Transmission of
Light through Eggshell
Simple coefficients of correlation between each variable and all other variables were calculated; of these coefficients T K W is highly significant, T K P is barely significant, and the others are not significant: r T W -0.04 f 0 . 0 8 r T p -0.04 i 0 . 0 8 -0.04 f 0.08
From these simple coeficients various coefficients of partial correlation were calculated : rKW,T
rKw,p
f0.74 =t 0.06 + 0 . 7 8 i 0.06
University of California, Sample No. 37 38 43 44
Berkeley, Calif.
@A
STUDY of factors influencing the candling appearance of eggs obviously requires investigation of those characteristics of the eggshell which influence the transmission of light through it. I n this investigation several properties of the shell have been measured which might be expected to affect light transmission, and their influence has been studied. The properties measured were the thickness, the water content, and the protein content of the shell. T h e relative surface brightness of the shell when illuminated was used as a measure of its light transmission. The eggs selected for measurement were day-old white-shelled hen’s eggs. They were of uniformly large size and had shells as free as possible of water spots. The membranes were carefully removed by peeling and then by scraping with a scalpel and with steel wool. The thickness was then measured by means of a micrometer caliper. Five measurements were made near the middle where the curvature was at a minimum, the average - value being recorded. The relative surface brightness was determined bv means of a Bunsen photometer. As standard luminous surface, a piece of white opaque glass 2.5 em. in diameter was used at one end of the photometer. It was illuminated by a 100-katt lamp directly behind it. The sample of shell was placed on the other end of the photometer, an area 2.5 em. in diameter being illuminated in the same manner. The “grease spot” was then adjusted unt,il it was illuminated equally from both ends of the photometer. The surface brightness of the shell was calculated by the usual procedure in terms of the standard glass which was given an arbitrary value of I. The water content was obtained by placing the weighed membrane-free shell in a desiccator and drying it at room temperature in vacuum over phosphorus pentoxide. Constant weight was obtained in 7 days. To determine the protein content, the shell was first decalcified with acetic acid; 30 cc. of water and 10 cc. of glacial acetic acid were added to each sample. After complete decalcification, the material was filtered through a Gooch crucible, washed carefully, dried, and weighed in the crucible. The crucible was then ashed in a mume furnace and again weighed. The weight of protein was taken as the loss of weight of the crucible on ashing. Complete measurements were obtained for seventy-eight shells.
a
I n the following table the mean values and standard deviations of each of the four variables studied are given :
Thkkness, mm., T Brightness K Percentagdof water, W Percentage of protein, P
*
rKP,w rKT,Wp
-0.36 & 0.07 -0.22 i 0.07
It is evident from the preceding figures that the water content of the shell is the most influential factor in affecting light transmission. I n order to illustrate further the specific effect of the water content, brightness measurements were made on a number of shells before and after drying. Typical results are as follows:
J. W. GIVENS, H. J. ALMQUIST, AND E. L. R. STOKSTAD
Mean Value 0.326 i 0.011 0.86 zt 0.02 1.49 0.03 1.72 j, 0.02
r K T -0.11 =t 0.07 r K + , +0.74 j, 0.06 r K P -0.27 i 0.07
rwp
Coefficient Standard of Deviation Variability 47.2 0,154 3t 0.008 0.20 3= 0.01 23.2 0.41 iz 0.02 27.5 0.30 & 0.02 17.4
Percentage of Water 1.31 1.60 0.90 1.84
K before Drying 0.92 0.97 0.82
1.14
K after Drying 0.55 0.48 0.61 0.57
It was thought that the membranes, while not properly parts of the shell, should have some effect on the light transmission. I n order to investigate this effect, brightness measurements were made on twenty-four shells before and after the removal of the membranes. Mean values of the brightness obtained are as follows: K , uith membranes K , nithout membranes Difference
0 75 2= 0 03 0 84 f 0 03 0 09 f 0.04
The smallness and nonsignificance of the difference on removal of the membranes is not surprising since the membranes are only 4 to 5 per cent of the whole shell. It is evident t h a t the membranes have a very slight influence on the light transmission by the whole shell. The present knowledge of the structure of the hen’s eggshell has been summarized by Stewart.’ The shell is known to consist of three layers. The inner layer adjacent to the membranes, called the mamillary layer, has been found to consist of knoblike formations of calcite. The second layer comprises about 80 per cent of the entire thickness and consists of small calcite crystals with a considerable amount of protein in the form of interlacing fibers. Calcium phosphate crystals are also found in both of these layers. The third layer, called the “cuticle,” is the outer covering of the shell. It consists of organic material, probably a protein, in relatively small quantities. The effect of water on light transmission through the shell is probably due to the filling in of the spaces between the calcite crystals with a medium of higher refractive index than that of air. Light which is being reflected from the crystal surfaces will, in the presence of water, be refracted in a direction perpendicular to the surface of the shell and, therefore, will pass more directly through the shell. This is comparable to the familiar effect of a drop of oil on a piece of paper. The factor next in importance is the protein content. The protein forms a very thin covering on the outside of the shell and is present between the crystals of the middle layer. I t s effect is t o fill some of the spaces between the calcite crystals with an opaque medium, thus hindering the passage of light through the shell. It is surprising that the thickness appears to have a minor influence on light transmission. The fact that even the coefficient of partial correlation between thickness and bright1
Stewart, G F , Poultry S a . , 14, 24 (1935).
