Abstract
The rheological behaviour of egg yolk after different storage periods and temperatures was investigated using rotational viscometer. The eggs were stored for 1, 2, 3, 4 and 8 weeks at constant temperatures: 4 °C, 8 °C, 12 °C and 16 °C. The apparent viscosity was measured as a function of shear rate. For given shearing rate, the viscosity was measured in dependence on shearing time. The decrease of viscosity with storage time is attributed to the balance of different osmotic pressure from yolk and albumen of fresh egg. The yolk (with higher dry matter then albumen) absorbs water from albumen through vitelline membrane. It was found that yolk samples exhibited shear-thinning and thixotropic behaviour. The shear-thinning behaviour was fitted well into simple models. The resting of sample at room temperature caused increase of viscosity. This effect is explained by drying off of testing yolk, the regeneration of yolk structures and changing chemical composition during resting. The time-dependant viscosity decreased rapidly with time and at lower share rates reached an equilibrium stage. The time-dependant viscosity was also found to decrease with storage time. The value of pH was changed (increased) during storing. No clear dependence between pH value and viscosity was confirmed.
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