TEXTURE AND MICROSTRUCTURE OF HEAT-FORMED EGG WHITE, EGG YOLK, AND WHOLE EGG GELS (WATER-BINDING).

摘要

The texture and microstructure of egg white, egg yolk, and whole egg gels were investigated by gel compression and scanning electron microscopy. Samples were treated by varying pH from 5 to 9, protein content from 9 to 13%, NaCl concentration from 0 to 1.00 M, temperature from 75 to 90(DEGREES)C, and heating time from 10 to 50 minutes. Gel strength generally increased with increasing pH, protein, temperature, and time of heating for all gels. Salt reduced the firmness of egg white gels, increased the firmness of egg yolk gels substantially, and caused slight increases in whole egg gel hardness. Gels from blends of white and yolk were 20 to 140% firmer than corresponding egg white or egg yolk gels due to product interaction.Freezing of gels caused structural artifacts due to ice crystal formation. Damage was most severe in egg white gels, slightly less severe in whole egg gels, and only slight in egg yolk gels. Gels were acceptably prepared for scanning electron microscopy by fixation in glutaraldehyde, post-fixation in O(,S)O(,4), hardening in uranyl acetate, dehydration in ethanol, and critical point drying.Egg white gel structure was affected by pH, heating time and temperature, salt, and yolk. Gels at pH 9 were uniform with small pores. Severe aggregation occurred at pH 5 and 6. Aggregation was enhanced by salt and reduced by 10% or more yolk in egg white. As time and temperature of heating were increased, egg white gels were more completely crosslinked. The microstructure of hard-cooked egg yolk was grainy and crumbly, while that of stirred egg yolk gels was smooth and continuous. Whole egg gels had a uniform structure which was only slightly affected by changing pH or adding salt.Water-binding ability was determined by expressing serum from frozen-thawed gels. Serum proteins were separated and identified by polyacrylamide gel electrophoresis. As gel hardness increased the amount of serum generally decreased. The best water-binding ability occurred for gels at pH 9, 13% protein, 80 to 85(DEGREES)C, and 30 minutes of heating. Salt caused slight reductions in serum of egg white and egg yolk gels, but it more effectively lowered serum in whole egg gels. Egg white, egg yolk and whole egg gels averaged 58, 7, and 27% expressed serum, respectively.