Egg white gelation . . .
native molecules become exposed on the protein surface, firm this result. In the presence of Ca2+, egg white gels were
and proteins aggregate mainly by formation of intermolecu- less homogeneous with particles clustered in random aggre-
lar -sheets (Li-Chan and Nakaï 1989). At pH 5, the protein gates, without “string of beads” structures (such as those ob-
net charge was low. Moreover, near their pI, proteins are the served in the absence of cations).
least hydrophilic. They randomly aggregated before unfold-
ing and retained little water. At pH 7 and 9, the negative net
charge of proteins was increased and thermocoagulation was
retarded (Ma and Holme 1982). Then, proteins unfolded be-
Conclusions
N CONCLUSION, THE USE OF A MULTIFACTORIAL ANALYSIS
through an experimental design in order to study complex
fore aggregating, leading to a gel with good viscoelastic prop- systems such as those we studied (protein mixture with pres-
erties and high water-holding capacity. Moreover, the higher ence of cations and anions) is helpful to quantify effects of
the pH, the greater the sulfhydryl disulfide interchange and factors and interactions through a reasonable number of ex-
the viscoelastic properties of the egg white gel (Handa and periments. We found again some known results on gelling
I
others 1998).
Whatever the pH, water release was maximum for the fects of pH, NaCl, Ca -ovalbumin, Mg -ovalbumin, and
properties of purified proteins and egg white (that is, the ef-
2
+
2+
3
+
highest NaCl concentration (120 mM); some differences on Fe -ovotransferrin binding). Unfortunately, we were unable
2
–
the scanning electron micrographs of egg white gels were to quantify the effects of HPO4 and citrate (expecting a
observed at pH 7. At this pH, electrostatic repulsion between slight effect on egg white gel viscosity at pH 7), and it may be
2
–
protein molecules was important. The binding areas of dena- a limitation of the experimental design. HPO
and citrate
4
tured egg-white proteins were limited to a small part of their are known to complex Ca2 , Mg , and Fe and should com-
surface, and linear aggregates were mainly formed at low pete with ovalbumin or ovotransferrin for binding. An expla-
NaCl concentrations (Kitabatake and others 1988). By in- nation for this could lie in the level of HPO42 and citrate fac-
creasing the NaCl concentration, repulsion between proteins tors. But in the conditions we used, these elements had a
+
2+
3+
–
3
+
2+
decreased by shielding the negative charges of the proteins. lower impact on gelling properties than pH, NaCl, Fe , Ca
,
Water-protein interactions were reduced in favor of protein- and Mg2
protein interactions. The aggregation step was privileged,
compared to the protein unfolding step: then, gels with more
+
.
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Vol. 67, Nr. 2, 2002—JOURNAL OF FOOD SCIENCE 613