1897.

[181] Weigmann, Milch Zeit., No. 50, 1889.

[182] Klein, Milch Zeit. (Hildesheim), No. 17, 1900.

[183] Adametz, Landw. Jahr., 18:256.

[184] Van Slyke and Hart, Bull. 214, N. Y. Expt. Stat., July 1902.

[185] Milch Zeit., 1898, No. 49.

[186] Lafar, Technical Mycology, p. 216.

[187] Adametz, Landw. Jahr., 18:228.

[188] Freudenreich, Landw. Jahr. d. Schweiz, 4:17; 5:16.

[189] Russell, 13 Rept. Wis. Expt. Stat., 1896, p. 95.

[190] Harrison and Connell, Rev. gen. du Lait, Nos. 4, 5, 6, 7 and 8, 1903-04.

[191] Lloyd, Bath and West of Eng. Soc. Rept., 1892, 2:180.

[192] Freudenreich, Landw. Jahr. d. Schweiz, 1900; Adametz, Oest. Molk. Zeit., 1899, No. 7.

[193] Russell, 14 Wis. Expt. Stat., 1897, p. 203. Harrison and Connell, Rev. gen. du Lait Nos. 4, etc., 1903-04.

[194] Babcock and Russell, 18 Rept. Wis. Expt. Stat., 1901. Dean, Harrison and Harcourt, Bull. 121, Ont. Agr'l. Coll., June 1902.

[195] Schaffer, Milch Zeit., 1889, p. 146.

[196] Adametz, Landw. Jahr., 18:261.

[197] Duclaux, Le Lait, p. 213.

[198] Adametz, Oest. Molk. Zeit., 1900, Nos. 16-18.

[199] Freudenreich, Landw. Jahr. d. Schweiz, 1897, p. 85.

[200] Weigmann, Cent. f. Bakt., II Abt., 1898, 4:593; also 1899, 5:630.

[201] Gorini, Abs. in Expt. Stat. Rec., 11:388.

[202] Babcock and Russell, 14 Rept. Wis. Expt. Stat., 1897, p. 161.

[203] Jensen, Cent. f. Bakt., II Abt., 3:752.

[204] Freudenreich, Cent. f. Bakt., II Abt., 1900, 6:332.

[205] Jensen, Ibid., 1900, 6:734.

[206] 17 Rept. Wis. Expt. Stat., 1900, p. 102.

[207] Jensen, Landw. Jahr. d. Schweiz, 1900.

[208] Babcock and Russell, 18 Rept. Wis. Expt. Stat., 1901.

[209] Cent. f. Bakt. 1899, p. 14.

[210] Bull. 128, Wis. Expt. Stat., Sept. 1905.

[211] Babcock and Russell, 18 Rept. Wis. Expt. Stat., 1901.

[212] Harding, Rogers and Smith, Bull. 183, N. Y. (Geneva) Expt. Stat., Dec., 1900.

[213] Guillebeau, Landw. Jahr., 1890, p. 27.

[214] Freudenreich, Fueehl. Landw. Ztg., 43:361.

[215] Harrison, Bull. 123 Ont. Agr'l. Coll., May, 1902.

[216] Bull. 183, N. Y. (Geneva) Expt. Stat., Dec. 1900.

[217] Connell, Bull. Canadian Dept. of Agr., 1897.

[218] Schmoeger, Milch Zeit., 1883, p. 483.

[219] De Vries, Milch Zeit., 1888, pp. 861, 885.

[220] Zeit. f. physiol. Chemie, 10:146.



INDEX.

Acid, effect of, on churning, 137; in butter-making, 138.

Acid test, 52.

Aeration of milk, 59.

Aerobic bacteria, 7.

Alcoholic fermentation in milk, 72.

Anaerobic bacteria, 7.

Animal, influence of, on milk infection, 34.

Animal odor, 56.

Anthrax, 94.

Antiseptics, 9, 88.

Aroma, of butter, 140.

