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 "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.