FOOTNOTES:

Fig. 14. Effect of cooling milk on the growth of bacteria.Fig. 14. Effect of cooling milk on the growth of bacteria.

Conn[42]is inclined to regard temperature of more significance in determining the keeping quality than the original infection of the milk itself. Milk which curdled in 18 hours at 98° F., did not curdle in 48 hours at 70°, and often did not change in two weeks, if the temperature was kept at 50° F.

Where kept for a considerable period at this low temperature, the milk becomes filled with bacteria of the undesirable putrefactive type, the lactic group being unable to form acid in any appreciable amounts. Running wellwater can be used for cooling, if it is possible to secure it at a temperature of 48°-50° F. The use of ice, of course, gives better results, and in summer is greatly to be desired. The influence of these lowered temperatures makes it possible to ship milk long distances[43]by rail for city supplies, if the temperature is kept low during transit.

Mixing night and morning milk.Not infrequently it happens when old milk is mixed with new, that the course of the fermentative changes is more rapid than would have been the case if the two milks had been kept apart. Thus, adding the cooled night milk to the warm morning milk sometimes produces more rapid changes in both. The explanation for this often imperfectly understood phenomenon is that germ growth may have gone on in the cooled milk, and when this material is added to the warmer, but bacteria-poor, fresh milk, the temperature of the whole mass is raised to a point suitable for the more rapid growth of all bacteria than would have occurred if the older milk had been kept chilled.

Number of bacteria in milk.The number of organisms found in milk depends upon (1) the original amount of contamination, (2) the age of the milk, and (3) the temperature at which it has been held. These factors all fluctuate greatly in different cases; consequently, the germ life is subject to exceedingly wide variations. Here in America, milk reaches the consumer with less bacteria than in Europe, although it may often be older. This is due largely to the more wide-spread use of ice for chilling the milken routeto market. Examinations have been made of various supplies with the following results: Sedgwick and Batchelderfound in 57 tests of Boston milk from 30,000-4,220,000 per cc. Jordan and Heineman found 30% of samples of Chicago milk to range from 100,000 to 1,000,000 while nearly one half were from 1-20,000,000 per cc. The germ content of city milks increase rapidly in the summer months. Park[44]found 250,000 organisms per cc. in winter, about 1,000,000 in cool weather and 5,000,000 per cc. in hot summer weather. Knox and Bassett in Baltimore report 1,500,000 in spring and nearly 4,500,000 in summer. Eckles[45]studied milk under factory conditions. He finds from 1,000,000 to 5,000,000 per cc. in winter, and in summer from 15-30 millions.

Bacterial standards for city supplies.It would be very desirable to have a hygienic standard for city milk supplies, as there is a butter fat and milk-solid test, but the wide spread variation in germ content and the impracticability of utilizing ordinary bacterial determinations (on account of time required) makes the selection of such a standard difficult. Some hold, as Park, that such a standard is feasible. The New York City Milk commission has set a standard of 30,000 bacteria per cc. for their certified milk and 100,000 per cc. for inspected milk. Rochester, N. Y. has attempted the enforcement of such a standard (limit, 100,000 per cc.) with good results it is claimed while Boston has placed the legal limit at 500,000 per cc. Quantitative standards would seem more applicable to "certified" or sanitary supplies than to general city supplies, where the wide range in conditions lead to such enormous variations that the bacterial standard seems too refined a method for practical routine inspection.

Other tests.Any test to be of much service must be capable of being quickly applied. The writer believes for city milk inspectors that the acid test would serve a very useful purpose. This test measures the acidity of the milk. There is, of course, no close and direct relationship between the development of acidity and the growth of bacteria, yet in a general way one follows the other at normal temperatures. Where the temperature is kept rather low, bacterial growth might go on without much acid development, but in the great majority of cases a high degree of acidity means either old milk, in which there has been a long period of incubation, or high temperature, where rapid bacterial growth has been possible. Either of these conditions encourages germ growth and thus impairs the quality of the milk.

