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Studies on colour of egg yolk Fadl , Essam Bahgat

Abstract

The effect of varying combinations of irradiation treatment, freezing procedures and duration of storage on the color of naturally and artificially pigmented egg yolk determined by two objective methods was studied. The relationship between the two methods of color evaluation was also determined. Naturally pigmented yolks were obtained from eggs laid by pullets of a single strain of Single Comb White Leghorn fed a standard diet. Artificially pigmented yolk was prepared by addition of either beta-carotene or canthaxanthin to the naturally pigmented material. Color of yolk was assessed: 1. On the basis of pigment concentration determined by absorbance of acetone extract and expressed as beta-carotene equivalent (BCE) and 2. On the basis of chromaticity coordinates (x,y), lightness (%Y), dominant wavelength (DWL) and excitation purity (EP) determined by reflectance spectrophotometry. Both irradiation dose (0, 0.5, 1.0 and 2.0 Mrad) and time of irradiation (before or after freezing) had significant effect on the chromaticity coordinates, BCE values and excitation purity of naturally and artificially pigmented yolk samples. Higher radiation doses and irradiation before freezing were associated with decreased chromaticity coordinates, BCE values and excitation purity. In artificially pigmented samples increases in irradiation dose and irradiation before freezing resulted in significant decreases in lightness. Samples frozen and stored at -10 F° had consistently higher mean chromaticity values and lower excitation purity than those at -35 F°. The temperature effect on BCE values was inconsistent among experiments. After 30 days storage mean x-values were lower and mean y-values were higher than after 10 days storage. These changes were associated with almost no change in DWL or EP. Nitrogen-packed samples had consistently lower BCE values than air-packed and this difference was significant in all but Experiment 1. No corresponding differences were found in chromaticity coordinates, lightness, DWL or EP. Correlation analyses revealed highly significant (P≤0.01) linear relationships between BCE and both chromaticity values and lightness ranging from + 0.09 to + 0.79.

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