AUGUST, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
ness (independent of water and protein content) has less than a clearly significant value emphasizes the relative unimportance of shell thickness as a factor in light transmission. The visibility of the yolk shadow in commercial grading of eggs by candling will depend on any property of the shell
973
which can control the direct passage of light through it. Such properties are the water content and the protein content of the shell, which, so far as present knowledge indicates, are independent of the true internal quality of eggs. RECEIVED
Jlarch
1935,
0
.*((I
#)t.
Foaming of Egg White 31. IRESE BAILEY
Columbia University, New York, N. Y.
*+((I
P
U B L I S H E D reports of quantitatively controlled experiments on the whipping of egg whites are few. Of two previously published articles, one gives results based on the use of a slotted-disk, handoperated beater ( 5 )while the results in the other report were obtained by means of a motordriven Dover type of beater (1). I n this investigation a different type of motor-driven beater was e m p l o y e d , yielding results in certain r e s p e c t s c o n t r a r y t o those published. One feature of the present i n v e s t i g a t i o n is that i t d e a l s w i t h m i x e d specimens of 70 to 220 pounds (31.8to 99.8 kg.) of egg white, t h u s e l i m i n a t i n g individual variations in eggs.
A new method for the determination of the foaming power of egg white and for testing the stability of the foam obtained has been developed. Unfrozen whites and thawed whites after frozen storage for short periods of time showed no pronounced difference in foaming power. Thick white had a higher foaming power (using the method described in this paper) than the thin white. Untreated egg white possessed a higher foaming power than did any of the treated egg-white mixtures tested (pH 5 , 6, 7, or 9.5). The addition of olive oil to egg white produced a greater decrease in foaming power than could be accounted for by the addition of the same amount of fat in the form of egg yolk. The stability of the foam varied with the treatment and the type of egg white used.
Procedure The Hobart C-10 mixer,‘ with an adapter which allowed for the use of a 3-quart (2.8-liter) bowl, was selected for the determination of the whipping power of egg whites. The regular Hobart wire whipper was used. The highest speed (No. 3) was found most satisfactory and was adopted for all experiments, the motor revolving a t a rate of 2300 r. p. m. The rotating whipper, eccentrically situated on a rotating disk, described a hypocycloid path; the rotation of the beater on its axis was 604 r. p. m., that of the rotating disk was 255 r. p. m. The axis of rotation of the whipper was 2.5 cm. from the axis of rotation of its supporting disk. One hundred and fift grams of the uniform mixture of egg white were placed in t i e 3-quart bowl of the Hobart mixer, the white was adjusted to room temperature to avoid large changes of temperature during whipping, the whipper was adjusted in place, and the machine was started. At the end of the desired time interval, measured by a stop watch, the machine was stopped. Immediately, a crystallizing dish [313/,0 inches (9.7 cm.) in diameter and 2 inches (5.1 cm.) in height] was fitled with the whipped white, the top of the extruding foam being leveled off by means of a straight edge. (The excess of whipped white in the beater bowl was discarded. For each time of whipping, a new specimen of egg white was used.) The crystallizing dish and its contents were then weighed to an accuracy of 0.5 gram in order to obtain the weight of the foam or ‘‘whipped 1
Manufactured by the Hobart Manufacturing Company, Troy, Ohio.
white.” This value was used to calculate the foaming power by means of the formula: F =
(+
100
) - 100
whereF = foaming power of egg white V = volume of dish, cc. W = weight of foam, grams
The specific gravity of egg white was taken as 1.04. The following procedure was adopted as a m e a s u r e of the s t a b i l i t y of the foam: After weighing the crystallizing dish with its contained foam, it was allowed to stand at 2 5 O C . for one h o u r , c o v e r e d by an inverted can [41/4inches (10.8cm.) in d i a m e t e r and 5 1 / s inches (14.9 cm.) in height] to inhibit evaporation. A t the end of one hour the volume of the liquid white that had “leaked” (separated owing to collapse of the foam) was measured. The percentage of leakage, L , was calculated by t h e formula:
L
1041 = -100
W where 1 = volume of leaked egg white, cc. The whipping times were generally 3, 6, 9, 12, 15, and 18 minutes. The foaming powers, F , and the per cent leakage, L , were plotted as ordinates against time in minutes as abscissas. From experience with 150 measurements it was found t h a t the weight of the whipped whites in the crystallizing dishes could be checked within h0.5 gram in 80 per cent of the cases, while 90 per cent checked * 1.0 gram or less. The greatest variation was k3.0 grams in two cases, both of which were 3-minute whips. Frequently, the foam a t the end of the 3minute whip was dry and for this reason i t wad difficult, if not impossible, t o fill the measuring dish free from voids.
Influence of Type of Whipper When this investigation was started, 8 motor-driven Dover beater was used. The twin beaters rotated a t 1035 r. p. m.,
