Outlines of Dairy Bacteriology Part 4

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 between taints 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 p.r.o.nounced 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 b.u.t.ter, 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 b.u.t.ter or sold as milk. Dean in Canada and Storch in Denmark have both experimented on the influence of aeration in b.u.t.ter 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 odors absorbed 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 "ga.s.sy" 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 pota.s.sium 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 pota.s.sium 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 cla.s.s. 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 c.u.mming, 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.