Chapter 4 FERMENTATIONS IN MILK AND THEIR TREATMENT.

Under the conditions in which milk is drawn, it is practically impossible to secure the same without bacterial contamination. The result of the introduction of these organisms often changes its character materially as most bacteria cause the production of more or less pronounced fermentative processes. Under normal conditions, milk sours, i. e., develops lactic acid, but at times this more common fermentation may be replaced by other changes which are marked by the production of some other more or less undesirable flavor, odor or change in appearance.

In referring to these changes, it is usually customary to designate them after the most prominent by-product formed, but it must be kept in mind that generally some other decomposition products are usually produced. Whether the organisms producing this or that series of changes prevail or not depends upon the initial seeding, and the conditions under which the milk is kept. Ordinarily, the lactic acid organisms grow so luxuriantly in the milk that they overpower all competitors and so determine the nature of the fermentation; but occasionally the milk becomes infected with other types of bacteria in relatively large numbers and the conditions may be especially suitable to the development of these forms, thereby modifying the course of the normal changes that occur.

The kinds of bacteria that find it possible to develop in milk may be included under two heads:

1. Those which cause no appreciable change in the milk, either in taste, odor or appearance. While these are frequently designated as the inert bacteria, it must not be supposed that they have absolutely no effect on milk. It is probably true in most cases that slight changes of a chemical nature are produced, but the nature of the changes do not permit of ready recognition.

2. This class embraces all those organisms which, as a result of their growth, are capable of producing evident changes. These transformations may be such as to affect the taste, as in the sour milk or in the bitter fermentations, or the odor, as in some of the fetid changes, or the appearance of the milk, as in the slimy and color changes later described.

Souring of milk. Ordinarily if milk is allowed to stand for several days at ordinary temperatures it turns sour. This is due to the formation of lactic acid, which is produced by the decomposition of the milk-sugar. While this change is well nigh universal, it does not occur without a pre-existing cause, and that is the presence of certain living bacterial forms. These organisms develop in milk with great rapidity, and the decomposition changes that are noted in souring are due to the by-products of their development.

The milk-sugar undergoes fermentation, the chief product being lactic acid, although various other by-products, as other organic acids (acetic, formic and succinic), different alcohols and gaseous products, as CO2, H, N and methane (CH4) are produced in small amounts.

In this fermentation, the acidity begins to be evident to the taste when it reaches about 0.3 per cent., calculated as lactic acid. As the formation of acid goes on, the casein is precipitated and incipient curdling or lobbering of the milk occurs. This begins to be apparent when the acidity is about 0.4 per cent., but the curd becomes more solid with increasing acidity. The rapidity of curdling is also dependent upon the temperature of the milk. Thus milk which at ordinary temperatures might remain fluid often curdles when heated. The growth of the bacteria is continued until about 0.8 to 1.0 per cent. acid is formed, although the maximum amount fluctuates considerably with different lactic acid species. Further formation then ceases even though all of the milk-sugar is not used up, because of the inability of the lactic bacteria to continue their growth in such acid solutions.

As this acidity is really in the milk serum, cream never develops so much acid as milk, because a larger proportion of its volume is made up of butter-fat globules. This fact must be considered in the ripening of cream in butter-making where the per cent. of fat is subject to wide fluctuations.

The formation of lactic acid is a characteristic that is possessed by a large number of bacteria, micrococci as well as bacilli being numerously represented. Still the preponderance of evidence is in favor of the view that a few types are responsible for most of these changes. The most common type found in spontaneously soured milk changes the milk-sugar into lactic acid without the production of any gas. This type has been described by various workers on European as well as American milks, and is designated by Conn as the Bact. lactis acidi type.[51] It is subject to considerable variation under different conditions.

