How Milk is Processed

The processing of raw milk for drinking or into other diary products requires stringent control to obtain a product that is hygienic and of high organoleptic quality.

Milk is produced on the dairy farm under good hygienic conditions, primarily as a result of using milking macines. The warm milk (30°C) is strained in the milk house, cooled and stored in cans or storage tanks to avoid sunlight. Milk is usually transported to the processing plant in cans or in the tank of a milk truck. Transportation of milk from mountains to valleys by pipelines made of polyethylene or PVC was first introduced in Austria, France and Switzerland.

The milk is first purified with a continuously-operated clarifier (centrifuge). Contaminants are removed in the sediment. Cream separation is often achieved simultaneously. The process is conducted at 40°C at 5,500-6,500 rpm. Such centrifuges have a flow production capacity of up to 20,000 kg/h. Back-mixing allows the milk fat content to be adjusted as desired.

The fluid milk is heated after clarification to improve its durability and to kill disease-causing microorganisms. Heat treatments used are:

• Pasteurization

The milk is treated: at high temperature (85°C for 2 sec); in a short-time, flash process (71-74°C for 15-40 sec) in plate heaters; or by the low temperature or holder process, in which it is heated at 62-65°C for at least 30 min, with stirring, and it is then cooled.

• Ultrahigh temperature (UHT) treatment

The process involves indirect heating by coils or plates at 135-140°C for 6-10 sec, or direct heating by live steam injection at 140-150°C for 2-4 sec, followed by aseptic packaging.

• Sterilization

Milk in retail packages is heated in autoclaves at 110-120°C for 10-20 min.

Heat treatment affects several milk constituents. Casein, strictly speaking, is not a heat-coagulable protein; it coagulates only at very high temperatures. Heating at 120°C for 5 h dephosphorylates sodium or calcium caseinate solutions (100% and 85%, respectively) and releases 15% of the nitrogen in the form of low molecular weight fragments.

However, temperature and pH strongly affect casein association and cause changes in micellular structure. An example of such a change is the pH-dependent heat coagulation of skim milk.

All pasteurization processes supposedly kill the pathogenic microorganisms in milk. The inactivation of the alkaline phosphatase is used in determining the effectiveness of pasteurization. At higher temperatures or with longer heating time, the whey proteins start to denature-this coincides with the complete inactivation of acid phosphatase.