Egg yolk gels: Sol-gel transition and mechanical properties as affected by oleuropein enrichment

The effect of oleuropein on the gelation properties of whole egg yolk at different pHs (2, 4.5 and 6.4) was studied. Egg yolk solutions were added with increasing amount of pure oleuropein, gelled by either thermal (pHs 4.5 and 6) or acid gelation (pH 2) and submitted to frequency sweep and stress relaxation tests. The experimental data obtained by the frequency sweep tests were modeled with a power law equation in order to get the coordination number z and the proportional coefficient A, which describe the network extension and its strength, while data obtained from stress-relaxation tests were fitted by a generalized Maxwell model with 5 elements. The addition of oleuropein affected the gelation behavior as well as the rheological properties of the gel network, and the effect was influenced by the pH of the systems. A shift of the gel point towards lower temperatures was observed in the presence of the phenolic compound, and this effect was significant in the systems at pH 4.5 (from 72.55 ± 0.07 °C to 70.43 ± 0.06 °C). According to the results obtained from the modelling of the frequency sweep test, gels at pH 6.4 and 2 were characterized by a weak network; on the contrary, at pH 4.5 the network was characterized by a low number of flow units (lower z, z = 16.52 on average) interacting with one another in a more strictly way (higher A, A = 5362 Pa s1/z on average). Such findings were successively confirmed by the stress relaxation results where it was shown that firmer gels were obtained at pH 4.5 after oleuropein enrichment.