The main purpose of this process is to remove impurities from milk. If process conditions are favorable, there will also be an efficient reduction in the amount of leucocytes and bacteria. All centrifuges perform as clarifiers, but normally only machines with a high hydraulic capacity are used for this application. The clarifier can operate either with cold milk below 46ºF (8ºC) or hot milk, 122 – 140ºF (50-60ºC).
Hot Milk Separation
The largest application for centrifuges in dairies is hot milk separation. The objective is to separate the globular milk fat from the serum, the skim milk. The separation process is normally incorporated into the pasteurization line and also combined with a direct, in line fat standardization system for both milk and cream. Separation normally takes place at 122-140ºF (50-60ºC). The fat content of the cream discharged from the separator can be regulated to a level between 20 and 70%.
To maintain optimum fate separation conditions during a long period of time, the fines should be removed from the whey before separation. The most efficient way to reduce the fines is to install a centrifugal hermetic clarifier ahead of the whey separator. The interval between sediment discharges from the fat separator can then be as long as that for the milk. Flow rate, fines content and skimming efficiency are important parameters for the choice of clarifier.
Cold Milk Separation
Cold milk separation at 39-41ºF (4-5ºC) is practiced when heating the milk is undesirable. The viscosity and characteristics of the cream allow temperatures preclude any other form except hermetic separation. Owing to less favorable processing conditions, the skimming efficiency of a cold milk separator is lower than that of the corresponding hot milk type. The fat content in the skim milk can vary between 0.1 and 0.2% and the maximum fat content in the cream at 39ºF (4ºC) is normally 45%.
Whey contains small amounts of fat which vary between 0.15 and 0.40% and have to be removed before further processing. The whey characteristics differ from those of milk in such a manner that the conditions of whey separation are more favorable. If the whey is pre-clarified, the skimming efficiency can be equal to that achieved in milk separation. Hermetic whey separators can also produce cream with a fat content higher than 40% and at temperatures below 95ºF (35ºC).