Sunday, October 5, 2008

Milk Processing

Milk Processing Industry

At a dairy plant, milk is tested to determine its milk-fat content. Different batches may then be blended to adjust the fat content. To meet the U.S. standards, fresh whole milk sold in stores must contain a minimum of 3.25% milk fat. Depending on the end use, therefore, bulk milk may be skimmed of much of its fat in a mechanical cream separator. Cream and skim milk, which are produce by separation, are then mixed in different proportions to produced lowfat milk (0.5% to 2.0% fat), whole milk, half and half (10.5% fat), coffee cream (18% fat), and whipping cream (30% fat).

Raw milk will sour quickly because of the presence of bacteria that convert lactose to lactic acid. Pasteurization, the heating of milk to at least 71.7oC (161oF) for 15 seconds destroys these bacteria, and refrigerated pasteurized milk will remain fresh for a week or more. UI-pasteurization, at 137.8 oC (280 oF) for 2 seconds, allows milk products to be stored at room temperature for several weeks.

To prevent formation of a cream layer, most whole milk undergoes Homogenization, a process in which hot milk is pumped through valves to break up and permanently disperse the fat globules. Off flavors that may have developed in the milk if the cows have eaten succulent weeds, such as wild onion, are removed by subjecting the hot milk to a vacuum in a closed chamber, volatilizing the feed flavor.

Vitamins A and D are contained in the cream component of milk, and both of these vitamins must be added to skim and lowfat milk. One quart of milk supplies 100% of the U.S. recommended daily adult requirement of vitamin D.

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