Changes in Color of Dehydrated Eggs during Storage H. J. DUTTON' AND B. G . EDWARDS TJ. S. Departm'ent of Agriculture, Albany, Ca1;f.
Western Regional Research Laboratory,
Color changes from yellow to brown during storage of dehydrated eggs have been studied spectrophotometrically by reflectance and absorption measurements. The influence of the simultaneous destruction of carotenoids and formation of brown aldehyde-amine substances upon the reflectance of egg powders is discussed. The lipide amine-aldehyde reaction rate is dependent on moisture content, whereas rate of carotenoid destruction is independent of moisture content. Both reactions are acceler-
ated by increasing temperature of storage. An approximately linear relation has been observed between'negative logarithm of the reflectance of residues aftel' ether extraction and fluorescence of salt extracts of the residues. This linearity confirms the conclusions previously drawn from experiments with model systems that the saltextractable fluorescent compound, which has been used as an index of palatability, is the brown "glucose-protein" reaction product.
T
ments can thus be used to study complex pigment changes and provide a method for the study of insoluble pigments. I n contrast to the obscure relation of light reflectance to pigment content, the relation of light absorption t o concentration of pigments can be given a precise description in terms of the BeerLambert law. Since the observed optical density of a mixture of pigments is the sum of the densities contributed by the individual pigments, mixtures of pigments frequently can be analyzed by measuring optical densities of the mixture a t various wave lengths, setting up corresponding algebraic equations, and simultaneously solving the equations (9). The color of light reflected from dehydrated egg samples and from residues after fat extraction is recorded here in spectral reflectance curves. These measurements were obtained with a Beckman spectrophotometer and reflectance accessory. I n this accessory, monochromatic light strikes the sample at normal incidence, and the light reflected a t approximately 45" is collected and measured photoelectrically. The reflectance of the sample a t a given wave length is expressed as the ratio of the intensity of light reflected from the sample to the intensity reflected from a standard magnesium oxide surface (10). The egg powder was placed in a brass cup 1 inch in diameter. Before a measurement was made, the surface of the powder was smoothed with a spatula. Despite the utmost care, the preparation of reproducible surfaces is the greatest source of error in color measurement. Duplicate determinations did not vary by more than 10%. Carotenoid and brown lipide amine-aldehyde products were determined upon ether extracts of the egg powders by the spectrophotometric and 5uorophotometr~c procedures previously published (6). The brown "glucose-protein" products were measured fluorometrically in salt extracts of the defatted residues
H E yellow color of freshly dehydrated egg powders is due primarily to the presence of two carotenoid pigments, lutein and zeaxanthin (7) and, to a lesser degree, to the presence of riboflavin (16). Changes in color of ether extracts of stored dehydrated eggs have been studied spectrophotometrically and ascribed t o destruction of carotenoids, the formation of brown lipide amine-aldehyde products, and the formation of brown polymerization products of unsaturated fatty acid groups (4, 6). I n addition, the proteins undergo discoloration (1, 14). Brown substances have been shown t o occur during storage of dried egg white and crystalline egg albumin as a result of reaction of the free amino groups of proteins with aldehyde groups, supposedly of glucose (11). The changes in color of dehydrated eggs during storage from a bright yellow to a darker brown color were studied by White and Grant (17) with a photoelectric colorimeter. This instrument employs filters for the isolation of various spectral regions. The intensity of light reflected by the sample is expressed as the percentage of that reflected by magnesium carbonate. The reflectance of unsltered white light from egg powders was found t o decrease with increasing time and jemperature of storage. While i n most spectral regions the reflectance was found to decrease, there were regions in which no change or a n actual increase in reflectance occurred. Aside from the suggestion that specific pigment alterations were involved, no explanation of the color changes was offered. The present paper reports observations obtained with a reflection spectr6photometer, and is thus an extension of those of White and Grant. Their apparently anomalous results are explained in terms of specific pigment changes determined spectrophotometrically and fluorophotometrically. The influence of moisture content on both loss of carotenoids and formation of brown lipide amine-aldehyde product during storage has been studied.