The rapid determination of acidity may be made in an approximate manner so as to serve as a test at the weigh-can or intake. The test is best made by the use of the well known alkaline tablet which is composed of a solid alkali, and the indicator, phenolphthalein. The tablets are dissolved in water, one to each ounce used. A number of white cups are filled with the proper quantity of the solution necessary to neutralize say, 0.2 per cent. lactic acid. Then, as the milk is delivered, the proper quantity is taken from each can to which is added the tablet solution. A retention of the pink color shows that there is not sufficient acid in the milk to neutralize the alkali used; a disappearance of color indicates an excess of acid. The standard selected is of course arbitrarily chosen. In our experience, 0.2 per cent. acidity (figured as lactic), has proven a satisfactory point. With carefully handled milk the acidity ought to be reduced to about 0.15 per cent.The acidity of the milk may be abnormally reduced if milk is kept in rusty cans, owing to action of acid on the metal.

Fig. 15.Fig. 15.

Apparatus used in making rapid acid test. A definite quantity of the alkali solution and indicator is placed in the white tea cup. To this is added the quantity of milk by means of the cartridge measure which would just be neutralized if the acidity was 0.2 per cent. A retention of the pink color shows a low acid milk; its disappearance, a high acid milk.

Kinds of bacteria in milk.The number of bacteria in milk is not of so much consequence as the kinds present. With reference to the number of different species, the more dirt and foreign matter the milk contains, the larger the number of varieties found in the same. While milk may contain forms that are injurious to man, still the great majority of them have no apparent effect on human health.In their effect on milk, the case is much different. Depending upon their action in milk, they may be grouped into three classes:

1. Inert group, those producing no visible change in the milk.

2. Sour milk forms, those breaking up the milk sugar with or without the formation of gas.

3. Digesting or peptonizing group, those capable of rendering the casein of milk soluble and more or less completely digested.

A surprisingly large number of bacteria that are found in milk belong to the first class. Undoubtedly they affect the chemical characteristics of the milk somewhat, but not to the extent that it becomes physically perceptible. Eckles[46]reports in a creamery supply from 20 to 55 per cent. of entire flora as included in this class.

By far the most important group is that embraced under the second head. It includes not only the true lactic acid types in which no gas is formed, but those species capable of producing gases and various kinds of acids. These organisms are the distinctively milk bacteria, although they do not predominate when the milk is first drawn. Their adaptation to this medium is normally shown, however, by this extremely rapid growth, in which they soon gain the ascendency over all other species present. It is to this lactic acid class that the favorable flavor-producing organisms belong which are concerned in butter-making. They are also indispensable in cheese-making.

The third class represents those capable of producing a liquefied or digested condition on gelatin or in milk. Theyare usually the spore-bearing species which gain access from filth and dirt. Their high powers of resistance due to spores makes it difficult to eradicate this type, although they are materially held in subjection by the lactic bacteria. The number of different kinds that have been found in milk is quite considerable, something over 200 species having been described more or less thoroughly. In all probability, however, many of these forms will be found to be identical when they are subjected to a more critical study.

Direct absorption of taints.A tainted condition in milk may result from the development of bacteria, acting upon various constituents of the milk, and transforming these in such a way as to produce by-products that impair the flavor or appearance of the liquid; or it may be produced by the milk being brought in contact with any odoriferous or aromatic substance, under conditions that permit of the direct absorption of such odors.

This latter class of taints is entirely independent of bacterial action, and is largely attributable to the physical property which milk possesses of being able to absorb volatile odors, the fat in particular, having a great affinity for many of these substances. This direct absorption may occur before the milk is withdrawn from the animal, or afterwards if exposed to strong odors.

It is not uncommon for the milk of animals advanced in lactation to have a more or less strongly marked odor and taste; sometimes this is apt to be bitter, at other times salty to the taste. It is a defect that is peculiar to individual animals and is liable to recur at approximately the same period in lactation.