Curiously enough if milk which has been drawn with special care is examined immediately after milking, the lactic organisms are not usually found. They are incapable of development in the udder itself, as shown by injections into the milk cistern. They abound, however, on hay, in dust, in the barn air, on the hairy coat of the animal, and from these sources easily gain access to the milk. In this medium they find an exceptionally favorable environment and soon begin a very rapid growth, so that by the time milk is consumed, either in the form of milk or milk products, they make up numerically the larger portion of the bacteria present.

Another widely disseminated, although numerically less prevalent, type is B. lactis aerogenes. This type forms gas in milk so that the soured milk is torn by the presence of gas bubbles. It also grows more luxuriantly in contact with the air.

Other types occur more or less sporadically, some of which are capable of liquefying the casein of milk while at the same time they also develop lactic acid. Conn and Aikman refer to the fact that over one hundred species capable of producing variable quantities of lactic acid are already known. It is fair to presume, however, that a careful comparative study of these would show that simply racial differences exist in many cases, and therefore, that they are not distinct species.

As a group these bacteria are characterized by their inability to liquefy gelatin or develop spores. On account of this latter characteristic they are easily destroyed when milk is pasteurized. They live under aerobic or anaerobic conditions, many of them being able to grow in either environment, although, according to McDonnell,[52] they are more virulent when air is not excluded.

While growth of these lactic forms may go on in milk throughout a relatively wide range in temperature, appreciable quantities of acid are not produced except very slowly at temperatures below 50° F.[53]

From the standpoint of frequency the most common abnormal changes that occur in milk are those in which gases of varying character are developed in connection with acids, from the milk sugar. Other volatile products imparting bad flavors usually accompany gas production. These fermentations are of most serious import in the cheese industry, as they are especially prone to develop in the manufacture of milk into certain types of cheese. Not often is their development so rapid that they appear in the milk while it is yet in the hands of the milk producer, but almost invariably the introduction of the causal organisms takes place while the milk is on the farm. Numerous varieties of bacteria possess this property of producing gas (H and CO2 are most common although N and methane (CH4) are sometimes produced). The more common forms are those represented by B. lactis aerogenes and the common fecal type, B. coli commune. The ordinary habitat of this type is dirt and intestinal filth. Hence careless methods of milk handling invite this type of abnormal change in milk.

It is a wide-spread belief that thunder storms cause milk to sour prematurely, but this idea has no scientific foundation. Experiments[54] with the electric spark, ozone and loud detonations show no effect on acid development, but the atmospheric conditions usually incident to a thunder storm are such as permit of a more rapid growth of organisms. There is no reason to believe but that the phenomenon of souring is wholly related to the development of bacteria. Sterile milks are never affected by the action of electric storms.

"Gassy" milks. Where these gas bacteria abound, the amount of lactic acid is generally reduced, due to the splitting up of some of the sugar into gaseous products. This type of germ life does not seem to be able to develop well in the presence of the typical lactic acid non gas-forming bacteria.

Fig. 16. Cheese made from "gassy" milk.

"Sweet curdling" and digesting fermentations. Not infrequently milk, instead of undergoing spontaneous souring, curdles in a weakly acid or neutral condition, in which state it is said to have undergone "sweet curdling." The coagulation of the milk is caused by the action of enzyms of a rennet type that are formed by the growth of various species of bacteria. Later the whey separates more or less perfectly from the curd, producing a "wheyed off" condition. Generally the coagulum in these cases is soft and somewhat slimy. The curd usually diminishes in bulk, due to the gradual digestion or peptonization of the casein by proteid-dissolving enzyms (tryptic type) that are also produced by the bacteria causing the change.

A large number of bacteria possess the property of affecting milk in this way. So far as known they are able to liquefy gelatin (also a peptonizing process) and form spores. The Tyrothrix type of bacteria (so named by Duclaux on account of the supposed relation to cheese ripening) belongs to this class. The hay and potato forms are also digesters. Organisms of this type are generally associated with filth and manure, and find their way into the milk from the accumulations on the coat of the animal.