(18).
T H E effect of moisture content upon the color changes of dried egg from yellow t o brown during storage was investigated. Portions of a batch of egg dried from the frozen state were adjusted to 0.7,1.2,3.0,3.7,and 4.3%moisture, and stored in air at 98"F. Reflectance of samples of each of these materials was determined before and after 2,4, and 8 weeks of storage. Figure 1 records the reflectance spectrum of the starting material a t the 0.7% moisture level. Although t6e unstored egg powders showed, in general, slightly higher percentage reflectance with increasing moisture, this effect was found to be well within the experimental error of the measurement in the range of moistures concerned. The curve given is therefore considered representhtive of the other four moisture levels at the beginning of the
T H E R E are two general physical methods for the study of color in food products: analysis of the color of light reflected from products (e.g. spectral reflectance measurements), and investigation of extracts of the product for concentrations and lightabsorbing properties of the pigments that contribute to color. Both procedures were used in the present study. I n general, t h e pigmept colitent of a given sample cannot be simply expressed i n terms of spectral refleatance properties (13). However, if factors such as particle size, refractive index, moisture content, etc., are constant, the light reflected from a given sample decreases with increasing pigment content. Reflectance measure1
Present address, Northern Regional Research Laboratdry, Peoria, 111.
341
INDUSTRIAL AND ENGINEERING CHEMISTRY
348
0
2
Vol. 38, No. 3
4
8
6
STORAGE T I M E IN W E E K S 350
400
450
550
500
WAVE L E N G T H
IN
600
650
70
MILLIMICRONS
Figure 1. Reflectance Spectra of Egg Powders before Storage and after 8 Weeks of Storage in Air at 98" F.
experiment. This curve exhibits minima characteristic of the carotenoid pigments a t wave lengths of approximately 430, 450, and 480 mp, which correspond to absorption maxima for these pigments in ether solution a t wave lengths 425,445, and 475 m p Figure 1shows also the reflectance spectra for samples stored in air a t 98' F. for 8 weeks. Three main changes in the spectra are apparent: (a)I n the region of carotenoid absorption, the reflectance for the 4.37, moisture powder decreased during storage; it increased in this region, however, for the 0.7 and 3.0% moisture powders. ( b ) The minima characteristic of carotenoids were less apparent in the stored samples than in the sample not stored. (c) Reflectance a t the short-wave-length end of the spectrum decreased with storage. The decrease was greater a t the higher moisture levels. The development of these complex changes in reflectance spectra during storage was followed by measurement of reflectance a t 450 and 380 mp (Figure 2 ) . The reflectance values a t 450 mp for all moisture levels passed through a maximum. The initial increase in reflectance was greatest in the low-moisture samples; the subsequent decrease in reflectance was greatest in the samples with highest moisture. At 380 mp (Figure 2b), the reflectance decreased a t all moisture levels to an extent which depended on the moistqre content. This behavior of the reflectance curve is explicable in terms of specific changes in pigment content. Carotenoid pigments are destroyed during storage, the rate being highest initially and decreasing with time (Figure 3). Within the experimental error of the determination, the rate of carotenoid destruction is independent of moisture content. The destruction of these pigments accounts for the initial in2reases in reflectance observed a t 450 mp (Figure 2a). Brown lipide amine-aldehyde substances formed during storage were measured fluorometrically (Figure 4). Their formation, in contrast to the carotenoid destruction, mas accelerated by increasing moisture content. From the reflectance curves for the ether-extracted residues of the 8-week samples, i t is apparent that part of the decrease in reflectance of the original powders arose from the formation of ether-insoluble brown materials (Figure 5). This process has been referred to as glucose-protein reaction. The observations of previous investigators (8,11, 14) that such a reaction is favored
Figure 2. Effect of Moisture Content and Duration of Storage at 98" F. on Reflectance of Egg Powders Curves above (a)represent reflectance at 460 mp, those below ( b ) at 380 mp. Open circles represent samples scored acceptable as scrambled eggs, with palatability score of 6 or higher (2); closed circles represent unacceptable samples.