The peculiar "cowy" or "animal odor" of fresh milk is an inherent peculiarity that is due to the direct absorption of volatile elements from the animal herself. This condition is very much exaggerated when the animal consumes strong-flavored substances as garlic, leeks, turnips and cabbage. The volatile substances that give to these vegetables their characteristic odor are quickly diffused through the system, and if such foods are consumed some few hours before milking, the odor in the milk will be most pronounced. The intensity of such taints is diminished greatly and often wholly disappears, if the milking is not done for some hours (8-12) after such foods are consumed.

This same principle applies in lesser degree to many green fodders that are more suitable as feed for animals, as silage, green rye, rape, etc. Not infrequently, such fodders as these produce so strong a taint in milk as to render it useless for human use. Troubles from such sources could be entirely obviated by feeding limited quantities of such material immediately after milking. Under such conditions the taint produced is usually eliminated before the next milking. The milk of swill-fed cows is said to possess a peculiar taste, and the urine of animals fed on this food is said to be abnormally acid. Brewers' grains and distillery slops when fed in excess also induce a similar condition in the milk.

Milk may also acquire other than volatile substances directly from the animal, as in cases where drugs, as belladonna, castor oil, sulfur, turpentine, jalap, croton oil, and many others have been used as medicine. Such mineral poisons as arsenic have been known to appear eight hours after ingestion, and persist for a period of three weeks before being eliminated.

Absorption of odors after milking.If milk is brought in contact with strong odors after being drawn from the animal, it will absorb them readily, as in the barn, where frequently it is exposed to the odor of manure and other fermenting organic matter.

It has long been a popular belief that milk evolves odors and cannot absorb them so long as it is warmer than the surrounding air, but from experimental evidence, the writer[47]has definitely shown that the direct absorption of odors takes place much more rapidly when the milk is warm than when cold, although under either condition, it absorbs volatile substances with considerable avidity. In this test fresh milk was exposed to an atmosphere impregnated with odors of various essential oils and other odor-bearing substances. Under these conditions, the cooler milk was tainted very much less than the milk at body temperature even where the period of exposure was brief. It is therefore evident that an exposure in the cow barn where the volatile emanations from the animals themselves and their excreta taint the air will often result in the absorption of these odors by the milk to such an extent as to seriously affect the flavor.

The custom of straining the milk in the barn has long been deprecated as inconsistent with proper dairy practice, and in the light of the above experiments, an additional reason is evident why this should not be done.

Even after milk is thoroughly cooled, it may absorb odors as seen where the same is stored in a refrigerator with certain fruits, meats, fish, etc.

Distinguishing bacterial from non-bacterial taints.In perfectly fresh milk, it is relatively easy to distinguish betweentaints caused by the growth of bacteria and those attributable to direct absorption.

If the taint is evident at time of milking, it is in all probability due to character of feed consumed, or possibly to medicines. If, however, the intensity of the taint grows more pronounced as the milk becomes older, then it is probably due to living organisms, which require a certain period of incubation before their fermentative properties are most evident.

Moreover, if the difficulty is of bacterial origin, it can be frequently transferred to another lot of milk (heated or sterilized is preferable) by inoculating same with some of the original milk. Not all abnormal fermentations are able though to compete with the lactic acid bacteria, and hence outbreaks of this sort soon die out by the re-establishment of more normal conditions.

Treatment of directly absorbed taints.Much can be done to overcome taints of this nature by exercising greater care in regard to the feed of animals, and especially as to the time of feeding and milking. But with milk already tainted, it is often possible to materially improve its condition. Thorough aeration has been frequently recommended, but most satisfactory results have been obtained where a combined process of aeration and pasteurization was resorted to. Where the milk is used in making butter, the difficulty has been successfully met by washing the cream with twice its volume of hot water in which a little saltpeter has been dissolved (one teaspoonful per gallon), and then separating it again.[48]

The treatment of abnormal conditions due to bacteria has been given already under the respective sources of infection,and is also still further amplified in following chapter.

