KETCHUP:
ELASTIC YIELD STRESS, THIXOTROPY,
SHEAR RATE AND FREQUENCY DEPENDENCE
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KETCHUP: ELASTIC YIELD STRESS, THIXOTROPY, |
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| These figures are profiles of the rheological properties of ketchup* measured under oscillatory flow in a 2 mm diameter tube at 22°C. Figure 1 shows decreasing viscosity and elasticity with increasing shear rate at 2 Hz. The maximum in the elastic stress shown in figure 2 is identified as the "elastic yield stress". At higher shear strains the elastic stress does not decrease significantly, indicating a dilatant tendency of the ketchup. |
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| Figure 3 shows the frequency response at a constant stress of 0.9
Pascal. Because of the continuing slope, it appears that over the frequency range of the
measurement, the viscoelasticity is primarily due to shorter relaxation time processes and
the influence of the long relaxation time processes is reduced. If the frequencies were
sufficiently lower, the long relaxation time processes would become dominant. In Figure 4,
the ketchup is subjected to a high stress of 7 Pascal and then to a lower stress of 0.4
Pascal. The slow recovery of the viscoelastic properties after the initiation of the lower
stress is a result of thixotropy. The time for complete recovery of structure is longer
than six minutes. *Ingredients: Tomato paste, water, corn syrup, sugar, vinegar, salt, dried onion, spice, dried garlic
Vilastic Scientific, Inc (01)512-327-4134 (c) 1998-2000 Vilastic Scientific, Inc. |
