The Egg Yolk Surface in Fresh Eggs*

T h e Egg Yolk Surface in Fresh Eggs* P. J. SCHAIBLE, J. A. DAVIDSON AND J. M. MOORE Section of Experiment Station Chemistry and Section of Poultry Husbandry, Michigan Agricultural Experiment Station, East Lansing (Presented for Publication October 31, 1935)

Y

It has been noted by the writers that "laid" eggs may or may not have a thin or intact peripheral layer of light yolk. When they do have, the yolk of the broken-out egg is observed to have an increased opacity. * Published with permission of the Director of the Experiment Station as Journal Article No. 239 (n.s.).

For this reason eggs from hens on the same feed and management or from the same birds at successive intervals may vary to a considerable extent in the intensity of the surface color of their yolks. This is explained by the fact that one is observing dark yolk color through a screen of light yolk of varying degrees of thicknesses. When the surface film does not uniformly cover the dark yolk, so called yolk spotting is produced. Depending on the magnitude of the imperfections in the peripheral film, variations are observed in the intensity of surface yolk color. The candler judges the interior quality of eggs by the visibility of the yolk shadow when the egg is illuminated. This has been found to be dependent upon a number of factors among which is the "condition" of the yolk. In scoring the latter our work points out the importance of the composition and structure of the yolk surface. To gain information on the nature of the yolk surface the following experiment was carried out: EXPERIMENTAL

Barred Rock hens in their first year of production were placed on an all-mash ration consisting of: Corn Barley Middlings Bran Meat Scraps . .

Per cent 33.5 1S.0 20.0 20.0 3.0

Per cent Skimmilk 3.0 Alfalfa 4.0 Salt 0.S Sardine Oil 1.0 Oyster Shell . .ad lib.

From December, 1934, eggs from the confined hens were collected at frequent intervals during each day for a period of two months. A record was kept of the approx-

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OLK index is used experimentally to a considerable extent as a measure of the interior quality or past history of eggs since it appears to be more sensitive and shifts earlier during deterioration than the percentage of firm white. However yolk indexes do not evaluate the nature of the surface of the yolk—a surface that is important to the candler as well as the housewife in the broken-out appearance of the egg. The yolk is not a uniform mass, but is made up of a central core of light yolk surrounded by concentric layers of dark and light yolk with a cylinder of light yolk extending from the core to the germinal disc. The layers of dark yolk are formed in the follicle during the day when the blood pressure is high and are wider than the layers of light yolk deposited at night during the shorter period of low blood pressure (Riddle, 1911). Light yolk represents less than 5 percent of the total and is of considerably different composition than the dark yolk—being higher in protein, extractives, ash and water and, what is particularly pertinent to this work, has been found to be less viscid at ordinary temperatures. As it leaves the ovary the yolk is described as having a thin layer of light yolk at its extreme periphery.

JULY,

1936.

VOL.

XV,

No.

4

RESULTS

Reference to Table 1 indicates that individual hens differed in the nature and condition of the yolk surface film formed during the experimental period. Yolks from the same hen had dark and light surfaces at different times. Sometimes they were spotted and at other times were not. An effort was therefore made to relate the nature of the surface to certain observations made at the time. Taking the group record of hens 300, 317, 358, and 369 which laid more than 48 eggs during the period and comparing it with that of 306, 318, 323, 332, and 331 which laid less than 31 eggs, it is found that the higher rate of production produced an average spotting of 1.2 compared to 1.8 in the latter group. Yolk surface color was not related to rate of production since the four birds 317, 335, 356, and 358 having the highest (61.2 percent) percentage of light yolk surfaces and at 82 percent production during the experimental period were equaled in production by hens 300,

302, 353, and 369 which laid only 14.7 percent of light yolks. Taking these points into consideration it appears that the hen is not constructing a different type of yolk surface because it is at a high rate of production or vice versa. Hens 306, 311, 332, 348, and 331 laid eggs weighing an average of 66 gms. while hens 300, 302, 317, 318, and 339 averaged 50 gms. per egg. The first group had an average spotting of 1.4 compared with 1.7 for the second group. Light yolk surfaces made up 12.5 percent of the first group and 22.7 of the second group. Comparing these results with those of other individuals in the pen it is apparent that yolk size is not in itself a causative factor. If light yolk is laid down at night during the period of low blood pressure, it would seem reasonable that the time of day the hen ovulated might designate the thickness and the type of yolk of the peripheral layer. The time of ovulation is approximately 24 hours before laying. Using this information, however, no such relationship was evident in this experiment. For simplification the data on the time of day the eggs were laid are not reported in Table 1. Eggs laid toward the end of a clutch were usually laid at a later hour of the day. Reference to Table 1 indicates that this difference in the time of day has no correlation with the character of the yolk surface as was true also with eggs laid by various hens during the same day. If the nature of the surface film is related to activity or rate of metabolism the mean daily temperature during the experimental period would be of interest. However no correlation of yolk spotting with the temperature was noted as reference to the table indicates. Hens 309 and 358 on 12-31-34 laid double yolk eggs of dark and light appearance respectively but in each case one yolk was spotted while the other was not. On

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imate time the eggs were laid. They were placed in a refrigerator and examined at intervals up to three days from the time of laying. The eggs were broken out in petri dishes and the intensity of yolk surface color and the degree of spotting of the easily visible top portion of the yolk noted. Intensity of yolk surface color was estimated by the term light (L) or dark (D) yolk depending on which layer predominated at the surface. Difficulty was experienced in placing yolks in only two classes because of the many combinations possible so that these observations are only an approximation. The degree of spotting was estimated with an arbitrary scale, 0, 1, 2, 3, 4, 5, representing no spotting and spotted areas 3 mm. in diameter up to combined areas of approximately one-half the yolk surface.

299

Av. Egg Wt.

—

gm. 300 52 302 54 306 63 309 311 66 315 57 317 52 318 46 323 327 61 329 62 331 65 332 66 335 60 336 62 338 61 339 44 344 62 348 68 353 57 356 60 358 56 369 55 376 55 378 60 382 56 392 61 Av. daily spotting Mean daily temp.

No. of Hen

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EGG YOLK SPOTTING—PEN 3. 1934-1935—All mask ration N-l

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During the course of this experiment it was observed that if a blunt glass stirring rod was applied with very slight pressure to the yolk surface, the white yolk film was pushed to one side producing a spot of dark yolk similar to those found naturally occurring in egg yolks. A slight amount of heat applied to the instrument was even more effective in producing these blemishes. This is consistent with the observed difference in physical character of the two types of yolk, the light being more plastic and less viscid than the dark yolk. The distinctive spotting of the yolks of hen 306 might then be explained by a pressure mechanism. A temporary or permanent imperfection in the follicle or oviduct could reasonably be expected to imprint itself on the yolk. According to the same theory, pressure of exceptionally large or hard chalazae during the descent of the oviduct might be undesirable although this relationship was not evident in this work. Apropos of this, Warren and Scott (1935) have recently shown that the yolk mass is subjected to considerable distortion by the musculature of the cephalic end of the magnum. These same workers report that the ovum released by the follicle is very loosely enclosed by the vitelline membrane permitting this distortion. Yolks of "laid" eggs on the

SCIENCE

contrary have the ability to some degree to retain their shape after removal from the shell. This points to the influx of a certain amount of material during the passage of the yolk down the oviduct. A slow penetration of water into the yolk sack was pointed out by Greenlee (1911) while Almquist and Lorenz (1932) reported that egg white as a whole is able in some cases to diffuse into the yolk and cause mottling. The latter workers were able to show this by obtaining the protein-fat ratio of mottled areas sliced from hard boiled yolks. In the present study the process of boiling destroys the structure of the spotted areas. This can be observed with the aid of the microscope so it is likely that the spotting referred to in this paper is not the mottling described by these investigators. In the present work by careful manipulation it is possible to show microscopically cellular differences in the surface film of untreated spotted yolks. These correspond to the well-known cellular structures of dark and light yolk (Lillie, 1919). Boiling destroyed this relationship. In this connection North (1934) mentions a condition in which the fresh appearance of egg yolks is affected by the presence or absence of "fat globules" or "fat islands." It is difficult to judge from the brief description whether this refers to the condition described by Almquist (1932) or to the spotted yolk condition under discussion. The influx of water through the yolk sack during and after the process of laying makes the light yolk film more fluid and reduces the opacity of the yolk surface. With the reduction in opacity the deeper color of the dark yolk is more apparent. This is particularly noticeable in storage eggs where the change in yolk surface appearance increases the visibility of the yolk shadow. It is also a factor in the candling appearance of eggs with "heated" yolks although there is an accompanying greater degree of liquefaction of the firm white. This change

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12-29-34 hen 327 laid two eggs on the same day, both of which were spotted but one had a light and the other a dark yolk appearance. Occasionally yolks had spots in which the yolk substance appeared coagulated or denatured. The spotting in these cases extended to a greater depth. The egg yolk of hen 306 between 1-22-35 and 2-1-35 had a large, easily distinguishable, narrow S-shaped area of dark yolk on a lighter background. Frequently it was noted that the chalaziferous membrane surrounding the yolk was decidedly opaque in certain regions thereby affecting the observed yolk surface color.

POULTRY

JULY,

193

6.

VOL.

XV,

No.

303

4

in character of the yolk surface is probably partly responsible for the common view held by egg handlers than yolks of eggs held under unsatisfactory temperature conditions deepen in intensity of yolk color. Spotted egg yolks are observable during candling because of the contrast of the colors of the different types of yolks on the yolk surface or because of the presence of opaque plaques of chalaziferous membrane.

similated by slight mechanical pressure on the yolk membrane. Another type of spotting observed in candling was due to an opaqueness of the chalaziferous membrane in definite areas. The condition of the egg yolk surface is discussed with relation to its importance in candling eggs and to the broken-out appearance of the egg.

SUMMARY

Almquist, H. J. and F. W. Lorenz, 1932. Some Possible Causes of Pink White. U. S. Egg and Poultry Magazine, May, 48-49. Greenlee, A. D., 1911. Deterioration of Eggs as Shown by Changes in the Moisture Content. Circular 83, Bureau of Chemistry, U. S. Dept. of Agr. Lillie, F. R., 1919. The development of the chick. P. 19, 2nd edition, Henry Holt and Co., New York. North, M. C , 1934. Bulletin 203, Wyoming Agr. Sta., p. 18. Riddle, O., 19H. On the Formation, Significance and Chemistry of the White and Yellow Yolk of Ova. Jour, of Morph., 22, 4SS-491. Warren, D. C. and H. M. Scott, 193S. The Time Factor in Egg Formation. Poul. Sci. 14, 195207.

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A particular spotting of yolks of fresh broken-out eggs has been described and found to be caused by nonuniformity in the peripheral light yolk layer. It has been observed and recorded semi-quantitatively for 2 7 hens over a period of eight weeks. During this period yolks from the same hens varied with respect to the presence and intensity of this condition. Extreme spotting can be detected in candling. Rate of production, atmospheric temperature, time of day egg was laid, the position of the egg in the cycle, and egg weight were found to be not related to this spotting. It may be

REFERENCES