by increasing time and moisture content can also be inferred from these curves. The course of change in reflectance a t 380 mp in the residues has been measured; the curves obtained me similar in shape to Figure 2b, though higher in value. The combination of browning reactions with loss of carotenoids caused the apparently anomalous behavior represented in Figures 1 and 2. Because of the overlapping of absorption bands of carotenoid and brown pigments, the loss of carotenoid pigments was in part compensated for by the formation of brown substances. At
0
I
I
2
4
I 6
8
WEEKS Figure 3. Change in Carotenoid Content of Egg Powder at Five Moisture Levels with Storage at 98" F. STORAGE T I M E I N
80,
0
2
4
6
e
STORAGE T I M E IN W E E K S
Figure 4. Change in Lipide Fluorescence Value of Egg Powder at Five Moisture Levels with Storage at 98" F. Open circles represent samples scored acceptable as scrambled eggs, with palatability score of 6 or higher (2); dosed circles represent unacceptable samples.
INDUSTRIAL AND ENGINEERING CHEMISTRY
March, 1946
349
of palatability in dehydrated eggs (16). The apparent correlation between color and quality suggests the possibility of using reflectance as an index of acceptability. Such reflectance measurements would have the advantages of ease and rapidity of direct measurements performed upon the egg powders. Attempts to find a quantitative relation between reflectance measurement and pigment content are now being made. Curves relating negative log reflectance to pigment content of stored egg powders and control samples thus far investigated have the shape of Figure 6 and are approximately linear for low values and concave , upward for increasing values of pigment.
/ k'
STORED 8 WEEKS
.4
350
400
I
I
I
I
450
500
550
600
WAVE LENGTH
IN
'i .3
650
MILLIMICRONS
Figure 5. Reflectance Spectra of Ether-Extracted Egg Powders before Storage and after 8 Weeks of StorageinAirat98' F. In the curvce for the residue. after ether extraation, aarotenoid minima are evident. Their presence reveals carotenoid removal to be incomplete even after 4 hours of continuous Soxhlet extraction and even though no further fat or pigment i s taken out by increasing the extraation time. Part of the lowered reflectance i n the region of carotenoid absorption i s probably due t o riboflavin (6). Minima attributable to this pigment have been observed in the reflectance spectra of lyophilized egg white.
high moisture levels the effect of the formation of brown substances nearly equaled the effect of loss of carotenoids upon the reflectance readings at 450 mp. The brown substances of the ether-extracted residues have recently been pointed out as being primarily responsible for the fluorescence of salt extracts of the residues ( 1 1 ) . This concept is substantiated by the approximately linear relation of the reflectance and fluorescence data shown in Figure 6. I n this graph the negative logarithm of the reflectance (log l/reflectance) rather than reflectance itself is plotted. The reciprocal function is used since i t increases with increasing pigment content, and the logarithmic function is used, since absorption, which is a logarithmic function in solution, also enters into the reflection phenomenon. The effect of storage temperature upon color of spray-dried (5% moisture) egg powders held for 9 months in air is recorded in the reflectance curves of Figure 7. The absorption and fluorescence characteristics of ether extracts of these samples were described in detail elsewhere (4). At the highest temperature of storage, the minima characteristic of carotenoid pigments are barely distinguishable superimposed upon the broad general absorption of the brown substances. Since browning reactions are accelerated both by increasing moisture and temperature, the shapes of these curves are similar to those of Figure 1 except i n the region of carotenoid absorption; they differ in this region, since rate of carotenoid destruction was increased by temperature ' but unaffected by moisture. The compensatory effect of simultaneous loss of carotenoids and formation of brown substances accounts for the previous .observations by White and Grant ( 1 7) that storage of egg powders decreases the intensity of light reflected in some spectral regions while in others it remains constant or actually increases, COLOR in itself is an important criterion of acceptability of food products (3). The changes in color of dehydrated eggs reported Bere are shown to occur simultaneously with loss of palatability. .The destruction of carotenoid pigments is indicative of oxidative change, The fluorescence of salt extracts, which is widely used as an index of palatability, was recently shown (11) and is confirmed here, to be excited in the biown "glucose-protein" reaction product. Other evidence indicates that the development of ibrown lipide amine-aldehyde compounds is closely related to loss
W
0
(3
0
J
.I
Figure 6. Relation between Negative Log Reflectance of Residues after Ether Extraction (X = 380 mp) and the Fluorescence of Salt Extracts
,With regard to the practical application of reflectance measurements to the eval,uation of stored egg powders, it is evident that determination of carotenoids on random samples cannot serve as an index of deterioration, since the carotenoid content of the original powders is highly variable. Determination of brown substance, however, has the advantage that the brown material occurs in freshly and properly dehydrated egg in only negligible quantities but is formed under adverse conditions of dehydration and storage.
