Heat-induced gelation of egg yolk as a function of pH. Does the type of acid make any difference?

Abstract

The objective of this study was to assess the influence of reducing pH from its native value on the heat-induced gelation behaviour of egg yolk by monitoring its linear viscoelastic properties. Three acids differing in their location in the Hofmeister series were used (hydrochloric, citric, and phosphoric acids) at pH between 2 and 6. The viscoelastic measurements were carried out under small amplitude oscillatory shear, using parallel plate geometry by means of: (i) stress sweep tests to delimit the linear viscoelastic range at different temperatures; (ii) temperature ramp tests to monitor egg yolk protein gelation; and (iii) frequency sweep tests at 20 °C, after the thermal cycle. The microstructure of gels was also evaluated by Cryo-scanning Electronic Microscopy (CryoSEM). Egg yolk undergo dramatic changes in rheological and microstructural properties, when processed at high temperature, depending on pH and the type of acid used. Generally, four different regions take place over heat treatment: (i) a fluid-like region showing a moderate decrease in viscoelastic properties with temperature (ii) a sol-gel transition region involving denaturation, aggregation of protein molecules and association of aggregates to form a gel network; (iii) a plateau region for G′ and G″ and (iv) a reinforcement of the gel network through the regeneration of physical interactions during cooling. This pattern may show a strong dependence on pH and the acid involved. Both effects tend to decrease as the thermal treatment proceeds. Heat treatment also reveals large differences in gel microstructure, depending on pH and on the type of acid used.