Conn[55] has separated the rennet enzym from bacterial cultures in a relatively pure condition, while Fermi[56] has isolated the digestive ferment from several species.

Duclaux[57] has given to this digesting enzym the name casease or cheese ferment. These isolated ferments when added to fresh milk possess the power of causing the characteristic curdling and subsequent digestion quite independent of cell development. The quantity of ferment produced by different species differs materially in some cases. In these digestive fermentations, the chemical transformations are profound, the complex proteid molecule being broken down into albumoses, peptones, amido-acids (tyrosin and leucin) and ammonia as well as fatty acids.

Not infrequently these fermentations gain the ascendency over the normal souring change, but under ordinary conditions they are held in abeyance, although this type of bacteria is always present to some extent in milk. When the lactic acid bacteria are destroyed, as in boiled, sterilized or pasteurized milk, these rennet-producing, digesting species develop.

Butyric acid fermentations. The formation of butyric acid in milk which may be recognized by the "rancid butter" odor is not infrequently seen in old, sour milk, and for a long time was thought to be a continuation of the lactic fermentation, but it is now believed that these organisms find more favorable conditions for growth, not so much on account of the lactic acid formed as in the absence of dissolved oxygen in the milk which is consumed by the sour-milk organisms.

Most of the butyric class of bacteria are spore-bearing, and hence they are frequently present in boiled or sterilized milk. The by-products formed in this series of changes are quite numerous. In most cases, butyric acid is prominent, but in addition to this, other organic acids, as lactic, succinic, and acetic, are produced, likewise different alcohols. Concerning the chemical origin of butyric acid there is yet some doubt. Duclaux[58] affirms that the fat, sugar and casein are all decomposed by various forms. In some cases, the reaction of the milk is alkaline, with other species it may be neutral or acid. This type of fermentation has not received the study it deserves.

In milk these organisms are not of great importance, as this fermentation does not readily gain the ascendency over the lactic bacteria.

Ropy or slimy milk. The viscosity of milk is often markedly increased over that which it normally possesses. The intensity of this abnormal condition may vary much; in some cases the milk becoming viscous or slimy; in others stringing out into long threads, several feet in length, as in Fig. 17. Two sets of conditions are responsible for these ropy or slimy milks. The most common is where the milk is clotted or stringy when drawn, as in some forms of garget. This is generally due to the presence of viscid pus, and is often accompanied by a bloody discharge, such a condition representing an inflamed state of the udder. Ropiness of this character is not usually communicable from one lot of milk to another.

Fig. 17. Ropy milk.

The communicable form of ropy milk only appears after the milk has been drawn from the udder for a day or so, and is caused by the development of various species of bacteria which find their way into the milk after it is drawn. These defects are liable to occur at any season of the year. Their presence in a dairy is a source of much trouble, as the unsightly appearance of the milk precludes its use as food, although there is no evidence that these ropy fermentations are dangerous to health.

There are undoubtedly a number of different species of bacteria that are capable of producing these viscid changes,[59] but it is quite probable that they are not of equal importance in infecting milk under natural conditions.

In the majority of cases studied in this country,[60] the causal organism seems to be B. lactis viscosus, a form first found by Adametz in surface waters.[61] This organism possesses the property of developing at low temperatures (45°-50° F.), and consequently it is often able in winter to supplant the lactic-acid forms. Ward has found this germ repeatedly in water tanks where milk cans are cooled; and under these conditions it is easy to see how infection of the milk might occur. Marshall[62] reports an outbreak which he traced to an external infection of the udder; in another case, the slime-forming organism was abundant in the barn dust. A defect of this character is often perpetuated in a dairy for some time, and may therefore become exceedingly troublesome. In one instance in the writer's experience, a milk dealer lost over $150 a month for several months from ropy cream. Failure to properly sterilize cans, and particularly strainer cloths, is frequently responsible for a continuance of trouble of this sort.