Aeration.It is a common belief that aeration is of great aid in improving the quality of milk, yet, when closely studied, no material improvement can be determined, either where the milk is made into butter or sold as milk. Dean in Canada and Storch in Denmark have both experimented on the influence of aeration in butter making, but with negative results. Marshall and Doane failed to observe any material improvement in keeping quality, but it is true that odors are eliminated from the milk during aeration. The infection of the milk during aeration often more than counterbalances the reputed advantage. Especially is this so if the aeration is carried out in an atmosphere that is not perfectly clean and pure.

In practice aeration differs greatly. In some cases, air is forced into the milk; in others, the milk is allowed to distribute itself in a thin sheet over a broad surface and fall some distance so that it is brought intimately in contact with the air. This latter process is certainly much more effective if carried out under conditions which preclude infection. It must be remembered that aeration is frequently combined with cooling, in which case the reputed advantages may not be entirely attributable to the process of aeration.

Infection of milk in the factory.The problem of proper handling of milk is not entirely solved when the milk is delivered to the factory or creamery, although it might be said that the danger of infection is much greater while the milk is on the farm.

In the factory, infection can be minimized because effective measures of cleanliness can be more easily applied.Steam is available in most cases, so that all vats, cans, churns and pails can be thoroughly scalded. Special emphasis should be given to the matter of cleaning pumps and pipes. The difficulty of keeping these utensils clean often leads to neglect and subsequent infection. In Swiss cheese factories, the custom of using home-made rennet solutions is responsible for considerable factory infection. Natural rennets are soaked in whey which is kept warm in order to extract the rennet ferment. This solution when used for curdling the milk often adds undesirable yeasts and other gas-generating organisms, which are later the cause of abnormal ferment action in the cheese (See page 186).

The influence of the air on the germ content of the milk is, as a rule, overestimated. If the air is quiet, and free from dust, the amount of germ life in the same is not relatively large. In a creamery or factory, infection from this source ought to be much reduced, for the reason that the floors and wall are, as a rule, quite damp, and hence germ life cannot easily be dislodged. The majority of organisms found under such conditions come from the person of the operators and attendants. Any infection can easily be prevented by having the ripening cream-vats covered with a canvas cloth. The clothing of the operator should be different from the ordinary wearing-apparel. If made of white duck, the presence of dirt is more quickly recognized, and greater care will therefore be taken than if ordinary clothes are worn.

The surroundings of the factory have much to do with the danger of germ infection. Many factories are poorly constructed and the drainage is poor, so that filth and slime collect about and especially under the factory. The emanations from these give the peculiar "factory odor" that indicates fermenting matter. Not only are these odorsabsorbed directly, but germ life from the same is apt to find its way into the milk. Connell[49]has recently reported a serious defect in cheese that was traced to germ infection from defective factory drains.

The water supply of a factory is also a question of prime importance. When taken from a shallow well, especially if surface drainage from the factory is possible, the water may be contaminated to such an extent as to introduce undesirable bacteria in such numbers that the normal course of fermentation may be changed. The quality of the water, aside from flavor, can be best determined by making a curd test (p. 76) which is done by adding some of the water to boiled milk and incubating the same. If "gassy" fermentations occur, it signifies an abnormal condition. In deep wells, pumped as thoroughly as is generally the case with factory wells, the germ content should be very low, ranging from a few score to a few hundred bacteria per cc. at most.

Harrison[50]has recently traced an off-flavor in cheese in a Canadian factory to an infection arising from the water-supply. He found the same germ in both water and cheese and by inoculating a culture into pasteurized milk succeeded in producing the undesirable flavor. The danger from ice is much less, for the reason that good dairy practice does not sanction using ice directly in contact with milk or cream. Then, too, ice is largely purified in the process of freezing, although if secured from a polluted source, reliance should not be placed in the method of purification; for even freezing does not destroy all vegetating bacteria.

FOOTNOTES:[1]Olson. 24 Rept. Wis. Expt. Stat., 1907.[2]Erf and Melick Bull. 131, Kan. Expt. Stat., Apr. 1905.[3]Storch (40 Rept. Danish Expt. Stat., Copenhagen, 1898) has devised a test whereby it can be determined whether this treatment has been carried out or not: Milk contains a soluble enzym known as galactase which has the property of decomposing hydrogen peroxid. If milk is heated to 176° F. (80° C.) or above, this enzym is destroyed so that the above reaction no longer takes place. If potassium iodid and starch are added to unheated milk and the same treated with hydrogen peroxid, the decomposition of the latter agent releases oxygen which acts on the potassium salt, which in turn gives off free iodine that turns the starch blue.[4]McKay, N. Y. Prod. Rev., Mch. 22, 1899.[5]Doane, Bull. 79, Md. Expt. Stat., Jan. 1902.[6]Harrison, 22 Rept. Ont. Agr'l Coll., 1896, p. 113.[7]Moore and Ward, Bull. 158, Cornell Expt. Stat., Jan. 1899; Ward, Bull. 178, Cornell Expt. Stat., Jan. 1900.[8]Harrison, 22 Rept. Ont. Agr. Coll., 1896, p. 108; Moore, 12 Rept. Bur. Animal Ind., U. S. Dept. Ag., 1895-6, p. 261.[9]Moore, Bacteria in Milk, N. Y. Dept. Ag., 1902.[10]Freudenreich, Cent. f. Bakt., II Abt., 10: 418, 1903.[11]Harrison, 22 Rept. Ont. Agr. Coll., 1896, p. 108.[12]Marshall, Bull. 147, Mich. Expt. Stat., p. 42.[13]Moore and Ward, Bull. 158, Cornell Expt. Stat., Jan. 1899.[14]Burr, R. H. Cent. f. Bakt., II Abt., 8: 236, 1902. Freudenreich, l. c. p. 418. Ward, Bull. 178, Cornell Expt. Stat., p. 277. Bolley (Cent. f. Bakt., II Abt., 1: 795, 1895), in 30 experiments found 12 out of 16 species to belong to lactic class. Harrison (Trans. Can. Inst., 7: 474, 1902-3) records the lactic type as most commonly present.[15]Ford, Journ. of Hyg., 1901, 1: 277.[16]Freudenreich, l. c. p. 421.[17]Stocking, Bull. 42, Storrs Expt. Stat., June, 1906.[18]Dinwiddie, Bull, 45 Ark. Expt. Stat., p. 57. Ward, Journ. Appld. Mic. 1: 205, 1898. Appel, Milch Zeit., No. 17, 1900. Harrison and Cumming, Journ. Appld. Mic. 5: 2087. Russell and Hastings, 21 Rept. Wis. Expt. Stat., 158, 1904.[19]Fokker, Zeit. f. Hyg., 9: 41, 1890.[20]Freudenreich, Ann. de Microg., 3: 118, 1891.[21]Hunziker, Bull. 197, Cornell Expt. Stat., Dec. 1901.[22]Freudenreich, Cent. f. Bakt., II Abt., 10: 417, 1903.[23]This general statement is in the main correct, although Ford (Journ. of Hyg., 1: 277, 1901) claims to have found organisms sparingly present in healthy tissues.[24]Backhaus, Milch Zeit., 26: 357, 1897.[25]Freudenreich, Die Bakteriologie, p. 30.[26]Stocking, Bull. 42, Storrs Expt. Stat., June 1906.[27]Harrison, Cent. f. Bakt., II Abt., 5: 183, 1899.[28]Drysdale, Trans. High. and Agr. Soc. Scotland. 5 Series, 10: 166, 1898.[29]Schuppan, (Cent. f. Bakt., 13: 155, 1893) claims to have found a reduction of 48 per cent. in the Copenhagen filters while in the more extended work of Dunbar and Kister (Milch Zeit., pp. 753, 787, 1899) the bacterial content was higher in the filtered milk in 17 cases out of 22.[30]Backhaus and Cronheim, Journ. f. Landw., 45: 222, 1897.[31]Eckles and Barnes, Bull. 159 Iowa Expt. Stat., Aug. 1901.[32]Dunbar and Kister, Milch Zeit., p. 753, 1899. Harrison and Streit, Trans. Can. Inst., 7: 488, 1902-3.[33]Doane, Bull. 88 Md. Expt. Stat., May 1903.[34]Eckles, Hoard's Dairyman, July 8, 1898.[35]Fraser, Bull. 91, Ill. Expt. Stat.[36]Fraser, Bull. 91, Ill. Expt. Stat., Dec. 1903.[37]Stocking, Bull. 42, Storrs Expt. Stat., June, 1906.[38]Backhaus. Ber. Landw. Inst. Univ. Königsberg 2: 12, 1897.[39]De Schweinitz, Nat. Med. Rev., April, 1899.[40]Conn, Proc. Soc. Amer. Bacteriologists, 1902.[41]Freudenreich, Ann. de Microg., 2:115, 1890.[42]Conn, Bull. 26, Storrs Expt. Stat.[43]New York City is supplied with milk that is shipped 350 miles.[44]Park, N. Y. Univ. Bull., 1: 85, 1901.[45]Eckles, Bull. 59, Iowa Expt. Stat., Aug. 1901.[46]Eckles, Bull. 59, Iowa Expt. Stat., Aug. 1901.[47]Russell, 15 Rept. Wis. Expt. Stat. 1898, p. 104.[48]Alvord, Circ. No. 9, U. S. Dept. Agric. (Div. of Bot.).[49]Connell, Rept. of Commissioner of Agr., Canada, 1897, part XVI, p. 15.[50]Harrison, Hoard's Dairyman, March 4, 1898.

