Introduction
A ready-to-use vinaigrette is a fast alternative to homemade dressings for salad preparation. Stores contain a broad variety of such products. Some of them are made not only of classical ingredients such as oil, vinegar and salt, but also contain herbs in suspension. In addition to taste, the aesthetics of a product play a great role in selection by the customer. In the case of a sauce containing herbs, its suspension gives a first impression about product quality, long before the dressing is tasted. A vinaigrette with settled herbs doesn’t look as yummy as the same one with suspended herbs. A dressing with suspended herbs contains such thickening agents as xanthan gum or carrageenan, which give structure to the sauce. This structure is indispensable for holding the particles in the suspension and preventing them from settling.
Experimental
Rheology allows for prediction of the stability of food products by quantifying their structure. In the following, we compare the rheological signals of a fresh salad dressing with herbs to one that is expired by three years. As shown in figure 1, the herbs of the expired vinaigrette are all on the bottom of the bottle.

Measurement Results
Figure 2 depicts the shear viscosity curves of the fresh and the expired salad dressings containing herbs. In the higher shear rate range, the two curves are similar and show the Shear ThinningThe most common type of non-Newtonian behavior is shear thinning or pseudoplastic flow, where the fluid viscosity decreases with increasing shear.shear-thinning behavior of the dressings: The higher the shear rate, the lower the shear viscosity. In practice, this means that the sauce feels “more liquid” if stirred faster. However, the two products differ in the low shear rate range. While the shear viscosity of the fresh product increases with decreasing shear rates, it reaches a Newtonian plateau for the expired one. In the first case, the sample has a Yield StressYield stress is defined as the stress below which no flow occurs; literally behaves like a weak solid at rest and a liquid when yielded.yield stress, i.e., requires a minimum StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress before it starts flowing. This is typical for products having a structure able to inhibit sedimentation. In contrast, the expired product has no Yield StressYield stress is defined as the stress below which no flow occurs; literally behaves like a weak solid at rest and a liquid when yielded.yield stress, but a zero-shear viscosity plateau, i.e., a shear viscosity at rest. Because of this absence of Yield StressYield stress is defined as the stress below which no flow occurs; literally behaves like a weak solid at rest and a liquid when yielded.yield stress, the sauce will not be able to hold the particles in the suspension anymore: They will settle down.

Quantification of a Structure: The Yield Stress
Figure 3 displays the shear viscosity curve of the fresh salad dressing together with the shear StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress curve. In the direction of the lower shear rates, the shear StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress decreases and tends to reach a plateau. The extrapolated shear StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress value in this plateau corresponds to the Yield StressYield stress is defined as the stress below which no flow occurs; literally behaves like a weak solid at rest and a liquid when yielded.yield stress. It is lower than 0.2 Pa.

The Yield StressYield stress is defined as the stress below which no flow occurs; literally behaves like a weak solid at rest and a liquid when yielded.yield stress can also be determined by a CreepCreep describes a time and temperature dependent plastic deformation under a constant force. When a constant force is applied to a rubber compound, the initial deformation obtained due to the application of the force is not fixed. The deformation will increase with time.creep test. For that, 5-minute CreepCreep describes a time and temperature dependent plastic deformation under a constant force. When a constant force is applied to a rubber compound, the initial deformation obtained due to the application of the force is not fixed. The deformation will increase with time.creep measurements were repeated on the same loading, using a starting StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress of 0.01 Pa and followed by tests with stresses increasing by a factor of 1.5. The temperature of the test was 25°C. Figure 4 depicts the resulting curves of such a test on the fresh salad dressing. At 0.10 Pa and 0.15 Pa, the curves overlay and tend to reach a plateau. The applied StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress doesn’t lead to any flowing. At higher shear StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress, the compliance increases with the shear StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress. The yield StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress is determined as a value between 0.15 Pa (no flowing) and 0.23 Pa (first shear StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress segment where flowing is detected). This accords well to the value detected above.

Prediction of Stability and Shelf Life: The Frequency Sweep
The stability of an emulsion or a suspension can also be investigated by the signals resulting from a frequency sweep, and particularly from the phase angle. Figure 5 displays the phase angle curves of both salad dressings during frequency sweep at 25°C. An increase in the phase angle toward lower frequencies indicates instability (expired sample, red curve).

During a CreepCreep describes a time and temperature dependent plastic deformation under a constant force. When a constant force is applied to a rubber compound, the initial deformation obtained due to the application of the force is not fixed. The deformation will increase with time.creep test, a constant shear StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress is applied and the resulting variations of shear StrainStrain describes a deformation of a material, which is loaded mechanically by an external force or stress. Rubber compounds show creep properties, if a static load is applied.strain are measured. The compliance J [Pa-1] is defined as:
J = Measured StrainStrain describes a deformation of a material, which is loaded mechanically by an external force or stress. Rubber compounds show creep properties, if a static load is applied.strain [%]/Applied StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress [Pa]
Conclusion
The stability of a suspension is closely related to the existence of a Yield StressYield stress is defined as the stress below which no flow occurs; literally behaves like a weak solid at rest and a liquid when yielded.yield stress, which can be predicted by rotational or oscillation tests. The shape of the shear viscosity curve toward lower shear rates (rotation test) as well as the shape of the phase angle in the frequency sweep toward the low frequencies (oscillation test) indicate whether a Yield StressYield stress is defined as the stress below which no flow occurs; literally behaves like a weak solid at rest and a liquid when yielded.yield stress is present or not.
The Yield StressYield stress is defined as the stress below which no flow occurs; literally behaves like a weak solid at rest and a liquid when yielded.yield stress can be determined by the minimum shear StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress in a shear viscosity curve, by the phase angle in a frequency sweep, or by a CreepCreep describes a time and temperature dependent plastic deformation under a constant force. When a constant force is applied to a rubber compound, the initial deformation obtained due to the application of the force is not fixed. The deformation will increase with time.creep test.