The slimy substance formed in milk comes from various constituents of the milk, and the chemical character of the slime produced also varies with different germs. In some cases the slimy material is merely the swollen outer cell membrane of the bacteria themselves as in the case of B. lactis viscosus; in others it is due to the decomposition of the proteids, but often the chief decomposition product appears to come from a viscous fermentation of the milk-sugar.

An interesting case of a fermentation of this class being utilized in dairying is seen in the use of "lange wei" (long or stringy whey) which is employed as a starter in Holland to control the gassy fermentations in Edam cheese. This slimy change is due to the growth of Streptococcus Hollandicus.[63]

Alcoholic fermentations. Although glucose or cane-sugar solutions are extremely prone to undergo alcoholic fermentation, milk sugar does not readily undergo this change. Where such changes are produced it is due to yeasts. Several outbreaks attributable to such a cause have been reported.[64] Russell and Hastings[65] have found these milk-sugar splitting yeasts particularly abundant in regions where Swiss cheese is made, a condition made possible by the use of whey-soaked rennets in making such cheese.

Kephir and Koumiss are liquors much used in the Orient which are made from milk that has undergone alcoholic fermentation. Koumiss was originally made from mare's milk but is now often made from cows' milk by adding cane sugar and yeast. In addition to the CO2 developed, alcohol, lactic acid, and casein-dissolving ferments are formed. Kephir is made by adding to milk Kephir grains, which are a mass of yeast and bacterial cells. The yeasts produce alcohol and CO2 while the bacteria change the casein of milk, rendering it more digestible. These beverages are frequently recommended to persons who seem to be unable to digest raw milk readily. The exact nature of the changes produced are not yet well understood.[66]

Bitter milk. The presence of bitter substances in milk may be ascribed to a variety of causes. A number of plants, such as lupines, ragweed and chicory, possess the property of affecting milk when the same are consumed by animals. At certain stages in lactation, a bitter salty taste is occasionally to be noted that is peculiar to individual animals.

A considerable number of cases of bitter milk have, however, been traced to bacterial origin. For a number of years the bitter fermentation of milk was thought to be associated with the butyric fermentation, but Weigmann[67] showed that the two conditions were not dependent upon each other. He found that the organism which produced the bitter taste acted upon the casein.

Conn[68] observed a coccus form in bitter cream that was able to impart a bitter flavor to milk. Sometimes a bitter condition does not develop in the milk, but may appear later in the milk products, as in the case of a micrococcus which Freudenreich[69] found in cheese.

Harrison[70] has traced a common bitter condition in Canadian milk to a milk-sugar splitting yeast, Torula amara which not only grows rapidly in milk but produces an undesirable bitterness in cheddar cheese.

Cream ripened at low temperatures not infrequently develops a bitter flavor, showing that the optimum temperature for this type of fermentation is below the typical lactic acid change.

Milk that has been heated often develops a bitter condition. The explanation of this is that the bacteria producing the bitter substances usually possess endospores, and that while the boiling or sterilizing of milk easily kills the lactic acid germs, these forms on account of their greater resisting powers are not destroyed by the heat.

Soapy milk: A soapy flavor in milk was traced by Weigmann and Zirn[71] to a specific bacillus, B. lactis saponacei, that they found gained access to the milk in one case from the bedding and in another instance from hay. A similar outbreak has been reported in this country,[72] due to a germ acting on the casein and albumen.

Red milk. The most common trouble of this nature in milk is due to presence of blood, which is most frequently caused by some wound in the udder. The ingestion of certain plants as sedges and scouring rushes is also said to cause a bloody condition; madders impart a reddish tinge due to coloring matter absorbed. Defects of this class can be readily distinguished from those due to germ growth because they are apparent at time of milking. Where blood is actually present, the corpuscles settle out in a short time if left undisturbed.

