The Story of Milk

by Johan Ditlev Frederiksen

CREAM

When new milk is left at rest the cream will rise to the top and after 12 to 24 hours a cream-line can be seen in the bottle. This cream-line is sharper and more easily seen in raw milk than in pasteurized milk and its absence is not always a sure sign of lack of richness or purity of the milk. By cooling the milk thoroughly so that it will keep, almost all the cream will be at the top in forty-eight hours and can be skimmed off. The cream can be used for coffee or on cereals or fruits or puddings; the skim milk left will still hold ½% or more of butter-fat and can be used to drink or for cooking.

The Separator.—On the farm or in the creamery the cream is no longer raised by gravity, that is, by letting the milk “set” either in shallow pans on the kitchen shelf or in deep cans in ice water, but the fresh, warm milk is run through the separator in a continuous stream.

Early conception of the separator

It was noticed that the rising of the cream due to the difference in specific gravity between the butter-fat and the milk-“serum” (the watery solution of the other constituents) might be greatly hastened by subjecting the milk to centrifugal force. This physical phenomenon was taken advantage of in the first conception of the separator where it was shown that if a pail of milk was whirled around like a stone in a sling the heavier milk-serum would be thrown towards the bottom of the separator pail with so much greater force than the lighter cream (butter-fat mixed with a small part of the serum) that the separation which would take 48 hours in the milk at rest, could be accomplished in a few minutes when exposed to centrifugal force. From this early crude attempt the continuous bowl-separator was developed and still later a number of divisions in the bowl were designed which increased the capacity and efficiency of the machine wonderfully. The most successful separator was designed by Dr. Gustaf De Laval of Sweden and the machines bearing his name are used all over the world where butter is made. But there are many other excellent separators on the market.

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:

1. From the test—fat-percentage—of the rich cream subtract the test desired.

2. From the test desired subtract the test of the milk used for diluting.

3. Divide the first difference by the second, and the result will be the number of pints (or pounds) of milk to be added for each pint (or pound) of the rich cream.

For instance, you may want to thin some 30% cream to 10% for making ice cream. The milk to be added is skim milk. Then:

1. 30 - 10 = 20.

2. 10 - 0 = 10.

3. 20 divided by 10 = 2. So for each pint of rich cream you may add 2 pints of skim milk.

Or you may wish to thin the 30% cream with whole milk, which has 4% butter-fat. Again:

1. 30 - 10 = 20.

2. 10 - 4 = 6.

3. 20 divided by 6 = 3⅓. So you may add 3⅓ pints of the whole milk to each pint of the 30% cream and still have a 10% cream.

Pasteurized Cream does not look as rich as raw cream, and fresh, sweet cream appears to be thinner than when it is 24 hours old and slightly ripened. So it is well, when buying cream, not to judge by appearances. Demand of the milkman that he furnish you cream of a certain percentage of butter-fat and see to it that you get what you pay for. If you have no Babcock tester the milk inspector will test the cream for you.

Whipped Cream.—For whipping, cream must be fairly rich, from 24% to 32%, and it must be cold. Fresh, sweet cream does not whip as readily as that which has been kept for 12 or 24 hours in ice water. There is no other secret connected with the process. Use a rich cream, suitably cooled and aged, and with a good beater there can be no trouble in getting a fine, stiff whipped cream. If the cream is too thin or too warm it may not become stiff. Sometimes, when it is beaten too long, it turns into butter and buttermilk.

Installation in a Danish creamery.
From the separator at the right the cream runs through the continuous pasteurizer which forces it up over the cooler whence it runs into the cream-vat at the left.
(From Boggild—Maelkeribruget i Danmark)

Emulsified Cream.—One of the recent additions to the already elaborate machinery used in the creamery, the milk supply or the ice cream business, is the Emulsifier. To be sure, emulsifiers were used thirty to forty years ago to mix animal and vegetable fats—oleomargarine oil, lard and cottonseed oil—into skim milk for “Filled Cheese” or for Butterine, but lately they are serving new purposes in the milk industry. By forcing melted butter-fat or oil mixed with water or skim milk through exceedingly small apertures under high pressure, or otherwise breaking up the mixture, an emulsion can be formed in which the fat globules are much finer even than those in natural milk or cream, and separation can be prevented. The force used in these emulsifiers may be produced by powerful pumps, or a steam jet, or centrifugal force under high speed; whichever system is used the machines answer the same purpose, to produce a permanent emulsion in which the oil or fat will stay in suspension even after cooling. In some milk supply plants and ice cream factories all the cream is emulsified and the system has especially been applied since the advent of the milk powder. It is now a not infrequent practice to run the new milk through the separator, make sweet, unsalted butter from the cream, and milk powder from the skim milk, and to ship or store these products separately where or until cream is wanted and then bring them together again by running them through the emulsifier with a suitable amount of water. At first thought this process would seem impractical and uneconomical. But it has proven good business because in many cases and places skim milk powder and butter keep better separately or can be shipped at long distance to better advantage than new milk or cream or condensed milk. In ice cream factories business may be dull in cold weather and cream is perhaps not provided and not available, when suddenly a hot spell brings orders for large quantities. With a stock of skim milk powder and butter on hand in the refrigerator, and an emulsifier to mix these products, cream can be produced on short notice and there will be no danger of shortage.