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.
White-of-egg consists chiefly of water and a substance called albumin (from a word meaning white). Albumin in its natural state is clear and soluble in water. In white-of-egg it seems sticky because enclosed by invisible cell-walls. When the white-of-egg is heated, the albumin hardens, or coagulates.1
Albumin is one of a class of foodstuffs called proteins, or collectively protein.
How did we show that starch contained carbon, hydrogen, and oxygen? (pp. 69-70.) We can prove in the same way that albumin contains these three elements. Let us see if albumin contains any element not found in starch.
1 Yolk albumin coagulates at a lower temperature than white albumin. In a "three-minute" boiled egg, however, the white is hard and the yolk nearly or quite raw, the heat not having had time to penetrate to the centre of the egg.
Heat some dried white-of-egg (albumin) with a little lime in a test-tube. Note odor of ammonia which comes off.
Ammonia contains nitrogen. There is no nitrogen in lime. Therefore the nitrogen must have come from the albumin. The presence of nitrogen in any protein may be shown by heating it with lime. A test for nitrogen in food is a test for protein.
Another test for protein is nitric acid (p. 151).
Proteins may serve as fuel, like carbohydrates, but besides this they do what carbohydrates cannot do. They build living tissues, such as muscle, blood, nerves. Without nitrogen they could not do this. This tissue-building power associated with nitrogen makes proteins so different from other foodstuffs that writers have sometimes divided all foodstuffs into two classes, nitrogenous and non-nitrogenous. The importance of protein is suggested by its name, which comes from a word meaning "first." Foods containing considerable protein are called protein foods.
The digestion of albumin begins in the stomach, which secretes for the purpose a fluid called gastric juice, containing pepsin and hydrochloric acid. Gastric juice softens solid proteins such as cooked white-of-egg and changes all proteins into new substances. (For completion of process, see Chap. 15.)
Label three test-tubes a, b, and c, respectively. Into a put about one teaspoonful of the finely chopped white of a hard-boiled egg; into b an equal quantity of the chopped white of a soft-cooked egg (see recipe), and into c a piece of hard boiled white, not chopped. Half fill the test-tubes with pepsin and dilute hydrochloric acid,1 and set them in warm water (about 98° F.).
U. S. Department of Agriculture Office of Experiment Stations A. C. True: Director
C. F. Langworthy
Expert in Charge of Nutrition Investigations
Composition Of Food Materials.
At the end of an hour and a half examine them. In which has the white-of-egg been most rapidly liquefied, i.e., digested? After five or six hours look at them again. If any of the egg is still undigested set it aside and look at it again the next day. Does white-of-egg digest more quickly in one piece or chopped? Which digests more quickly, hard-boiled or soft-cooked albumin?
Eggs are completely digestible. They contain no waste. They are most quickly and easily acted upon by gastric juice when cooked at a temperature not higher than 180. They are probably hardest to digest when fried.
Eggs are about three-fourths water. But of the nutritive material in them more than half is protein. This makes them one of the richest of protein foods, and so one of the most valuable as a tissue-builder. The mineral matter includes valuable compounds of calcium, iron, and phosphorus. One-third of the yolk is fat in the form of oil. What class of foodstuffs do eggs lack? What foods commonly eaten with eggs supply this lack?
Make a table showing the composition of eggs similar to that showing the composition of potatoes on p. 62. Potatoes contain a little albumin. It shows as froth on the water squeezed out of them, and coagulates if the water be boiled. Try this, and if you observe the albumin, note it in your potato table.