20. JUDGING THE QUALITY OF EGGS IN THE HOME.--After eggs have been received in the home, several simple tests for determining their freshness can be applied in addition to the ones already mentioned. A rather indefinite test, but one that is sometimes applied to determine the freshness of an egg, is to shake it. However, to be able to carry out this test successfully, it is well to understand the interior structure of an egg. Fig. 2 illustrates this clearly. At a is shown the air space previously mentioned; at b, the spiral cords that run from the yolk to each end of the egg and hold the yolk in place; at c, the yolk; and at d, the white. When the water inside the shell evaporates, the yolk and white shrink so much that they can be felt moving from side to side when the egg is shaken. The staler the egg, the more pronounced does the movement become. This method should be applied only immediately before the egg is to be used, as the thin membrane between the yolk and the white and the spiral cords that hold up the yolk are liable to be disturbed by the shaking. If they are broken, the yolk will settle and finally adhere to the shell in case the egg is stored for any length of time after that.
21. If nothing has been done to preserve eggs, the simple test for freshness illustrated in Fig. 3, which consists in placing the eggs in a glass containing water, will be found effective. A perfectly fresh egg will sink when it is put into the water, but if the egg is 3 weeks old the broad end will rise slightly from the bottom of the glass. An egg that is 3 months old will sink into water until only a slight portion of the shell remains exposed; whereas, if the egg is older or stale, it will rise in the water until nearly half of it is exposed.
22. The test known as candling, which is usually applied to eggs before they are put on the market, can also be practiced by the housewife in the home. This method of determining the freshness of eggs consists in placing a piece of cardboard containing a hole a little smaller than an egg between the eye and a light, which may be from a lamp, a gas jet, or an electric light, and holding the egg in front of the light in the manner shown in Fig. 4. The rays of light passing through the egg show the condition of the egg, the size of its air space, and the growth of mold or the spoiling of the egg by any ordinary means.
In Fig. 5 is shown how an egg at various stages of freshness appears when candled. When an egg is fresh, it will appear as in (a); that is, the yolk will be barely distinguishable from the white except as a slightly darker area in the center of the egg, and the entire egg will appear clear and bright and free from spots. In an egg that is a little older, candling will reveal a slightly darker yolk, a cloudy white, and a larger air space, as in (b). In a watery egg, or one that is beginning to spoil, various dark spots and blotches usually develop, as view (c) indicates. When an egg is rotten, the contents of the shell will look dark in candling and the yolk will appear to be mixed with the white, as in (d).
23. If the housewife does not wish to resort to candling, she may determine the condition of an egg by breaking it into a saucer and examining it carefully. If the egg is newly laid, no odor will be detected and the white will be clear, elastic, and rather thick; also, where it joins the yolk it will be almost solid. The yolk of such an egg will have an even yellow color, without lighter or darker spots and, as shown in Fig. 6 (a), will stand up well from the surface of the white. Sometimes a small spot of blood may be detected on the yolk of a perfectly fresh egg, but, while this is not pleasant to look at, it does not affect the quality of the egg. When an egg that is not real fresh is broken into a saucer, the yolk will lie flat, as in (b). In an egg that is quite stale, the membrane surrounding the yolk is easily destroyed, so that even when such an egg is broken carefully the yolk and the white are likely to run together.