35. Sources and control of bacteria in milk.—Most of the bacterial infection of milk is due to lack of care in handling. Some of the common sources[15] of contamination are: the air in the stable; the cow's body; the milker; the utensils; the method of handling the milk after it is drawn from the cow; unclean cheese factory conditions.
Since bacteria cause various kinds of fermentation, not only in the milk but in the products manufactured from it, the question of their control is of prime importance. There are two ways in which the bacterial growth in milk used for cheese-making may be controlled: (1) prevention of infection; (2) the retardation of their development when present. The former is accomplished by strict cleanliness, the latter by adequate cooling.
36. The cow.—The body of the cow may be a source of bacterial contamination. Bacteria adhere to the hair of the animal, and to the scales of the skin, and during the process of milking these are very liable to fall into the milk. To prevent this, the cow should be curried to remove all loose material and hair. Just before milking, the udder and flank should be wiped with a damp cloth; this removes some of the material, and causes the remainder to adhere to the cow.
37. Stable air.—If the air of the stable is not clean, it will be a source of contamination. Particles of dust floating in the air carry more or less bacteria, and these fall into the milk during the process of milking. To keep the stable air free from dust at milking time, all operations which stir up dust, such as feeding, brushing the cows, cleaning the floor, should be practiced after milking or long enough before so that the dust will have settled. It is a good plan to close the doors and to sprinkle the floor just before milking.
38. The milker himself may be a source of contamination. He should be clean and wear clean clothing. The hands should not be wet with milk during milking.
Fig. 3.—Types of small-top milk pails.
39. Utensils.—The utensils are an important source of bacterial contamination. The bacteria lodge in the seams and corners unless these are well-flushed with solder. From these seams they are not easily removed. When fresh warm milk is placed into such utensils, the bacteria begin to grow and multiply. All utensils with which milk comes in contact should first be rinsed with cold water and then thoroughly washed and finally scalded with boiling water, and drained or blown absolutely dry. They should then be placed in an atmosphere free from dust until wanted for use again. If an aërator is used, this should be operated in pure air, free from odors and dust. One of the greatest sources of bacterial contamination of cheese milk is the use of the milk-cans to return whey to the farms for pig feed. Frequently, sour whey is left in the cans until ready to feed. These cans are then not properly washed and scalded. The practice of pasteurizing the whey at the cheese factory is a great help in preventing this source of infection and the spreading of disease.