Bacillus: definition of, 2. acidi lactici, 64; cyaneo-fuscus, 188; cyanogenus, 74; foetidus lactis, 157; lactis aerogenes, 65; lactis erythrogenes, 74; lactis saponacei, 67; lactis viscosus, 71; nobilis, 162, 174; prodigiosus, 74; rudensis, 188; synxanthus, 75; tuberculosis, 84.

Bacteria: on hairs, 35; kinds in milk, 63; in barn air, 42; in milk pails, 27; in butter, 154; classification of, 4; in cheese, 160; culture of, 17; in cream, 128; discovery of, 1; external conditions affecting, 8; form of, 2; in butter, 142; in butter-making, 127; in centrifuge slime, 39; In fore milk, 28; in rennet, 163; In separator slime, 39; manure, 37; number of, in milk, 50. Distribution of: milk of American cities, 50; European cities, 50; in relation to cheese, 168. Of disease: anthrax, 94; cholera, 98; diphtheria, 99; lockjaw, 94; toxic, 100; tuberculosis, 84; typhoid fever, 98. Methods of study of: culture, 15; culture media, 13; isolation, 14.

Bitter butter, 158; cheese, 189; milk, 72.

Bloody milk, 74.

Blue cheese, 191; milk, 74.

Bovine tuberculosis, 84.

Brie cheese, 182.

Butter: bacteria in, 154; bitter, 158; "cowy," 157; fishy, 159; lardy, 157; moldy, 158; mottled, 156; oily, 158; putrid, 156; rancid, 155; tallowy, 157; turnip flavor in, 157. Making: aroma, 140; flavor in, 140; pure culture, 143; in ripening of cream, 136.

Butyric acid fermentation, 69.

By-products of factory, methods of preserving, 25.

Casease, 68.

Caseone, 68.

Centrifugal force, cleaning milk by, 38.

Cheese: bacterial flora of, 168; bitter, 189; blue, 187; Brie, 182; Edam, 72, 162; Emmenthaler, 185; flavor of, 179; gassy fermentations in, 183; Gorgonzola, 180; molds on, 191; mottled, 189; "nissler," 185; poisonous, 192; putrid, 190; ripening of moldy, 180; ripening of soft, 181; Roquefort, 180; rusty spot in, 188; Stilton, 180; Swiss, 185. Making and curing: chemical changes in curing, 166; influence of temperature on curing, 169; influence of rennet, 177; physical changes in curing, 165; prevention of defects, 193; starters in, 161; temperature in relation to bacterial influence, 169. Theories of curing: digestive, 173; galactase, 175, 177; lactic acid, 174.

Chemical changes in cheese-ripening, 166.

Chemical disinfectants in milk: bleaching powder, 81; corrosive sublimate, 81; formalin, 80; sulfur, 80; whitewash, 81; vitriol, 81.

Chemical preservatives, 80.

Children, milk for, 45.

Cholera in milk, 98.

Classification by separator, 38.

Coccus, definition of, 2.

Cold, influence on bacteria, 8, 48.

Contamination of milk through disease germs, 95, 191.

Covered milk pails, 41.

Cream, bacterial changes in, 135; mechanical causes for bacteria in, 135; pasteurized, 113; restoration of consistency of pasteurized, 132. Ripening of, 136; advantage of pure cultures in, 144; by natural starters, 142; characteristics of pure cultures in, 145; objections to pure cultures in, 146; principles of pure cultures in, 143; propagation of pure cultures, 151; purity of commercial starters, 150; home-made starters in, 146.

Creaming methods, 134.

Curd test, 76.

Dairy utensils a source of contamination, 21.

Diarrhoeal diseases, 100.

Digesting bacteria, 67.

Digestibility of heated milk, 111.

Diphtheria, 99.

Dirt in milk, 34.

Dirt, exclusion of, 36.

Disease germs in milk, 95; effect of heat on, 91; origin of, 83.

Disinfectants, 9: carbolic acid, 81; chloride of lime, 81; corrosive sublimate, 81; formalin, 80; sulfur, 80; vitriol salts, 81; whitewash, 79.

