Elements of the theory and practice of cookery

the table from six to ten minutes. Break the eggs and compare their contents. In which is the white hard and the yolk unchanged? In which is the white hard and the yolk sticky or partly dry? In which is the white a tender jelly and the yolk thick? 155. An egg put into boiling water and removed from the heat is, at the end of about ten minutes, evenly cooked through, the temperature of the water falling during this time to about 168° F. The tem- perature of the egg averages about 185° F. 156. Experiment to show why eggs should be cooked below the boiling-point of water. — Put a little white-of-egg in a test-tube ; hold the test-tube and chemical thermometer in a vessel of water. Heat the water gradually. How does the white-of-egg look at 150° F. ? At 180*^? Stir with glass rod or a stick to show degree of solidity. Note appearance and degree of solidity at 212°. Keep the water boiling for several minutes ; then take out some of the white- of-egg and examine it. Albumin 157. Composition and food value of eggs. — The most important foodstuff in eggs is albumin (from albus, white), a sticky, glairy substance, found in both the white and the yolk. The white is nearly pure albumin and water ; the yolk contains much fat in the form of oil.i Eggs also contain a small quantity of valuable mineral salts. What important foodstuffs do eggs lack ? Name some foods that may be eaten with eggs to supply this lack. Albumin is a tissue -builder, the first foodstuffs we have learned about that contains nitrogen, the element 1 For distinction between oil and fat see § 342. 86 THEORY AND PRACTICE OF COOKERY essential to tissue-building. Plants draw nitrogen from the soil ; animals obtain it from their food, neither being able to make use of it as it exists uncombined in the air. Foodstuffs containing nitrogen, also foods hav- ing a considerable amount of such foodstuffs, are called nitrogenous ; others are called non-nitrogenous. Potatoes contain a little albumin, distinguishable as froth on the water squeezed out in analyzing them. (§ 110.) Make a table showing the composition of eggs, similar to that showing the composition of potatoes on p. 62. 158. Tests for albumin. — A common test for albumin is heat. If potato water or other albuminous liquid be boiled, the albumin can be recognized by its solidifying. 159. The chemical test for nitrogenous substances is nitric acid, which turns albumin and other nitrogenous food- stuffs yellow. Being a dangerous liquid it must he used only by experienced persons. 160. The solidifying of albumin is called coagulation. Have you known more substances to be changed from liquid to solid, or from solid to liquid, by heat ? 161. Digestion of albumin; gastric juice. — The diges- tion of albumin begins in the stomach, which secretes for the purpose a fluid called gastric juice, containing pepsin and hydrochloric acid. These together change tissue-building foodstuffs, including albumin, into more soluble substances. Experiments to show how eggs are digested. — Label three test- tubes a, h, and c, respectively. Into a put about one teaspoonf ul of the finely chopped white of a hard-boiled egg ; into h an equal TISSUE-BUILDING FOODS: EGGS AND MILK 87