[1]Olson. 24 Rept. Wis. Expt. Stat., 1907.

[2]Erf and Melick Bull. 131, Kan. Expt. Stat., Apr. 1905.

[3]Storch (40 Rept. Danish Expt. Stat., Copenhagen, 1898) has devised a test whereby it can be determined whether this treatment has been carried out or not: Milk contains a soluble enzym known as galactase which has the property of decomposing hydrogen peroxid. If milk is heated to 176° F. (80° C.) or above, this enzym is destroyed so that the above reaction no longer takes place. If potassium iodid and starch are added to unheated milk and the same treated with hydrogen peroxid, the decomposition of the latter agent releases oxygen which acts on the potassium salt, which in turn gives off free iodine that turns the starch blue.

[4]McKay, N. Y. Prod. Rev., Mch. 22, 1899.

[5]Doane, Bull. 79, Md. Expt. Stat., Jan. 1902.

[6]Harrison, 22 Rept. Ont. Agr'l Coll., 1896, p. 113.

[7]Moore and Ward, Bull. 158, Cornell Expt. Stat., Jan. 1899; Ward, Bull. 178, Cornell Expt. Stat., Jan. 1900.

[8]Harrison, 22 Rept. Ont. Agr. Coll., 1896, p. 108; Moore, 12 Rept. Bur. Animal Ind., U. S. Dept. Ag., 1895-6, p. 261.

[9]Moore, Bacteria in Milk, N. Y. Dept. Ag., 1902.

[10]Freudenreich, Cent. f. Bakt., II Abt., 10: 418, 1903.

[11]Harrison, 22 Rept. Ont. Agr. Coll., 1896, p. 108.

[12]Marshall, Bull. 147, Mich. Expt. Stat., p. 42.

[13]Moore and Ward, Bull. 158, Cornell Expt. Stat., Jan. 1899.

[14]Burr, R. H. Cent. f. Bakt., II Abt., 8: 236, 1902. Freudenreich, l. c. p. 418. Ward, Bull. 178, Cornell Expt. Stat., p. 277. Bolley (Cent. f. Bakt., II Abt., 1: 795, 1895), in 30 experiments found 12 out of 16 species to belong to lactic class. Harrison (Trans. Can. Inst., 7: 474, 1902-3) records the lactic type as most commonly present.

