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A study of some physical and chemical properties of egg white Beveridge, Herbert James Thomas

Abstract

A three-part investigation is described in which gel filtration was utilized to detect electrostatic interactions claimed to be important in determining functional properties of egg white; viscometry and ultracentrifugation were employed to detect changes which occur during aging of the egg; and beating of egg white for foam formation was studied utilizing bubble size analysis, gel electrophoresis and gel chromatography. A method for the gel filtration of whole egg white on columns of Sephadex G-150 was developed. Dilution of egg white 1:1 with 0.01 M phosphate buffer, pH 7.0, containing 2% (w/v) NaCI and 0.6% (v/v) 2-mercaptoethanol caused rapid and complete dissolution of the thick white gel and resulted in a solution amenable to gel filtration. Elution was accomplished with 0.01 M phosphate buffer, pH 7.0, containing 1% NaCI and 0.3% 2-mercaptoethanol. Four distinct regions were discerned in the elution profile. Disc gel electrophoresis of the regions and the elution volume of the purified proteins were used to identify the components in the various regions. Region 1 contained high molecular weight material eluting at the V[sub o] position where purified ovomucin eluted. Region 2 contained ovalbumin and conalbumin whereas regions 3 and k contained ovalbumin and lysozyme respectively, as major constituents. Lysozyme decreased in region h and increased in region 3 when the salt concentration was decreased to 0.25%. A rotational viscometer and an ultracentrifuge were used to evalute the changes which occur in egg white and in ovomucin on aging. Egg white was found to be a time-dependent pseudoplastic fluid exhibiting decreasing apparent viscosity with shearing time. The time-dependent characteristic was fitted by an equation A = A[sub o] - n.log t and the initial shear stress index (A[sub o]) was found useful as an index of egg white thinning. Potassium bromate gave slightly lower values of A[sub o] compared to an untreated control whereas KIO₃ gave higher values of A[sub o]. Sulfhydryl (-SH) levels were unaffected by KBrO₃ but KIO₃ reduced them from 59 to 54 uM/g protein. Blockage of up to 25% of the -SH groups of egg white with mercuric chloride or p-chloromercuribenzoate resulted in reduction of rate of change of A[sub o] with incubation time, however, more extensive blockage with the same reagents caused increases in A[sub o] suggestive of aggregation reactions. Ultracentrifugation of ovomucin isolated from -SH blocked egg white showed that the relative increase in α-ovomucin which occurs on aging untreated egg white was inhibited, but not completely, by blocking some -SH groups. The beating of egg white for foam formation was studied utilizing bubble size analysis, gel electrophoresis and gel chromatography. Bubble size distributions were found to be fitted best by a lognormal distribution curve for all beating times tested up to 10 minutes and all aging times up to 20 minutes. The average bubble size increased on aging and the change was considerably faster for foam beaten 8 minutes than for foam beaten 2 minutes. In addition the polydispersity of the foam as indicated by the standard deviation of the distribution curve increased much more rapidly for 8 minute foam than for 2 minute foam. Quantitative data detailing these changes are presented. Disc gel electrophoresis of the material drained from the foam showed progressive depletion of lysozyme and the G₂ and G₃ globulins as beating time increased. After 8 minutes beating the globulins had essentially disappeared from the drained material. The depletion of lysozyme was confirmed qualitatively by an enzyme assay. Chromatography on Sephadex G-150 showed changes reflecting those observed by electrophoresis and also confirmed observations by other authors that ovomucin is transferred from egg white to the foam when egg white is beaten. Essentially all of the material eluting at the V[sub o] position was removed in 2 minute beating with only a slight further decrease by 8 minute beating.

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