Disinfectants in milk: alkaline salts, 106; boracic acid, 106; formalin, 106; preservaline, 107; salicylic acid, 106.

Domestic pasteurizing apparatus, 119.

Drugs, taints in milk due to, 56.

Drying, effect of, 8.

Edam cheese, 72, 162.

Emmenthaler cheese, 185.

Endospores, 3.

Enzyms, 10.

Factory by-products, 22; treatment of, 25.

Farrington alkaline tablet, 52.

Fecal bacteria, effect of, on butter, 35.

Fermentation: In cheese: gassy, 183. In milk: alcoholic, 72; bitter, 72; blue, 74; butyric, 69; digesting, 67; gassy, 66; kephir, 72; koumiss, 72; lactic acid, 63; lange-wei, 72; red, 74; ropy, 69; slimy, 69; soapy, 73; souring, 63; sweet curdling, 67; treatment of, 75. Tests, 76; Gerber's, 76; Walther's, 76; Wisconsin curd, 76.

Filtration of milk, 38.

Fishy butter, 159.

Flavor: of butter, 140; of cheese, 179.

Foot and mouth disease, 93.

Fore milk, 28.

Formaldehyde, 80.

Formalin, 80.

Fruity flavor in cheese, 188.

Galactase in cheese, 175.

Gassy fermentations: in cheese, 183; in milk, 67; in Swiss cheese, 167.

Glaesler, 185.

Gorgonzola cheese, 180.

Growth of bacteria, essential conditions for, 4; in milk, 46.

Hair, bacteria on, 35.

Heat, influence on bacterial growth, 8.

Heated milk: characteristics of, 109; action toward rennet, 112; body, 110; digestibility, 111; fermentative changes, 111; flavor, 110; hydrogen peroxid test in, 23; Storch's test, 23.

Hygienic milk, bacteria in, 45.

Infection of milk: animal, 34; dairy utensils, 21; fore milk, 28; milker, 36.

Isolation of bacteria, methods of, 14.

Kephir, 72.

Koumiss, 72.

Lactic acid: fermentation in milk, 63; theory in cheese-curing, 174.

Lange-wei, 72.

Lardy butter, 157.

Light, action on bacteria, 9.

Manure, bacteria in, 33.

Methods: of isolation, 14; culture, 15.

Micrococcus casei amari, 189.

Microscope, use of, 17.

Milk: a bacterial food medium, 19; bacteria in, 48. Disease organisms in: anthrax, 94; cholera, 98; diphtheria, 99; foot and mouth disease, 93; poisonous, 101; ptomaines, 101; scarlet fever, 99; tuberculosis, 84; typhoid fever, 98. Contamination, 20: from air, 42; from animal odors, 55; dirt, 34; distinction between bacterial and non-bacterial, 57; fore milk, 28; infection in factory, 59; milker, 36; relative importance of various kinds, 43; utensils, 21.

Milk fermentations: alcoholic, 72; bitter, 72; bloody, 74; blue, 74; butyric acid, 69; gassy, 66, 167; kephir, 72; koumiss, 72; lactic acid, 63; red, 72; ropy, 69; slimy, 69; soapy, 74; souring, 63; sweet curdling, 67; tests for, 76; treatment of, 75; yellow, 75.

Milk, heated: action towards rennet, 112; digestibility, 111; flavor of, 110; fermentative changes in, 111; hydrogen peroxid test, 110.

Milking machines, influence of, on germ content, 37.

Milk preservation: chemical agents in, 106; condensation, 107; freezing, 108; heat, 108; pasteurization, 113; sterilization, 112.

Milk-sugar as bacterial food, 19.

Mold, in butter, 158; in cheese, 191.

Mottled cheese, 189.

"Nissler" cheese, 185.

Odors, direct absorption of, in milk, 55.

Oidium lactis, 159.

Oily butter, 158.

Pasteurization of milk; acid test in, 128; bacteriological study of, 124, 126, 149; for butter, 147; for cheese, 162; for direct use, 113; of skim milk, 25; details of, 128; temperature and time limit in, 118.

