Processing of Milk
Milk as delivered to the processing plant, is first clarified while cold. Clarification consists of passing milk through a centrifuge similar to a cream separator but operated at low speed. This treatment is sufficient to separate out dirt and sediment that might be present, depositing them as a layer on the inner surface of the centrifuge bowl. The clarifier is not operated at sufficient speed to separate the cream from the milk. After clarification, the milk is usually pumped into a storage tank equipped with an agitator. While in the storage tank, the milk is sampled, and butterfat content is determined. It is then standardized by adding enough cream or skim milk (milk from which cream has been removed) to provide the fat content required by state regulations. The milk is then fortified with vitamin D at the rate of 400 USP units per quarts (0.95 liter).
The next step in fluid milk processing is to pasteurize it. During pasteurization, milk must be heated in all parts to 145 degree F and held at this temperature for 30 min, or it must be quickly heated to 161 degree F and held at this temperature for 15 sec, heated to 191 degree F and held for 1 sec or heated to 194 degree and held for 0.5 sec. then cooled. Pasteurizing at temperatures above 145 degree F is called the high temperature short time (HTST) method, and both heating and cooling are carried out over a short period of time. Milk may be pasteurized in insulated vats heated by coils carrying hot water, or in vats heated by hot water sprayed within a jacket surrounding the sides and bottom of the vat. With low temperature pasteurization, the milk is ordinarily agitated during heating and cooling. Plate heater and cooler or tubular heaters may be used to pasteurize and cool milk. When tubular heaters are used, the product travels in one direction through an inner tube while the hot water for heating or the refrigerated liquid for cooling passes in the opposite direction though an outer tube surrounding that which carries the product.
Processing of Milk