There are a number of chromogenic or color-producing bacteria that are able to grow in milk, but their action is so slow that generally they are not of much consequence. Moreover their development is usually confined to the surface of the milk as it stands in a vessel. The most important is the well-known B. prodigiosus. Another form found at times in milk possessing low acidity[73] is B. lactis erythrogenes. This species only develops the red color in the dark. In the light, it forms a yellow pigment. Various other organisms have been reported at different times.[74]

Blue milk. Blue milk has been known for many years, its communicable nature being established as long ago as 1838. It appears on the surface of milk first as isolated particles of bluish or grey color, which later become confluent, the blue color increasing in intensity as the acidity increases. The causal organism, B. cyanogenes, is very resistant toward drying,[75] thus accounting for its persistence. In Mecklenberg an outbreak of this sort once continued for several years. It has frequently been observed in Europe in the past, but is not now so often reported. Occasional outbreaks have been reported in this country.

Other kinds of colored milk. Two or three chromogenic forms producing still other colors have occasionally been found in milk. Adametz[76] discovered in a sample of cooked milk a peculiar form (Bacillus synxanthus) that produced a citron-yellow appearance which precipitated and finally rendered soluble the casein. Adametz, Conn, and List have described other species that confer tints of yellow on milk. Some of these are bright lemon, others orange, and some amber in color.

Still other color-producing bacteria, such as those that produce violet or green changes in the milk, have been observed. In fact, almost any of the chromogenic bacteria are able to produce their color changes in milk as it is such an excellent food medium. Under ordinary conditions, these do not gain access to milk in sufficient numbers so that they modify the appearance of it except in occasional instances.

Treatment of abnormal fermentations. If the taint is recognized as of bacterial origin (see p. 57) and is found in the mixed milk of the herd, it is necessary to ascertain, first, whether it is a general trouble, or restricted to one or more animals. This can sometimes be done by separating the milk of the different cows and noting whether any abnormal condition develops in the respective samples.

Fermentation tests. The most satisfactory way to detect the presence of the taints more often present is to make a fermentation test of one kind or another. These tests are most frequently used at the factory, to enable the maker to detect the presence of milk that is likely to prove unfit for use, especially in cheese making. They are based upon the principle that if milk is held at a moderately high temperature, the bacteria will develop rapidly. A number of different methods have been devised for this purpose. In Walther's lacto-fermentator samples of milk are simply allowed to stand in bottles or glass jars until they sour. They are examined at intervals of several hours. If the curdled milk is homogeneous and has a pure acid smell, the milk is regarded as all right. If it floats in a turbid serum, is full of gas or ragged holes, it is abnormal. As generally carried out, no attempt is made to have these vessels sterile. Gerber's test is a similar test that has been extensively employed in Switzerland. Sometimes a few drops of rennet are added to the milk so as to curdle the same, and thus permit of the more ready detection of the gas that is evolved.

Wisconsin curd test. The method of testing milk described below was devised at the Wisconsin Experiment Station in 1895 by Babcock, Russell and Decker.[77] It was used first in connection with experimental work on the influence of gas-generating bacteria in cheese making, but its applicability to the detection of all taints in milk produced by bacteria makes it a valuable test for abnormal fermentations in general.

In the curd test a small pat of curd is made in a glass jar from each sample of milk. These tests may be made in any receptacle that has been cleaned in boiling water, and to keep the temperature more nearly uniform these jars should be immersed in warm water, as in a wash tub or some other receptacle. When the milk is about 95° F., about ten drops of rennet extract are added to each sample and mixed thoroughly with the milk. The jars should then remain undisturbed until the milk is completely curdled; then the curd is cut into small pieces with a case knife and stirred to expel the whey. The whey should be poured off at frequent intervals until the curd mats. If the sample be kept at blood heat (98° F.) for six to eight hours, it will be ready to examine.

Fig. 18.

Improved bottles for making curd test. A, test bottle complete; B, bottle showing construction of cover; S, sieve to hold back the curd when bottle is inverted; C, outer cover with (D H) drain holes to permit of removal of whey.]