.I .6
2
20 3 W -I
A
a .3
.e
'
I
I
I
I
I
I
I
I
350
400
450
500
550
600
660
70
WAVE LENGTH
IN
MILLIMICRONS
Figure 7. Reflectance Spectra of 5% Moisture Egg Powders Stored for 9 Months at 15", 70°, and 98" F.
INDUSTRIAL AND ENGINEERING CHEMISTRY
350
ACKNOWLEDGMEXT
The authors are indebted to H. L. Fevold for samples used, and wish to acknowledge the interest and suggestions of other members of the staff of this laboratory who are investigating the causes of deterioration of dehydrated eggs. LITERATURE CITED
Balls, A. K., and Swenson, T. L., Food Resewch, 1,319 (1936). Boggs, M. M., and Fevold, H. L., unpublished results. Dutton, H. J., Bailey, G. F., and Kohake, E., ISD.ENG.C H ~ M , , 35, 1173 (1943).
Dutton, H. J., and Edwards, B. G., ISD. ENG.CHEM.,37, 1123 (1948).
Dutton, H. J., and Edwards, B. G., ISD.ENG.CHEM.,ASAL. ED.,18, 38 (1946).
Vol. 38, No. 3
(6) Edwards, B. G., and Dutton, H . J., Ibid..37, 1131 (1945). (7) Kuhn, R., and Brockrnann H. B E F 65B 894 (1932). (8) Maillard, L.c.,A ~them:, , 5,'258 ;191~;, (9) Miller, E. S., "Quantitative Biological Spectroscopy", Minneapolis, Burgess Pub. Co., 1939. (10) Xat. Bur. of Standards, Letter Circ. 547 (1939). (11) Olcott, H. S.,and Dutton, H.J., IND.EbG. C H E X . , 37, 1119 (1945). (12) Pearce, J. A., Thistle, M . W., and Reid, M . , Can. J . Research, 0 2 1 , 341 (1943). (13) Saunderson, J. R., J . Optical SOC.A m . , 32, 727 (1942). (14) Stewart, G . F., Best, L.R., and Lome, B., Proc. I n s t . Food Tech., 1943, 7 7 . (15) u. s. D W . k-., C b c . 583, 57 (1941). (16) Western Regional Research Lab., unpublished results. (17) White, W. H., and Grant, G. A., Can. J . Research, F22, 73 (1944).
SPecific Heats of Vegetable Oils from 0"to 280" C. PAUL E. CL>4RK, C. R. WALDELAND, AND ROBERT P. CROSS Washington and Jefferson College, Washington, Pa.
T h e specific heats of hjdrogenated cottonseed, castor, soybean, tung, linseed, and perilla oils have been determined otcr the temperature range 0 ' to 280" C. in a batch calorimeter. For each run the specific heat w a s calculated from measurements of the weight of the oil, the actual temperature rise of the oil, and the heat energy supplied to the oil corrected for the heat capacitj of the calorimeter. The heat capacity of the calorimeter was determined at various temperatures by liquid diphenyl. Within the temperature range studied, the specific heats of each oil increase with increasing temperatures arid all values fall within the range 0.40 to 0.70 calorie/gram/ ' C. In general, the specific heat-temperature curiea for the different oils hale about the same shape and slope and are displaced toward lower value6 as the iodine numbers of the oils increase. The values are compared with those of other intestigators.