[15]Ford, Journ. of Hyg., 1901, 1: 277.

[16]Freudenreich, l. c. p. 421.

[17]Stocking, Bull. 42, Storrs Expt. Stat., June, 1906.

[18]Dinwiddie, Bull, 45 Ark. Expt. Stat., p. 57. Ward, Journ. Appld. Mic. 1: 205, 1898. Appel, Milch Zeit., No. 17, 1900. Harrison and Cumming, Journ. Appld. Mic. 5: 2087. Russell and Hastings, 21 Rept. Wis. Expt. Stat., 158, 1904.

[19]Fokker, Zeit. f. Hyg., 9: 41, 1890.

[20]Freudenreich, Ann. de Microg., 3: 118, 1891.

[21]Hunziker, Bull. 197, Cornell Expt. Stat., Dec. 1901.

[22]Freudenreich, Cent. f. Bakt., II Abt., 10: 417, 1903.

[23]This general statement is in the main correct, although Ford (Journ. of Hyg., 1: 277, 1901) claims to have found organisms sparingly present in healthy tissues.

[24]Backhaus, Milch Zeit., 26: 357, 1897.

[25]Freudenreich, Die Bakteriologie, p. 30.

[26]Stocking, Bull. 42, Storrs Expt. Stat., June 1906.

[27]Harrison, Cent. f. Bakt., II Abt., 5: 183, 1899.

[28]Drysdale, Trans. High. and Agr. Soc. Scotland. 5 Series, 10: 166, 1898.

[29]Schuppan, (Cent. f. Bakt., 13: 155, 1893) claims to have found a reduction of 48 per cent. in the Copenhagen filters while in the more extended work of Dunbar and Kister (Milch Zeit., pp. 753, 787, 1899) the bacterial content was higher in the filtered milk in 17 cases out of 22.

[30]Backhaus and Cronheim, Journ. f. Landw., 45: 222, 1897.

[31]Eckles and Barnes, Bull. 159 Iowa Expt. Stat., Aug. 1901.

[32]Dunbar and Kister, Milch Zeit., p. 753, 1899. Harrison and Streit, Trans. Can. Inst., 7: 488, 1902-3.

[33]Doane, Bull. 88 Md. Expt. Stat., May 1903.

[34]Eckles, Hoard's Dairyman, July 8, 1898.

[35]Fraser, Bull. 91, Ill. Expt. Stat.

[36]Fraser, Bull. 91, Ill. Expt. Stat., Dec. 1903.

[37]Stocking, Bull. 42, Storrs Expt. Stat., June, 1906.

[38]Backhaus. Ber. Landw. Inst. Univ. Königsberg 2: 12, 1897.

[39]De Schweinitz, Nat. Med. Rev., April, 1899.

[40]Conn, Proc. Soc. Amer. Bacteriologists, 1902.

[41]Freudenreich, Ann. de Microg., 2:115, 1890.

[42]Conn, Bull. 26, Storrs Expt. Stat.

[43]New York City is supplied with milk that is shipped 350 miles.

[44]Park, N. Y. Univ. Bull., 1: 85, 1901.

[45]Eckles, Bull. 59, Iowa Expt. Stat., Aug. 1901.

[46]Eckles, Bull. 59, Iowa Expt. Stat., Aug. 1901.

[47]Russell, 15 Rept. Wis. Expt. Stat. 1898, p. 104.

[48]Alvord, Circ. No. 9, U. S. Dept. Agric. (Div. of Bot.).

[49]Connell, Rept. of Commissioner of Agr., Canada, 1897, part XVI, p. 15.

[50]Harrison, Hoard's Dairyman, March 4, 1898.

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 caseinis 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 theBact. lactis aciditype.[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 isB. 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 CO2are most common although N and methane (CH4) are sometimes produced). The more common forms are those represented byB. lactis aerogenesand 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 phenomenonof 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.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 andsomewhat 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 namecaseaseor 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. Whenthe 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 othersstringing 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.

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