Pasteurizing apparatus: continuous flow, 122; coolers, 131; Danish, 123; domestic, 119; Farrington, 122; intermittant flow, 121; Miller, 122; Potts, 121; regenerator, 122; Reid, 126; Russell, 121; testing rate of flow, 124.

Penicillium glaucum, 159, 180, 190.

Pepsin, 10.

Physical changes in cheese-ripening, 165.

Poisonous bacteria: in cheese, 192; in milk, 100, 101.

Preservaline, 167.

Preservation of milk: by exclusion, 103; chemical agents, 106; condensing, 107; filtration, 38; freezing, 108; pasteurization, 112; physical agents, 107; sterilization, 112.

Ptomaine poisoning, 101.

Pure cultures, 15.

Pure culture starters: advantages of, 144; characteristics of, 145; home-made cultures compared with, 146; propagation of, 151.

Putrid cheese, 190; butter, 156.

Rancidity in butter, 155.

Red milk, 74.

Rennet: action in heated milk, 112; bacteria in, 163; influence of, on cheese-ripening, 177.

Restoration of consistency in pasteurized cream, 132.

Ripening of cheese: moldy cheese, 180; soft cheese, 181. Of cream, 136; artificial starters, 143; natural starters, 142; principles of pure culture starters in, 143.

Ropy milk, 69.

Roquefort cheese, 180.

Rusty spot in cheese, 190.

Rusty cans: effect of, on acidity, 53.

Sanitary milk, 45, 104.

Sanitary pails, 41.

Scarlet fever in milk, 99.

Separator slime: bacteria in, 39; tubercle bacillus in, 93.

Scalded layer, resistance of bacteria in, 91.

Skim-milk, a distributor of disease, 96.

Slimy milk, 69.

Soapy milk, 74.

Soft cheese, ripening of, 186.

Sources of contamination in milk: barn air, 42; dairy utensils, 21; dirt from animals, 34; factory cans, 25; fore-milk, 28; milker, 36.

Souring of milk, 63.

Spirillum, definition of, 2.

Spores, 3.

Starters: in cheese-making, 161; in butter-making, 142; propagation of, 151; pure cultures in cream-ripening, 143.

Sterilization of milk, 112.

Streptococcus Hollandicus, 72, 162.

Stilton cheese, 181.

Storch's test, 23.

Sulfur as a disinfectant, 81.

Sweet curdling milk, 68.

Sweet flavor in cheese, 188.

Swiss cheese, 177; gassy fermentations in, 24, 185.

Taints, absorption of, 55.

Taints, bacterial vs. physical, 58.

Taints in milk, absorption of, 55.

Taints, use of starters in overcoming, 79.

Taints in butter: putrid, 156; rancidity, 155; turnip flavor, 157.

Tallowy butter, 157.

Temperature: effect on bacterial development, 6, 48; effect of low, 108; effect of high, 108; and time limit in milk pasteurization, 113.

Tests for milk: fermentation, 76; Storch's, 23; acid, 52.

Theories in cheese-curing: digestive, 171; galactase, 175, 177; lactic acid, 174.

Trypsin, 10.

Tubercle bacillus: in milk, 88; in separator slime, 93; thermal death limits, 117.

Tuberculin test, 86.

Tuberculosis, bovine, 84.

Turnip flavor in butter, 157.

Typhoid fever, 98.

Tyrogen, 162.

Tyrotoxicon, 101, 190.

Udder: artificial introduction of bacteria into, 32; milk germ-free in, 19; infection of, 28; washing, 89; tuberculosis in, 87.

Viscogen, 132.

Water: as a source of infection, 61.

Whey, pollution of vats, 23; method of preserving, 25; treatment of, in vats, 25.

Whitewash, 81.

Wisconsin curd test, 76.

Yeasts: alcoholic ferments in milk, 73; fruity flavor in cheese, 186; gassy due to yeasts, 186; in bitter cheese, 189; in canned butter, 159; kephir, 72.

THE END