T
HE lack of adequate specific heat data in the literature for vegetable oils and the importance of such data in industry, as well as in phase studies of fats and oils, were discussed recently by Gudheim (4) and Bailey, Todd, Singleton, and Oliver (I). Specific heats of the various types of vegetable oils may also be of value in studying the mechanism of the heat-induced polymerization of drying oils. Deaglio and b4ontu ( 2 ) used a n electrical comparison method, and reported the specific heat of castor oil as 0.505 a t 50" C. and 0.525 a t 70". Long, Reynolds, and Kapravnik (5) reported the specific heats of alkali-refined linseed oil (iodine number, 181.3) as 0.504 to 0.665 in the range 75" t o 290" C.; tung oil (iodine number, 156.7) as 0.516 to 0.644 in the range 69" to 155' C.; and soybean oil (iodine number, 131) as 0.568 to 0.765 in the range 75' to 287" C. Their apparatus consisted of an insulated thermos flask used as a batch calorimeter. They calculated specific heats from a measured amount of heat energy supplied to a known quantity of oil and the actual temperature rise of the oil. They stated that their results are reasonably correct in the lower temperature ranges,and that the heat capacity is a linear function of temperature in the lower ranges from 70" to about 170" C. Delaplace (3) used the method of mixtures, and reported
specific heats of castor oil as 0.424 to 0.553 in the range 0" t o 210" C. Gudheim (4)also used the method of mixtures and reported the specific heats of refined cottonseed oil as 0.524 * 0.010 in the range 45' to 50" C., and 0.535 * 0.010 in the range 95' to 100' C., and refined and bleached palm oil as 0.515 * 0,010 in the range 45" to 50" C. Bailey, Todd, Singleton, and Oliver (1) used a n all-metal batch calorimeter of special design for low-temperature determinations with which they obtained results accurate within 1%. With the same apparatus they also reported (8)specific heats of cottonseed oil (iodine number, 108.3) as 0.471 to 0.499 in the range 15' to 60" C., and of hydrogenated cottonseed oil (iodine number, 59.5) as 0.497 to 0.514 in the range 40" to 70" C. Oliver and Bailey ( 7 ) used the same apparatus and reported the specific heats of highly hydrogenated cottonseed oil (iodine number, 0.85) as 0.524 to 0.537 in the range 67" to 82" C. The purpose of the present study was to determine specific heats of various types of vegetable oils over much of the temperature range in which they are commonly heated in industry. To obtain uniformly good results in the range 0' to 280" C., a modification of the apparatus and technique employed by Long, Reynolds, and Napravnik (5)was used. All oils were furnished by the Armstrong Cork Company. Before use they were heated in a round-bottom flask maintained a t 100" C.for 3 to 6 hours under a pressure of 2-3 mm. of mercury to remove moisture. After this heating their iodine numbers (Wijs) were as follows: hydrogenated cottonseed, 6.5; castor, 83.0; soybean, 128.3; tung, 154.4; linseed, 172.1; perilla, 186.2. The diphenyl used in determining the heat capacity of the calorimeter was Eastman Kodak Company's highest purity, melting point 69.5-70.5' C. APPARATUS AND PROCEDURE
The calorimeter was a silvered, evacuated Dewar flask of about one liter capacity, closed with a gasketed '/r-inch brass plate equipped with four brass tubes for heater leads, thennometer, motor-driven glass stirrer, and inert gas conduit. The gasket was a ring of either '/:-inch gasket cork or 1,'a-inch synthetic rubber. The brass tubes were about 3 inches long, had an inside diameter of about 3/8 inch, and were brazed to the brass plate.
