This section is from the book "Elements Of The Theory And Practice Of Cookery", by Mary E. Williams. Also available from Amazon: Elements Of The Theory And Practice Of Cookery; A Textbook Of Domestic Science For Use In Schools.
A. With a spoon remove the cream from one pint of milk that has stood overnight. Drop a little of the cream on unglazed paper. Examine the paper after it has dried for a time. Can you tell from the spot one foodstuff that is present in milk?
B. Test a little of the milk with iodine. Is there any starch in milk?
C. Boil the rest of the milk. What do you see on the top of it ? What do you think this skin is? Is there water in milk? Air? How do you know?
D. Remove the skin and put a little of the milk in a test-tube. Add a few drops of vinegar. What happens? Strain the milk through a cloth, and examine the solid substance (curd) and the watery liquid (whey).
F. Dry some of the milk-curd, heat it with lime, and note the odor of ammonia. What must the curd contain?
Theory And Practice Of Cookery
U. S. Department of Agriculture Office of Experiment Stations A. C. True: Director
C. F. Langworthy
Expert in Charge of Nutrition Investigations
Cow's milk contains fat, albumin, and a substance which is coagulated by vinegar but not by heat. This is casein,l a protein. Milk also contains a carbohydrate, milk-sugar, or lactose, and mineral matter. All of these except the fat are dissolved in water,2 which forms almost nine-tenths the bulk of the milk. Name one element contained in casein that is lacking in milk-sugar. What difference does this make in its work in the body?
Milk serves all the purposes of food and drink. The protein in it builds all kinds of tissue. The protein, fat, and sugar all give heat and energy, and if not needed for the immediate use of the body they may be changed into fatty tissue. The mineral matter supplies calcium to harden bones and tissues, and phosphorus, some of which helps to build the nerves and brain. Some of the phosphorus is combined with casein. One can live upon milk for a long time. It is a good food to grow on, and it does not produce some of the acid and poisonous waste that meat does. We should do well to use more milk and less meat. (See pp. 142-143.) Mention some foods commonly eaten with milk. What foodstuff or foodstuffs, lacking in milk, do these foods supply? Remember that milk is food, not drink merely; less of other food is needed at a meal with which milk is drunk. Drink it slowly. It is more readily digested when taken in sips.
1 Coagulated casein, when dried, is a hard, horny, yellow solid. It can be so toughened as to resemble celluloid, a state in which it is made into buttons and similar articles.
2 The albumin is in true solution, the casein in partial solution only. In this state it is called by chemists caseinogen (casein-maker).
Fresh, unskimmed milk should be creamy white, and cream should rise on standing. There should be no dirt nor other sediment in it. Milk is one of the hardest foods to keep clean, pure, and sweet. It may look all right and yet be unfit for food. In most places where the milk sold comes from a distance, the law requires it to be up to a certain standard of quality and purity. Only bottled milk is safe to buy in cities.
Skim milk is much cheaper than whole milk. Of course it contains little fat, but it contains more protein, sugar, and mineral matter than an equal quantity of whole milk.
Put milk as soon as you get it in the coolest place you have. Wipe the mouth of the bottle before removing the cap. After pouring out what milk you need, cover the bottle, and set it away. Do not mix old milk with new. Keep milk away from anything with an odor.
G (continued from page 93). Let fresh milk stand in a warm room 24 hours, or until it thickens. Stir it, and notice the separation of the curd from the whey. What substance, added to milk, makes it separate like this? Taste the milk. How does it taste ?
Sour milk. Lactic acid. - When milk is kept at the ordinary temperature, some of its sugar turns into lactic acid (milk acid), which gives the milk a sour taste, and like the acid of vinegar, coagulates the casein. As butter is usually made from sour cream (p. 100), buttermilk is sour. Either buttermilk or milk purposely soured by a special process makes a good drink, better for some people than sweet milk. (See Fermented milks, p. 332.)
The formation of lactic acid from milk-sugar is caused by the action of certain bacteria called lactic acid bacteria. Other kinds of bacteria spoil it in other ways, producing sliminess, bad odor, and other unpleasant effects. A few of these bacteria are disease germs. Bacteria grow rapidly in milk at ordinary temperatures. They get into it from unclean surroundings and from the air. Therefore in order to have milk when delivered as free as possible from bacteria, it must be drawn in a cleanly way, cooled, and kept cold in clean vessels protected from the air.
Pasteurization is a process in which milk is heated and then rapidly cooled. Its purpose is to kill any disease germs that may be present and to reduce the number of other microorganisms without injuring the taste or lessening the food value of the milk. The process is named after Pasteur, the eminent French bacteriologist. Pasteurization cannot make dirty milk clean. It can make clean milk safe. Properly pasteurized milk will keep longer than unpasteurized milk. It sours in time because not all the lactic acid forming bacteria have been killed.
Pasteurize the milk in bottles before opening. Pasteurizers may be bought, but one can be contrived more cheaply. You will need a pail or kettle, a tin pie-plate, an accurate thermometer, and a clean bath-towel or other thick cloth. Punch a few holes in the pie-plate, and put it upside down in the kettle, to keep the bottles from touching the bottom. Punch a hole through the cap of one of the bottles, and insert the thermometer. Set the bottles on the plate in the pail, and fill the pail with water nearly to the level of the milk. Heat until the thermometer registers 145° F.1 Remove the bottles. Take out the thermometer. Replace the punctured cap by a whole one. Cover the bottles closely with the cloth and let them stand from twenty to thirty minutes. Cool quickly by placing them in water. Have the water warm at first and run cold water into it to avoid breaking the bottles. After cooling, keep them on ice or in the coldest place you have. Pasteurized milk requires the same care as raw milk. (For more about milk see pp. 314, 315.)
1 The temperature may go to 150° F. without harm, but no higher.