More convenient types of this test than the improvised apparatus just alluded to have been devised by different dairy manufacturers. Generally, they consist of a special bottle having a full-sized top, thus permitting the easy removal of the curd. The one shown in Fig. 18 is provided with a sieve of such construction that the bottles will drain thoroughly if inclined in an inverted position.

Interpretation of results of test. The curd from a good milk has a firm, solid texture, and should contain at most only a few small pin holes. It may have some large, irregular, "mechanical" holes where the curd particles have failed to cement, as is seen in Fig. 19. If gas-producing bacteria are very prevalent in the milk, the conditions under which the test is made cause such a rapid growth of the same that the evidence of the abnormal fermentation may be readily seen in the spongy texture of the curd (Fig. 20). If the undesirable organisms are not very abundant and the conditions not especially suited to their growth, the "pin holes" will be less frequent.

Fig. 19. Curd from a good milk. The large irregular holes are mechanical.

Sometimes the curds show no evidence of gas, but their abnormal condition can be recognized by the "mushy" texture and the presence of "off" flavors that are rendered more apparent by keeping them in closed bottles. This condition is abnormal and is apt to produce quite as serious results as if gas was formed.

Overcoming taints by use of starters. Another method of combatting abnormal fermentations that is often fruitful, is that which rests upon the inability of one kind of bacteria to grow in the same medium in competition with certain other species.

Some of the undesirable taints in factories can be controlled in large part by the introduction of starters made from certain organisms that are able to obtain the ascendency over the taint-producing germ. Such a method is commonly followed when a lactic ferment, either a commercial pure culture, or a home-made starter, is added to milk to overcome the effect of gas-generating bacteria.

Fig. 20. Curd from a badly tainted milk. Large ragged holes are mechanical; numerous small holes due to gas. This curd was a "floater."

A similar illustration is seen in the case of the "lange wei" (slimy whey), that is used in the manufacture of Edam cheese to control the character of the fermentation of the milk.

This same method is sometimes applied in dealing with certain abnormal fermentations that are apt to occur on the farm. It is particularly useful with those tainted milks known as "sweet curdling." The ferment organisms concerned in this change are unable to develop in the presence of lactic acid bacteria, so the addition of a clean sour milk as a starter restores the normal conditions by giving the ordinary milk bacteria the ascendency.

Chemical disinfection. In exceptional instances it may be necessary to employ chemical disinfectants to restore the normal conditions. Of course with such diseases as tuberculosis, very stringent measures are required, as they are such a direct menace to human life, but with these abnormal or taint-producing fermentations, care and cleanliness, well directed, will usually overcome the trouble.

If it becomes necessary to employ chemical substances as disinfecting agents, their use should always be preceded by a thorough cleansing with hot water so that the germicide may come in direct contact with the surface to be disinfected.

It must be borne in mind that many chemicals act as deodorants, i.e., destroy the offensive odor, without destroying the cause of the trouble.

Sulfur is often recommended as a disinfecting agent, but its use should be carefully controlled, otherwise the vapors have but little germicidal power. The common practice of burning a small quantity in a room or any closed space for a few moments has little or no effect upon germ life. The effect of sulfur vapor (SO2) alone upon germ life is relatively slight, but if this gas is produced in the presence of moisture, sulfurous acid (H2SO3) is formed, which is much more efficient. To use this agent effectively, it must be burned in large quantities in a moist atmosphere (three lbs. to every 1,000 cubic feet of space), for at least twelve hours. After this operation, the space should be thoroughly aired.

Formalin, a watery solution of a gas known as formaldehyde, is a new disinfectant that recent experience has demonstrated to be very useful. It may be used as a gas where rooms are to be disinfected, or applied as a liquid where desired. It is much more powerful in its action than sulfur, and it has a great advantage over mercury and other strong disinfectants, as it is not so poisonous to man as it is to the lower forms of life.

Bleaching powder or chloride of lime is often recommended where a chemical can be advantageously used. This substance is a good disinfectant as well as a deodorant, and if applied as a wash, in the proportion of four to six ounces of the powder to one gallon of water, it will destroy most forms of life. In many cases this agent is inapplicable on account of its odor.

Corrosive sublimate (HgCl2) for most purposes is a good disinfectant, but it is such an intense poison that its use is dangerous in places that are at all accessible to stock.

For the disinfection of walls in stables and barns, common thin white wash Ca(OH)2 is admirably adapted if made from freshly-burned quick lime. It possesses strong germicidal powers, increases the amount of light in the barn, is a good absorbent of odors, and is exceedingly cheap.

Carbolic acid, creosote, and such products, while excellent disinfectants, cannot well be used on account of their odor, especially in factories.

For gutters, drains, and waste pipes in factories, vitriol salts (sulfates of copper, iron and zinc) are sometimes used. These are deodorants as well as disinfectants, and are not so objectionable to use on account of their odor.

These suggestions as to the use of chemicals, however, only apply to extreme cases and should not be brought into requisition until a thorough application of hot water, soap, a little soda, and the scrubbing brush have failed to do their work.

FOOTNOTES:

[51] Günther and Thierfelder, Arch. f. Hyg., 25:164, 1895; Leichmann, Cent. f. Bakt., 2:281, 1896; Esten, 9 Rept. Storrs Expt. Stat., p. 44, 1896; Dinwiddie, Bull. 45, Ark. Expt. Stat., May, 1897; Kozai, Zeit. f. Hyg., 38:386, 1901; Weigmann, Hyg. Milk Congress, Hamburg, 1903, p. 375.

[52] McDonnell, Inaug. Diss., Kiel. 1899, p. 39.

[53] Kayser, Cent. f. Bakt. II. Abt. 1:436.

[54] Treadwell, Science, 1894, 17:178.

[55] Conn, 5 Rept. Storrs Expt. Stat., 1892, p. 396.

[56] Fermi, Arch. f. Hyg., 1892, 14:1.

[57] Duclaux, Le Lait, p. 121.

[58] Duclaux, Principes de Laiterie, p. 67.

[59] Guillebeau (Milch Zeit., 1892, p. 808) has studied over a dozen different forms that possess this property.

[60] Ward, Bull. 165, Cornell Expt. Stat., Mch., 1899; also Bull. 195, Ibid., Nov., 1901.

[61] Adametz, Landw. Jahr., 1891, p. 185.

[62] Marshall, Mich. Expt. Stat., Bull. 140.

[63] Milch Zeit., 1899, p. 982.

[64] Duclaux, Principes de Laiterie, p. 60. Heinze and Cohn, Zeit. f. Hyg., 46: 286, 1904.

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

[66] Freudenreich, Landw. Jahr. d. Schweiz, 1896, 10; 1.

[67] Weigmann, Milch Zeit., 1890, p. 881.

[68] Conn, 3 Rept. Storrs Expt. Stat., 1890, p. 158.

[69] Freudenreich, Fühl. Landw. Ztg. 43: 361.

[70] Harrison, Bull. 120 Ont. Agr'l. Coll., May, 1902.

[71] Milch Zeit. 22:569.

[72] Marshall, Bull. 146, Mich. Expt. Stat., p. 16.

[73] Grotenfelt, Milch Zeit., 1889, p. 263.

[74] Menge, Cent. f. Bakt., 6:596; Keferstein, Cent. f. Bakt., 21:177.

[75] Heim, Arb. a. d. Kais. Gesundheitsamte, 5:578.

[76] Adametz, Milch Zeit., 1890, p. 225.

[77] 12 Rept. Wis. Expt. Stat., 1895, p. 148; also Bull. 67, Ibid., June, 1898.

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