Dr. Gustaf De Laval of Sweden, inventor of the separator bearing his name
In the machine the milk is separated instantaneously by centrifugal force and runs out through two tubes, one for cream and the other for skim milk. A small modern hand machine will take care of from 200 to 1,000 lbs. of milk an hour, and power machines are built to separate 6,000 lbs. or more. By regulating the cream-outlet a heavier or lighter cream can be produced.
Hand separator
Suppose we are separating some milk containing 4% butter-fat. If ⅛ of this milk is separated out as cream and contains all the butter-fat, the cream will be eight times as rich in butter-fat as the original milk; 32% of the cream will be butter-fat. Such cream is called “32% cream.” If we take ¼ as cream, we get a cream four times as rich as the milk, a “16% cream.” So out of 100 lbs. of milk we can take 12½ lbs. (⅛) of 32% cream, 25 lbs. (¼) of 16% cream, etc.
Although the skim milk is really valuable as a food, it is worth but little commercially; the cream containing the butter-fat is the expensive part of the milk, and we must be prepared to pay for the cream all that the milk would have cost.
Percentage of Butter-Fat.—The “richness” of cream or milk and their value depend upon the amount of butter-fat in them. So cream or milk is often called “30% cream” or “20% cream” or “4% milk” according as 30% or 20% or 4% of the fluid is butter-fat; 30% cream is quite rich; ordinary market cream varies between 18% and 40%, though it may fall below 18%, or it may be sold as a very expensive article as high as 60%. Rich milk may contain more than 6% of butter-fat and skim milk less than 0.1 of one per cent. The average for good whole milk is between 3% and 4½%.
For a long time scientists and scientific dairymen were the only ones to speak of milk and cream in terms of percentage of butter-fat. Now, however, people are beginning to realize how valuable a part of the milk the butter-fat is and are paying more attention to the actual percentage of butter-fat in the cream or milk they use. So it is no longer unusual to see a dairyman advertise cream of a certain percentage or to hear a housewife ask for it specifically.
Standardizing Cream.—For ice cream or for preparing modified milk for babies, it is often desirable to dilute rich cream to a certain lower standard. The following simple steps can be taken to find out how much milk to add for diluting: