UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA RELATION OF THE CANDLING APPEARANCE OF EGGS TO THEIR QUALITY H. J. ALMQUIST BULLETIN 561 NOVEMBER, 1933 UNIVERSITY OF CALIFORNIA BERKELEY, CALIFORNIA Digitized by the Internet Archive in 2012 with funding from University of California, Davis Libraries http://www.archive.org/details/relationofcandli561almq RELATION OF THE CANDLING APPEARANCE OF EGGS TO THEIR QUALITY 1 H. J. ALMQUIST 2 The closer commercial grading of eggs in recent years has brought to greater prominence the apparent tendency of eggs, originally placed in the same grade, to change into several grades during shipment or stor- age, instead of remaining together in a narrow range of quality. It has been suggested that most of this apparent tendency is caused by a lack of uniformity in the grading of eggs and that efforts should be made to develop a quality test more accurate than the present candling method. It is well, therefore, to review the candling method of grading eggs in the light of recent investigations. In the following discussion the present technical knowledge on the physical and chemical structure of eggs, as related to the problem of grading eggs, has been collected and summarized. The information given includes some original results. The various factors which contribute to yolk shadow, air space, and shell texture, as these are revealed by can- dling, have been considered. It is hoped that the discussion, although necessarily incomplete, will foster a better general understanding of this complex problem. Egg-quality changes such as incipient incubation or the onset of molds or rots will not be treated, since these are of a more accidental nature and readily subject to control by care and sanitation. Rather, the more unavoidable physical and chemical alterations and variations common to the edible egg will be the chief concern. In certain respects the conclusions oppose or modify current ideas of the interpretation which may be placed on the candling appearance of eggs. This should stimulate thought and investigation on the part of those interested in egg-quality problems. Since close adherence to fact is the only safe way to avoid the artificial difficulties which often arise when commercial practice is guided by arbitrary standards, such re- search on egg problems should be of great practical value. •» Keceived for publication April 14, 1933. 2 Junior Poultry Husbandman in the Experiment Station. [3] University of California — Experiment Station YOLK SHADOW The yolk shadow, or the visibilitj^ of the yolk when the egg is illumi- nated, as in candling, is commonly used as a major index to the quality of the egg ; for example, distinct yolk shadows are usually assumed to be associated with greater degrees of liquefaction of the white. Upon closer examination of this candling characteristic of eggs it may be seen, however, that a multitude of factors contribute to the yolk shadow, although many of them are not quality factors in a true sense of the word. Among these is the yolk color, which has been given the most complete investigation. In entering upon a discussion of yolk color, its variation with feed, age, and storage conditions must be considered. YOLK COLOR Variations of Yolk Color with Feed. — The variation of the color of the yolk in normal fresh eggs produced from differently pigmented feed- stuffs is conveniently shown by a figure given by Parker, Gossman, and Lippincott (1926) . In this figure, reproduced in figure 1, are plotted the percentage distributions in various yolk-color grades of eggs produced from the rations indicated. The color scale at the bottom of the figure expresses a color range from 28, a pale, yellowish white, to 12, a deep orange-red. Birds fed white corn and no greens produced very pale- yolked eggs, while birds fed yellow corn or greens or both produced highly colored yolks. Different kinds of greens had equivalent effects when fed in equivalent amounts. The increase in depth of yolk color was found to be related to the amount of greens fed, a correlation co- efficient of 0.51 ± 0.10 being found. It is possible to produce great variations in yolk color by abnormal feeding practices. The excessive feeding of greens may result in very dark or even greenish yolks. Green weeds usually have darkening effects on yolk color. Crude cottonseed oil and cheeseweed have a strong tend- ency to cause the development of pink whites and dark yolks in stored eggs (Almquist and Lorenz, 1932a). On the other hand, it is possible to produce yolks that are nearly white by withholding all yolk-coloring substances from feeds. The pigments in the yolk of the hen's egg are composed almost en- tirely of the xanthophylls, which are a group of complex organic sub- stances of the same general constitution and very closely related in their physical and chemical behavior. Their color ranges from yellow to red. Bul. 561] Candling Appearance of Eggs Xanthophylls exist in the leaves of green plants and in the tissues of many animals as well as in the yolks of eggs. Another group of pig- ments, the carotins, are usually associated in varying proportions with the xanthophylls in plants and animals. While the structure of the carotins is similar to that of the xanthophylls (the xanthophylls appear to be oxides of the carotins) , it has not been found possible for one to be S3 ez 16 fh/e 16 Dork SO Medium Yofk Co/or Fig. 1. — Percentage distribution of yolk color in eggs produced on differently pigmented rations. (From Parker, Gossman, and Lippincott, 1926.) converted into the other or to replace the other. The color of egg yolks increases when hens are fed rations high in xanthophylls. Carotins in- fluence yolk color only slightly, and are found in the body fat of hens and yolks of eggs in relatively small quantities. Conclusive evidence exists that at least one of the carotins is a pre- cursor of vitamin A and that hens and chicks can convert carotins into vitamin A. The feeding of xanthophylls has shown no vitamin-A value or vitamin-A generative power in this class of pigments. A recent pub- lication on this subject, confirming other investigations, is that of Kline, Schultze, and Hart (1932). A very complete review of the subject of yolk pigments has been prepared by Mattikow (1932) . References to the numerous researches on this subject may be found in this review. 6 University of California — Experiment Station It has often been assumed that a relation exists between the yellow pigmentation of certain foodstuffs, butter, corn, egg yolks, etc., and their vitamin-A content. Particular interest has been attached to the possible relation of yolk color to its vitamin-A value, and statements have been made that darker yolks are richer in vitamin A. In some cases TABLE 1 Relation of Yolk Shadow to Yolk Color in* Fresh and in Stored Eggs, and the Change in Yolk Shadow and Yolk Color* Age of eggs when graded Age of eggs when graded 1 day old 4 weeks old 1 day old; same eggs as graded 4 weeks old 1 day old 4 weeks old Number of eggs in sample Average yolk shadowf Average yolk colorj Test No. 1 : White corn — very little greens Yellow corn — no greens Kale 8.68±0 05 8.10±0 07 7.51±0.08 7.60±0.09 7.71=1=0.09 7.61=1=0.05 7.85±0.04 9.72±0.06 8.35±0.12 7.66±0.15 8 22db0 11 7.26=1=0.14 6.12d=0 14 4 78d=0 15 5.52±0.15 5.43=b0.14 5.25±0 09 5.70±0.07 9.22±0 13 7.57±0.25 5.70±0 19 6 83d=0 16 9 00±0 06 8 21±0.08 7.43=b0.08 7.75±0.10 7.87±0.07 7.69±0 05 7.97±0 04 21.45±0.18 19.24=1=0. 14 17.47±0.10 17.77i0.16 18.04±0.12 17.76±0.08 18.57±0 09 24 17=1=0.10 20 56±0.18 17 49=1=0.15 19.52±0,21 1988±0.21 17.95±0.08 16.94±0.06 17.29=1=0.12 17.25±0.08 17.16±0.06 17. 70=1=0. 06 22.50±0 17 19.61±0.17 17 68±0.14 19.07±0.16 34 42 51 Growing oats Alfalfa 52 53 Total with fresh greens Average, 1st test Test No. 2: 156 232 18 23 53 Average, 2nd test 94 * Data from: Parker, S. L. Studies on egg quality. III. Effect of age of eggs on their candling appear- ance and yolk color. Third World's Poultry Congress Proc, Ottawa, Canada, p. 402-405. 1927. t Graded by arbitrary standards starting from 1, with very deep-red yolk shadow, to 10, with no yolk shadow visible. % Graded by arbitrary standards starting from 13, with deep-orange yolk, to 27, with almost white yolk. the apparent relation of color to vitamin A has been disproved; for example, the removal of color from butter did not decrease its vitamin-A value. The darkest butter may be less potent in vitamin A than a color- less cod-liver oil. Vitamin A is itself colorless. Very light-colored yolks may possess adequate vitamin A for growth and reproduction in rats and in chickens. The assumed relation between darker yolks and higher vitamin-A values is, therefore, not well founded and requires further investigation. Variation of Yolk Color with Age. — Studies by Parker (1927) pro- vide some information on the effect of the age of eggs on their yolk color Bul. 561] Candling Appearance of Eggs and yolk shadow. The data secured by Parker are given in table 1. The method of storage in this work consisted of keeping the eggs in a moder- ately cool room without protection from shrinkage. It is evident that darkening of yolk shadow and a comparatively small darkening of yolk color resulted during storage under these rather unfavorable conditions, but the relative amounts of darkening were not in the same proportion, the yolk shadow having increased about three times as much as might have been expected. The increase in yolk shadow was greater in the eggs TABLE 2 Change in Color of Yolks During 5 Months of Cold Storage Group Average color of fresh yolks Number of fresh eggs examined Average color of stored yolks Number of stored eggs examined Change in color No. 1 16 3 133 16.1 51 -0.2 2 18 .7 117 18 7 53 0.0 3 18 .6 76 18 4 61 -0 2 4 18 7 87 18 8 61 +0.1 5 18.6 86 18 9 69 +0.3 6 18 9 80 18 9 54 7 18.6 85 19 48 +0.4 having the darker yolks, while the eggs with very light yolks showed relatively little increase in yolk shadow. Since the increase in yolk shadow was not proportional to the increase in yolk color, it might be assumed that the candling appearance of the stored eggs depended to a larger extent upon factors other than yolk color. These factors were probably in operation in all of the stored eggs, but had less apparent effect on the yolk shadow where the yolks were light in color. In a series of studies on the effect of different laying rations on the quality of eggs, both fresh and stored, 3 it was found that yolks in the normal color range do not change perceptibly in color during cold stor- age. The same color standards used by Parker, Gossman, and Lippincott (1926) in producing the results given in figure 1 and by Parker (1927), whose data are given in table 1, were used in estimating the change in color of yolks during cold storage. The average yolk color of fresh eggs produced by groups of about 30 birds was determined weekly over a period of ten weeks. During this period, eggs from the same birds were also oil-dipped and placed in storage at 30-32° Fahrenheit. After 5 months of storage, the average color of the yolks of the stored eggs was determined in the same way as before. The range covered by this investi- s Almquist, H. J., and F. W. Lorenz. Unpublished data from experiment conducted in 1932. 8 University of California — Experiment Station gation extended from medium to moderately dark-colored yolks. The results are given in table 2. The differences found in the average yolk colors of fresh and stored eggs in any group were so small that they would be scarcely perceptible to the unaided eye. Some differences were positive and some negative, indicating that they might be due merely to experimental errors. It is probably correct to state that, ordinarily, the color of the yolks of eggs does not change to any important extent during proper cold storage and that the increase in the visibility of the yolk in stored eggs is due to other factors, which may include liquefaction of the whites, change in the position of the yolk, and increase in yolk size. In a later investigation 4 oiled and unoiled eggs were stored at 86° F for more than two weeks. The oiled eggs showed a small tendency for the yolk color to lighten, which may be due to the diffusion of water into the yolk and a consequent dilution of the yolk color. Very little water was lost from the oiled eggs. The unoiled eggs showed a slight tendency for the yolk color to darken in conjunction with greater shrinkage. A simi- lar association has been reported by Stewart, Gans, and Sharp (1932a) in commercial eggs. The darkening which accompanies shrinkage is due only to a concentration of the yolk color with the loss of water. This may account for some or all of the darkening found by Parker (1927). It appears to be a common idea among poultrymen that holding eggs at room or atmospheric temperatures causes a darkening of the pigments of yolks, or "burned" yolks. Such a change, however, does not appear to take place, even at temperatures as high as 86°. Relation of Yolk Color to Yolk Shadoiu. — The information from sev- eral sources is concordant in showing a distinct effect of yolk color on yolk shadow. Parker, Gossman, and Lippincott (1926) calculated co- efficients of correlation of yolk color with yolk shadow ranging from 0.44 ± 0.05 to 0.71 ± 0.03. Correlation coefficients of this magnitude indicate that there is a strong trend of yolk shadow with yolk color and that yolk color in normal fresh eggs can be estimated with moderate accuracy by candling. A brief investigation gave results agreeing with others. A number of fresh eggs were candled and classified according to yolk shadow into two groups, one showing light or no yolk shadows, the other showing distinct yolk shadows. The eggs were broken out and the color of the yolks expressed in three grades, pale, medium, and dark, corresponding to those of figure 1. A summary of the data obtained is given in table 3. 4 Almquist, H. J., and F. W. Lorenz. Unpublished data from an experiment con- ducted in 1932. BUL. 561] Candling Appearance of Eggs 9 The table shows an almost exact reversal of the distribution of yolk shadow in the yolk-color groups on passing from one extreme of color to the other, while the distribution of yolk shadow is about even in the middle color group. In a number of other experiments dealing with eggs of a medium yolk color there has been found almost no association of yolk shadow with yolk color, probably because of the fact that the range of yolk color variation was much smaller. TABLE 3 Relation of Yolk Color to Yolk Shadow in Fresh Eggs Color of yolk Per cent light yolk shadow Per cent distinct yolk shadow Pale 76 53 23 24 47 Dark 77 TABLE 4 Ratio of Protein to Fat in Various Kinds of Egg Yolk Source of yolk Protein-fat ratio 46 47 0.58 62 Mottled Yolks. — Mottled yolks, often observed in stored eggs and oc- casionally in fresh eggs, are due to causes as yet practically unknown. These mottled yolks can be detected by expert candlers if the yolk shadow is sufficiently visible. The small shadows cast by the chalazas, however, are sometimes confused with mottles on the yolk. Yolks which are mottled appear to have membranes that are not able to prevent a slow penetration by the egg-white proteins. Ordinarily, yolks absorb only water from the white, but in some cases the egg white as a whole is able to diffuse into the yolk, causing a change in the yolk color at the area of penetration. That egg white may diffuse into the yolk was proved by Almquist and Lorenz (1932a), who showed by chemical analyses that the protein-fat ratio in yolks from stored eggs produced on a ration containing crude cottonseed fat was far greater than the normal ratio in stored eggs or in fresh eggs from the cottonseed-fat ration. The ratio of protein to fat in the yolk remains very constant in fresh or well-preserved eggs. A distinct increase in this ratio shows that 10 University op California — Experiment Station the protein content of the yolk has been increased by the admixture of protein from the white. In table 4 are given values for the ratio of pro- tein to fat in fresh egg yolks, normal stored yolks, "pink white" stored yolks, and mottled-yolk areas of stored eggs. In securing the last value, the mottled areas were sliced from the yolks of boiled eggs, and an analysis made of the material collected. The yolk membrane was not in- cluded in this material. The protein-fat ratio found is even higher than that in the yolks from "pink white" stored eggs. This fact proves that the diffusion of egg white protein into the yolk may take place, in certain cases, and indicates at least one process by which yolks become mottled. Outer liquid white- _-^sss== -__^_ — sas^ ^Shell and shell membranes Inner liquid white' Chalazae Interior arrangement of the hen's egg. POSITION OF THE YOLK Position of the Yolk as Affected by the Structure of Egg White. — In a recent article on the arrangement of egg white, Almquist and Lorenz (1933) discussed several facts which probably have a bearing on the yolk shadow. The white of a fresh egg can be found in three distinct layers, an outer watery layer, a middle firm or gelatinous layer, and an inner liquid layer. This inner layer usually has about the same volume as the outer liquid layer and is often more fluid. At first, the total-solids con- tent of each of these layers is distinctly different, increasing toward the center of the egg, but after a few days all layers attain the same solids content and the same density, although retaining their characteristic appearances. Normally, 50-60 per cent of the white is in the firm layer, the rest is distributed about equally between the two liquid layers. The yolk has a thin covering of fibrous material, which extends out in the form of the chalazas and aids in holding the yolk in place in the center of the egg. The distribution of these various components of the egg is probably somewhat as represented in figure 2. The proportions of this figure have been taken from actual data. It should be noticed that the inner liquid Bul. 561 Candling Appearance op Eggs 11 layer, although having about the same total volume as the outer liquid layer, has much greater average depth because it is contained in a smaller space. The yolk is not embedded in a mass of firm white, as TABLE 5 Volume and Solids Content of the Different Layers of Egg White- Hen Age of egg, days Layer of white Weight of each layer, in grams Weight of each layer in per cent of total weight Per cent of solids in each layer Average per cent of solids in the liquid layers Difference in per cent of solids in the liquid layers 1 2 3 4 5 6 7 8 D 811 1 3 5 7 8 7 4 19 6 7.5 9.5 19 8 8 7 7.4 18 2 22 21 57 20 26 54 25 22 53 10.80 \ 11.90 J 11 50 11 60 \ 12 10 / 11.90 1 1 40 1 11.70/ 11 55 11.35 11.85 11.55 1 10 Middle firm 0.50 Middle firm Outer liquid Inner liquid Middle firm 0.30 E846 1 3 5 Outer liquid Inner liquid Middle firm 8 1 7 5 14 9 11 .3 9.2 10 3 13 9 8 2 8 2 27 25 48 37 30 33 46 27 27 10.65 \ 13.15/ 11.80 11 30 \ 11.80/ 11 55 12.05 \ 12.05/ 12.05 11.90 11 55 12.05 2 50 50 Middle firm Outer liquid 00 Middle firm D 639 1 3 5 Outer liquid 7.7 10 3 14 6 9.9 11 1 14 .3 9.5 10 12.1 24 32 44 28 31 41 30 32 38 11.65 \ 13 45 / 12.65 12.35 \ 13.10/ 12.75 12.55 ) 12.95/ 12 70 12 55 12 73 12.75 Inner liquid Middle firm Outer liquid 1 80 75 Middle firm Outer liquid Inner liquid 40 Middle firm * Data from: Almquist, H. J. 12:83-89(1933). and F. W. Lorenz. The solids content of egg white. Poultry Science popularly supposed, but floats in the inner liquid white. The chalazas are often much smaller than those illustrated, and sometimes only vestiges of them may be seen. The outer ends of the chalazas are usually fastened to the firm white layer. Representative data on the composition of egg white are given in table 5. 12 University of California — Experiment Station When the yolk is firmly held in the center of the egg, very little yolk shadow can be seen by the candler. If the yolk is not firmly held, it may move through the inner liquid white to a position which allows a dis- tinct, but not heavy, yolk shadow to be seen. Often yolks are not exactly centered in fresh eggs ; this causes a distinct shadow on one side of the egg, and practically none on the other. It is important to realize that the yolk may have considerable freedom of movement, although there may have been no liquefaction of the firm white. Such loose yolks have little or no effect on the appearance of the egg when it is broken out. Relation Between Yolk Shadow and Factors Other than Yolk Color. — The independent variation of yolk shadow with other factors, such as the state of liquefaction of the white or the position or mobility of the yolk, was investigated by making use of a mass of data already accu- mulated throughout a number of egg-quality studies. The data included information on the yolk shadow, yolk color, and percentage of firm white in fresh and stored eggs. Eggs were grouped into two classes, according to whether they showed a light, scarcely discernible yolk shadow or a distinct yolk shadow which often allowed the outline of the yolk to be seen. The differentiation in yolk shadow thus achieved was about that between IT. S. Specials and U. S. Extras or Standards in the United States standards of quality for individual eggs. Eggs selected for distinct yolk shadows in this study would have been given a definitely lower grade in commercial grading. The actual color of the yolks was estimated by means of the color stand- ards previously described. Since it was desired to study variations in yolk shadow independently of variations in yolk color, data were used only from eggs of practically the same yolk color, corresponding to the medium color range in the commercial egg. This procedure reduced the available data but definitely eliminated the variation of yolk shadow with yolk color, making it possible to determine the variation of yolk shadow with other factors. The percentage of the total white existing in the form of firm white was taken as a measure of a state of liquefaction of the white, or "wateriness." All eggs were sound and free from molds, rots, or abnormalities. In table 6 are given the mean percentage of firm white in each group and the standard deviation from the mean. The data of this table show mathematically that practically no differentiation in the condition of the white was made by the yolk-shadow classification. The difference in the means of the fresh-egg groups is less than its probable error, indi- cating that the difference is insignificant. The same statement may be made about the stored eggs. The means of the fresh-egg groups are very Bul. 561 Candling Appearance oe Eggs 13 similar to a mean value of 56.44 obtained from a group of 1,228 fresh eggs in which the yolk shadow and yolk color varied. The only real difference is that of the mean percentages of firm white in the stored eggs as compared to those of the fresh eggs. This difference comes as a result of the partial liquefaction of the firm white in the stored eggs. The standard deviations from the means are also the same in com- parable groups ; this indicates that the distribution of the percentage of firm white about the mean value is practically identical in each group. The slightly higher standard deviations for the stored eggs are probably TABLE 6 Mean Percentage of Firm White and the Standard Deviation from the Mean Percentage of Firm White in Eggs of the Same Yolk Color but with Different Yolk Shadows Group Number of eggs Mean percentage of firm white Standard deviation 178 169 174 194 57.55±0.45 58.18±0.46 48.91±0.53 49.42±0.49 8.88 Fresh eggs, light shadow 8.83 Stored eggs, distinct shadow 10.41 10.10 due to a greater spread of these groups caused by a variation in the rates of liquefaction of individual eggs. With the yolk color held constant, the expected variation of the con- dition of the white with the yolk shadow did not manifest itself in the slightest. This fact leads to the conclusion that the observed variation in yolk shadow was due chiefly to other factors, such as the location or mobility of the yolk. In some eggs the yolks probably were not well centered or well anchored. Further investigations of the relation of the percentage of firm white to the shadow of the yolk have given the same result; that is, these characteristics appear to be almost entirely unrelated. Eggs were class- ified, as before, into two grades of yolk shadow. The yolk color varied over a small range, but the eggs were not segregated on this basis. The data are summarized in table 7. It may be noted from tables 6 and 7 that, in every case, the eggs of distinct shadows in comparable groups are a trifle lower in percentage of firm white. This difference, however, is so small as to be unimportant. It may be taken as a measure of the extent to which the shadow of the yolk is actually correlated with the wateriness of the white in fresh eggs. The preceding discussion throws much doubt on the value of the yolk shadow as a general criterion for the detection of "watery white" eggs. 14 University of California — Experiment Station It is well known, however, that eggs vary greatly in respect to the rela- tive proportions of the liquid and firm phases of the white. Hoist and Almquist (1931) found that the percentage of firm white in fresh eggs varied from 45 to 90 per cent of the total white with mean values in the neighborhood of 55 to 60, and pointed out that this characteristic was linked up with the individuality of the hen. Lorenz, Taylor, and Alm- quist (1933), have secured proof that the percentage of firm white in TABLE 7 Mean Percentage of Firm White in Fresh Eggs of Light and Distinct Yolk Shadows and Variable Yolk Color Group Number of eggs Mean percentage of firm white a. Random selection: 39 20 25 22 88 51 24 24 24 24 56 5 55 8 b. Random selection: 57.4 56 2 c. Random selection: 55.2 54.2 d. Selected for extremes of yolk shadow: 59 7 58 6 e. Selected for extremes of yolk shadow: 62.2 60.0 eggs is a hereditary characteristic and may be raised by selective breeding. The wateriness of egg white may progressively increase through physico-chemical processes in the egg. Almquist and Lorenz (1932&) have shown that liquefaction of the firm phase of egg white may take place by a disintegration of its fibrous structure (true liquefaction) or by a shrinking of the fibrous structure (syneresis) . Almquist and Lorenz (1933) pointed out that though liquefaction is not complete, the en- velope of firm white may be reduced to such a point that the layers of liquid white may mix readily, particularly when the egg is broken out of the shell. The result is a very "watery" appearing egg. In figure 3 is shown the effect on the appearance of an egg caused by a rupture of the firm white layer. Although these photographs were made of a fresh egg, Bul. 5C1] Candling Appearance op Eggs 15 they demonstrate in a striking fashion a type of apparent liquefaction which is purely mechanical. If the quantity of firm white in a fresh or otherwise well-preserved egg is about 40 per cent or less of the total, or Fig. 3. — The increased watery appearance of an egg caused by a rupture of the layer of firm white. The upper illustration shows the egg just as broken out. The lower shows the apparent liquefaction of the white which results when the inner layer is allowed to run out. (From Almquist and Lorenz, 1933.) if the firm white envelope has been weakened or ruptured by liquefac- tion, the probability of this abrupt increase in watery appearance be- comes very great. While this is a very rare process in fresh eggs, it is common in stored eggs, both unbroken and broken. 16 University of California — Experiment Station The shocks and vibrations incident to the transportation of eggs un- doubtedly are important factors tending to loosen the yolk, particularly in eggs transported across the continent. The protection of eggs from rough handling, jarring, etc., is highly important in preserving their candling grade when the basis of grading involves the yolk shadow, since loose yolks may approach the shell more closely and thus cast darker shadows without, however, being accompanied by any loss of firmness in the white or yolk. It is a common practice in candling to give eggs a sharp twist in order to observe the yolk "spin." The manner in which the yolk "spins" is con- sidered to be an indication of the condition of the white. Since the yolk turns in a very fluid medium — the inner liquid layer of white — the manner in which the spin takes place refers to the anchorage of the yolk rather than to the condition of the white. A well-anchored yolk will spin rapidly to regain its former position with respect to the rest of the egg, while a yolk that is less firmly held in place will spin more slowly. Investigation has suggested yet another type of variation in yolk shadow, resulting from a variation in the size and shape of the egg. Yolks of about the same color and size appear to give a more noticeable shadow in smaller eggs or very narrow eggs, probably because of the shorter distance traveled by light from the yolk to the shell. Stuck Yolks. — Stuck yolks are caused when the yolk moves through the white and becomes fastened to the inner shell membrane. In this case the yolk shadow becomes dark and immovable. Sharp and Powell (1929) have shown that the difference in densities of the yolk and the white at ordinary temperatures is sufficient to account for the distinct tendency of the yolk to move through the white, usually upward. The densities of yolk and white become more nearly the same at cold-storage temperatures, hence, yolks in cold-storage eggs have less tendency to rise. The more rapid liquefaction of the firm white and the more rapid diffu- sion of water into the yolk at ordinary temperatures are changes which assist the rising of the yolk. As Sharp and Powell (1929) have pointed out, the rising of the yolk is more readily detected during candling if the yolk moves to the small end of the egg rather than to the large end. For this reason, eggs packed with the large end and air space upward present a better candling ap- pearance after storage or shipment. Bul. 561] Candling Appearance of Eggs 17 CLOUDY WHITES Although egg white is usually transparent, it is often found to be slightly cloudy. The cloudiness, in practically every case, is due to a slight excess of carbon dioxide, which is retained in considerable amounts by the newly laid egg. The condition usually disappears after the carbon dioxide is allowed to escape, more slowly if the cloudiness has existed for a long time. The cloudiness of the white influences the can- Fig. 4.— The effect of ex- cess carbon dioxide on liquid and firm egg white. The tube at the left contains liquid white, the tube at the right, firm white. dling appearance of the egg, especially the yolk shadow. Often it is er- roneously termed "white rot" or taken to indicate a diseased condition of the hen, whereas it actually represents a harmless and often a very fresh condition. A laboratory illustration of the effect of excess carbon dioxide is shown in figure 4. The tube at the left contained liquid white, the one at the right, firm white. They were kept in an atmosphere of pure carbon dioxide gas for several weeks. The cloudy mass shown in the tube at the right is the original fibrous structure of the firm white. This became opaque and shrunk to about half its former volume, expressing a clear, watery fluid, but retaining its jelly-like properties. No similar effect was to be noticed in the liquid white. While the change shown in the illus- 18 University of California — Experiment Station tration is an exaggerated one, because of the high concentration of car- bon dioxide used, the same effect is often encountered to varying degrees in whole eggs. This type of liquefaction of egg white is identical with syneresis, a general process by which colloidal gels shrink to smaller volumes. Although only a physical change, it produces an egg with an unattractive appearance and a lower commercial value. It is character- istic of liquefaction by syneresis that the firm white remains well defined, but with a much smaller volume, and often adheres to the yolk. The oil-processing of eggs tends to prevent the escape of carbon di- oxide, hence eggs that are processed when very fresh may retain much of the carbon dioxide which they originally contained. When this is the case, a cloudy appearance of the white may develop and persist for a long time, particularly in cold storage, where much less carbon dioxide is needed for the same effect on the appearance of the white. These state- ments are supported by the results of experiments. It seems possible that the processing of very fresh eggs may favor liquefaction in cold storage and that there may be an optimum holding period between the time of lay and the time of processing, during which period the egg loses carbon dioxide to a point sufficient for its best preservation in cold storage. This average optimum time might be de- termined by noting the effect of different holding periods before pro- cessing on the retention of quality after processing. A number of other variables, such as temperature, shell porosity, and carbon dioxide con- tent of the atmosphere, must be taken into account, since they also govern the loss of carbon dioxide. AIR SPACE CHANGE IN THE SIZE OF THE AIR SPACE Change of Air-Space Size with Temperature. — The air space of an egg normally is located between the inner and outer shell membranes at the large end of the egg. It forms as a mechanical response to the con- traction of the liquid egg on cooling from a temperature of about 105° F, as laid, to the ordinary room and cold-storage temperatures. In order to estimate the size of the air space resulting from temperature changes only, calculations were made of the dimensions of the air space in a normal 2-ounce egg. The calculations were based on specific-gravity data for egg yolk and egg white as given by Sharp and Powell (1929), and determined at this laboratory. According to Benjamin (1925), no air space exists at the instant an egg is laid. This was verified by a brief Bul. 561J Candling Appearance of Eggs 19 investigation. The calculated dimensions of the air space in a normal 2-ounce egg were as follows : At 60° l 1 At30°F Diameter of air space 1.49 cm (0.59 inch) 1.92 cm (0.76 inch) Depth of air space 0.28 cm (0.11 inch) 0.36 cm (0.14 inch) The calculations indicate that, in a 2-ounce egg, an air space more than V2 i nc h * n diameter at 60° F and more than % inch m diameter at 30° may result merely from cooling. Correspondingly, the depth of the air space will be slightly less than % inch at 60° and slightly more at 30°. These dimensions are about the same as those actually found, except for the fact that a curvature of the inner air-space wall will often tend to decrease the diameter and increase the depth. The size of the air space will also vary with the size of the egg, a fact which must be borne in mind when grading eggs on this basis. When eggs are brought from cold stor- age to room temperature, there will be an equal tendency for a decrease in the size of the air space as the egg warms up. Change of Air-Space Size with Shrinkage. — The increase of the size of the air space over the normal size at the temperature of the egg may be taken as a rough measure of the loss of water, or shrinkage. It has already been pointed out that the solids content of the different layers of the white soon reaches and remains at a common value throughout all the layers. This equality of solids concentration persists in spite of shrinkage and the diffusion of water from the white into the yolk. The loss of water from egg white is, therefore, evenly distributed among the different layers of white. On the basis of the above facts, Hoist and Almquist (1931) pointed out that shrinkage alone is not a contributing factor in the processes by which the contents of an egg become liquefied, or "watery." This state- ment was experimentally supported. It was found easy to establish con- ditions such that eggs would shrink at rapid rates but deteriorate in other respects with comparative slowness, and vice versa. In recent studies by Stewart, Gans, and Sharp (1932a-), it was found that the liquid white increased by only 5 per cent over the entire range of in- creasing air-space sizes. Nevertheless, commercial candlers grading the same eggs tended strongly to lower their rating of the egg-white quality as the air space increased in size. Eggs which undergo greater rates of shrinkage, however, are those which are more susceptible to several of the real deteriorative processes. In other words, shrinkage, liquefaction, and infection by microorganisms are often found to be associated be- cause two of the numerous controlling factors, temperature and shell porosity, are common to each process. 20 University of California — Experiment Station Recent work by Spanswick (1930) has shown that the mustiness which sometimes develops in eggs is due to a microorganism rather than to any of the purely physico-chemical processes that have been mentioned. On the other hand, studies by Pennington (1932) on the flavor of storage eggs indicate an association between shrinkage and the loss of freshness. It seems reasonable to conclude that shrinkage, except in extreme cases, cannot be used as a reliable guide to egg quality. This does not mean that the conservation of the water content of an egg does not merit a good deal of attention commercially, but that shrinkage is far from being an absolute measure of the interior quality of an egg. DEFECTS OF THE AIR SPACE Air spaces that are tremulous, movable, or entirely loose are commonly found, even in fresh eggs, and are almost as commonly taken as sufficient cause for grading the eggs down as "watery whites," since they tend to create an impression of wateriness in the mind of the candler. Tremulous and movable air spaces may be caused by sharp jars or vibrations which tend to separate the shell membranes at the air space. This may readily be demonstrated. If an egg is held in the hand and merely shaken vigorously once or twice, a normal air space can often be dislodged and made tremulous, movable, or entirely loose. No doubt nearly all of these defects of air spaces arise, similarly, through mechan- ical stresses such as might be encountered in transportation. While the air space is usually located between the inner and outer shell mem- branes at the large end of the egg, it is occasionally formed inside of both membranes, in which case it is entirely loose and may move to any part of the egg which is uppermost. Obviously, such changes are purely mechanical and possess little significance concerning other quality char- acteristics of the egg. The examination of the interior condition of many fresh eggs which were graded by commercial candlers as "watery" or "weak" whites, chiefly because of these defects of the air space, has not revealed a justifiable basis for such classification. For example, table 8 gives the results of the examination of 100 fresh eggs having defective air spaces and of 198 fresh eggs with perfect air spaces, both lots having been selected from a number of places and examined together. These defects include tremulous or somewhat movable air spaces and air spaces that are entirely loose and sometimes broken into bubbles. This variation in the condition of air spaces is one of degree rather than one of kind. The interior condition of the eggs was expressed by measuring the volumes of liquid and firm white and calculating the percentage of the total white Bul. 561] Candling Appearance of Eggs 21 in the firm white phase. This gives a measure of the state of liquefaction of the white. The percentage of firm white in fresh eggs has been found to vary from about 45 to about 90, but is usually found to average be- tween 55 and 60. The data of table 8 show that, from the standpoint of the condition of the white, the interior quality of the eggs with defective air spaces was not noticeably inferior to that of normal fresh eggs. TABLE 8 Relation of Defective Air Spaces in Fresh Eggs to Interior Quality as Expressed by the Percentage of Firm White Class of eggs Number of eggs Mean firm white as a per cent of total white Standard deviation from the mean Defective air spaces 100 198 56.12±0.56 56.60±0.32 8.3 Normal air spaces 6.6 SHELL TEXTURE MOTTLED SHELLS The uneven transmission of light which gives rise to a mottled-appear- ing shell when the egg is candled has been recently investigated by Hoist, Almquist, and Lorenz (1932). This mottled appearance of the shell, illustrated in figure 5, has often been a main criterion considered in grading for "shell texture." The particular variety of "shell texture" referred to was found to be due to an uneven distribution of moisture in the shell. Significant differ- ences were found in the moisture content of shells of good and poor "texture," although the moisture content was low and in the neighbor- hood of 1 to 3 per cent. The common explanations for this mottling, such as variation of shell thickness, variation of crystal structure, fat spots, etc., were found to be incorrect. Since the mottling of the shell is due to water, the appearance of the shell in this respect is variable and influenced by the storage conditions of the egg. Eggs of an excellent initial "texture" may, in a few hours or days, acquire many of the water spots if stored in a humid atmosphere. In a drying atmosphere, egg shells tend to lose their spots, changing toward a texture which the candler would call perfect. (See fig. 6.) The storage of eggs under optimum conditions has, in fact, a tendency to increase the mottled appearance of the shell. Hoist, Almquist, and Lorenz (1932) investigated the rates of shrink- age and deterioration in storage of eggs of widely different degrees of 22 University of California — Experiment Station water-spot mottling'. Practically no differences were found. Slight trends toward higher rates of shrinkage in the more mottled shells were found at incubation temperatures. The mottled character of the shell was not found to be related to the true shell porosity. Fig. 5. — Illustrations of the even and uneven shell trans- lucency referred to in the text. No. 1 shell is often called good and No. 2 poor "shell texture." (From Hoist, Almquist, and Lorenz, 1932.) Fig. 6. — Change in candling appearance of the shell caused by drying at a low temperature. A, The original appearance of the shell; B, the ap- pearance after drying. An abnormal type of shell which appears to be covered with many small, translucent spots has been investigated by Burmester and Alm- quist. 5 The candling appearance of these shells (fig. 7) is distinctly different from that of the mottled shells described by Hoist, Almquist, and Lorenz (1932). In the latter case the more translucent areas are much longer and less numerous. Shells of the type shown in figure 7 emit a musical clink when tapped, like that of a glass vessel, while normal 5 Burmester, B. E., and H. J. Almquist. Characteristics of an abnormal type of egg shell. (Manuscript in course of preparation.) Bul. 501 J Candling Appearance op Eggs 23 shells give only a dull sound. Hence, these abnormal shells have been referred to as "glassy" shells. Glassy shells appear to be more fragile than normal shells. It is a common belief that they are also much more porous. Eggs with glassy shells are viewed with suspicion by the commercial candler and placed in low grades. The porosity of glassy shells was found to be extremely low, contrary to popular opinion, while the shrinkage of glassy-shelled eggs was also Fig. 7. — Comparison of the candling appearance of "glassy" shells with that of a perfect shell. much lower than that of normal eggs under the same conditions of stor- age. The interior quality of glassy-shelled eggs was much superior to that of normal eggs at the end of the storage period. In cold storage, however, glassy-shelled eggs were found to have a tendency to develop cloudy whites and to undergo liquefaction through syneresis. The percentage of shell, the thickness of shell, and the percentage of shell membrane were definitely lower in the glassy-shelled eggs. Com- parative data are given in table 9. The breaking strength of the glassy shells was the same as that of normal shells when placed on the basis of the same shell thickness. From a technical standpoint the glassy shells were superior to normal shells except in one respect, a tendency toward thinness, which causes a greater liability of breakage. When oil-processed the glassy shells were found to be definitely inferior to similarly treated normal shells from the standpoint of shrinkage. It appears that process- ing oils are much less effective on glassy shells. 24 University of California — Experiment Station SHELL POROSITY The porosity of egg shells must be recognized as an important factor in their preservation. It is a common belief that highly porous shells can be detected by candling, although actual shell porosity is not discernible to the unaided eye. The basis for the detection of supposedly more porous shells is sometimes the uneven shell translucency already discussed, which is not related to true porosity, and sometimes the numerous small pits to be observed on the exterior of any normal egg shell. It is neces- sary, therefore, to include shell porosity in a discussion of the candling appearance of eggs, if a proper conception of this shell characteristic is to be formed. TABLE 9 Comparison of Glassy Shells with Normal Shells Average shell porosity Average thickenss of the shell proper, in mm Average per cent weight lost per egg per day Average shell proper as a per cent of total egg weight Average shell membrane as a per cent of total egg weight Average whole shell as a per cent Unoiled eggs Oiled eggs of total egg weight 1.5 7 3 292 0.315 0.343 0.580 111 0.026 7.63 8.78 0.29 0.42 7 92 9 20 Nathusius (1868) found that the inorganic material of the shell proper was present in the form of crystals cemented together by an organic material. He found, also, that this layer of dense material was traversed by numbers of small, definitely tube-like passageways. These passageways, or pores, were closed at the outer end by the cuticle, or "bloom," which appeared as a nonporous, structureless deposit of a thin and delicate character. • Kizzo (1899) showed that the number of pores in hen's eggs is very large, varying in six eggs from 5,935 to 9,735 pores per egg. His ex- perimental procedure, however, was such as to detect the maximum porosity which a shell might develop, rather than the actual number of shell channels which were open and serving as free pores at the time. As pointed out by Almquist and Hoist (1931 ) , it may be assumed that a number of outside influences might partly or wholly remove the ex- ternal seal or cuticle, in this way opening the already existing and numerous channels. The actual porosity of a shell, therefore, does not correspond to the discernible number of small pits on the exterior of a shell, although these are probably the outer ends of the pore channels. Bul. 561 J Candling Appearance of Eggs 25 Almquist and Hoist (1931) were able to secure a measure of the actual porosity of shells by immersing them in a dye solution which penetrated the open pores. It was found by this method that the actual porosity of normal fresh egg shells varied over a wide range but was only a fraction of the maximum porosity possible when all pores were effective. The porosity of eggshells was found to increase, usually, with the age of the eggs, although no easily visible evidence of this change existed. The different degrees of porosity found showed a close relation to the rates of shrinkage of the eggs. This variation was such as to indicate that Fig. 8. — Illustrations of actual egg-shell porosity as detected by the penetration of a dye solution. Both halves of each shell are shown. (From Almquist and Hoist, 1931.) the rate of shrinkage was controlled, largely, by the number of pre- formed, open pores, but that a shell of zero porosity would still show a small rate of shrinkage. This is not surprising, since it is probable that a small loss of moisture may proceed through the entire shell surface in much the same manner that a relatively impervious rock can lose moisture. In figure 8 are shown a few examples of actual shell porosity in fresh eggs. The small dye spots on the insides of the half shells represent open pores. The dye spreads somewhat throughout the shell membranes, hence the size of the spots in no way indicates the size of the pore, which is microscopic. Each spot has opposite to it on the outer part of the shell one of the numerous small pits ; however, the converse of this condition is seldom completely, and usually only slightly, realized. The spots on the shells of figure 8 in no way correspond to those of figures 5, 6, and 7, since they represent an entirely different characteristic of egg shells. A recent microscopic study of egg shells by Burmester and Alm- quist 6 has verified the conception of egg-shell porosity as developed by Nathusius, Rizzo, and Almquist and Hoist. Egg shell from which the e Burmester, B. B., and H. J. Almquist. A study of egg shell pores. (Manuscript in course of preparation.) 26 University of California — Experiment Station external cuticle or "bloom" and the shell membranes have been removed contains 1.5 to 3.0 per cent of organic matter in the form of a matrix of interlacing fibres. The removal of mineral matter bv decalcification Fig. 9. — Microphotograph of a section cut lengthwise through a shell pore, after decalcification of the shell, showing the bloom, shell matrix and pore, and shell membranes (enlarged 600 times). causes much shrinkage of this matrix, but by careful technique it can be made to retain its original form. In figure 9 is shown a microphotograph of a section cut lengthwise through a shell pore after decalcification of the shell. The dark-colored region at the top shows the external cuticle, which has absorbed a stain. The grey area next to the cuticle consists of the fine, organic matrix of the original shell proper. Through this matrix passes a pore channel, Bul. 561 Candling Appearance of Eggs 27 somewhat deformed by the contraction during decalcification. Under this grey area is a darker one showing the shell membranes. The fibrous character of the organic matter in the shell proper and in the shell mem- Fig. 10. — Microphotograph of a section cut across a shell pore through the organic shell- matrix layer (enlarged 600 times.) Fig. 11. — Photograph of the edge of a broken shell, showing two pores partially filled with the external cuticle or bloom (enlarged 60 times). branes is clearly evident, while the cuticle appears quite structureless. In figure 10 is shown a transverse section of a shell pore cut through the organic shell matrix layer. Figure 11 is an enlarged photograph of the broken edge of a shell. Two pores filled with the external cuticle at their outer ends are clearly shown. 28 University of California — Experiment Station GENERAL REMARKS ON THE COMMERCIAL GRADING OF EGGS The eggs received at a packing plant come from many sources ; hence, it will probably always remain necessary to inspect every egg. The ob- vious external characteristics of color, size, shell condition, dirt, etc., are not sufficient as a basis for grading ; many additional quality fea- tures, such as checks, meat spots, blood, mold, rot, excessive shrinkage, advanced liquefaction, and off-colored yolks and whites must also be detected. In view of the number of widely different factors that con- tribute to the quality of an egg, it appears extremely unlikely that any grading method can be devised which will possess as much versatility, accuracy, and speed as the candling method. A more exact measure of egg quality may possibly be developed, but it will be subject to several very serious objections, among which are the time and labor required for proper sampling and making tests, and the unavoidable destruction of the eggs tested. The examination of every egg will necessarily be impractical, and the examination of samples will determine only the average lot quality. As a matter of fact, methods of measuring egg quality which are more direct than candling are now available, but these are suitable for experimental work only. While the candling of eggs is not reliable in making the fine distinc- tions in quality that are supposed to be discernible in intermediate egg grades, it is to be expected that the agreement of candling grade with actual quality will be good for eggs of very high and very low quality. Recent studies by Stewart, Gans, and Sharp (1932& and following articles) have shown plainly that experienced candlers may disagree widely when classifying eggs of intermediate quality, but agree much better on eggs at both extremes of the quality range. The accuracy with which quality features are detected will always be largely dependent upon the candler. The proper balancing of accuracy against speed in candling must remain one of the many problems of the grading-room manager. Details such as the candle power and color of lights may be decided usually by individual preference; however, a universal standardization of the candling equipment would be very valuable. It may be well to keep in mind the possibility that the great variation that is found in the commercial grading of eggs of the better grades may result largely from the fact that the distinctions that are attempted are Bul. 561] Candling Appearance of Eggs 29 often not founded upon actual fact or real conditions. With particular reference to "watery whites" in very fresh eggs, it seems that much of the supposed trouble may be fictitious and caused by exaggerated ideas of the possibilities of candling with an overemphasis on less essential characteristics of eggs. The egg shell should provide a minimum of interference with the visibility of its contents when candled. This minimum is achieved only with white-shelled eggs. The detection of deterioration is hindered to a considerable extent by tinted or brown shells, and the grading of eggs with such shells is correspondingly less accurate. Experimental work has disclosed several facts which show that eggs are not "created equal" or, if they are, they do not long remain so. Initial quality and keeping quality may vary greatly from egg to egg but are found to vary to a relatively small extent in the eggs from the same hen. It appears, also, that even with perfectly consistent grading by can- dling, or by any other method, eggs originally all of one grade, because of the wide difference in their keeping powers, will be found distributed in part through several lower grades after storage or shipment. It is im- portant to point out that this variation in quality cannot be ascribed to, or eliminated by, the present methods of grading or by any other grading methods that may be invented. 30 University of California — Experiment Station LITERATURE CITED Almquist, H. J., and W. F. Holst 1931. Variability of shell porosity in the hen's egg. Hilgardia 6:61-71. Almquist, H. J., and F. W. Lorenz 1932a. Some possible causes of pink white. U. S. Egg and Poultry Magazine, May, p. 48-49. 1932&. Liquefaction of egg whites. Nulaid News, March. 1933. The solids content of egg white. Poultry Science 12:83-89. Benjamin, E. W. 1925. Marketing poultry products. 2nd ed., 332 p., John Wiley & Sons, New York. Holst, W. F., and H. J. Almquist 1931. Measurement of deterioration in the stored hen's egg. Hilgardia 6:49-60. Holst, W. F., H. J. Almquist, and F. W. Lorenz 1932. A study of shell texture of the hen's egg. Poultry Science 11:144-149. Kline, O. L., M. O. Schultze, and E. B. Hart 1932. Carotene and xanthophyll as sources of vitamin A for the growing chick. Jour. Biol. Chem. 97:83-91. Lorenz, F. W., L. W. Taylor, and H. J. Almquist 1933. Per cent firm white of eggs as an inherited characteristic. Poultry Science (in press). Mattikow, M. 1932. A critical review of the literature on the coloring matter in e^gg yolk. Poultry Science 11:83-93. Nathusius, W. von 1868. Ueber die Hiillen, welche den Dotter des Vogeleies umgeben. Zeit. Wiss. Zool. 18:225-270. Parker, S. L. 1927. Studios on egg quality III. Effect of age of eggs on their candling ap- pearance and yolk color. Proc. Third World's Poultry Congress, Ottawa, Canada, p. 402-405. Parker, S. L., S. S. Gossman, and W. A. Lippincott 1926. Studies on egg quality I. Variations in yolk color. Poultry Science 5:131-145. Pennington, M. E. 1932. Flavor and eating quality. U. S. Egg and Poultry Magazine. September, p. 28-31. Rizzo, A. 1899. Sul numero e sulla distributions dei pori nel guscio dell'ovo de gallina. Ricerchie del Anat. Lab. Roma 7:171-199. Bul. 561] Candling Appearance of Eggs 31 Sharp, P. F., and C. K. Powell 1929. Significance of the changes of temperature in preventing stuck yolks. XT. S. Egg and Poultry Magazine, December. Spanswick, A. 1930. The cause of mustiness in eggs. Amer. Jour. Pub. Health 20:73-74. Stewart, G. F., A. E. Gans, and P. F. Sharp 1932a. The relation of air cell size to the interior quality of eggs by candling and from the opened egg. U. S. Egg and Poultry Magazine, December, p. 28-31. 1932/;. Average candling grade for each dozen eggs as determined by four different candlers. U. S. Egg and Poultry Magazine, May, p. 31-34. 13m-12.'33