Viscosity is measured by rotating the spindle at different speeds and recording the value. General observation for dressings is that viscosity reduces as rotational speed increases. This means that there is less resistance to flow the faster the dressing moves. The graph in Figure 4 shows data for two dressings. The x-axis parameter is “shear rate,” which is proportional to rotational speed. Shear rate accounts for the shape of the spindle and the ratio of spindle diameter to container diameter. The curves for both dressings look similar; the premium brand is slightly higher in value than the non-brand.
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Traditional use of viscosity flow data might have led to the conclusion that the two dressings were relatively similar. Use of yield stress measurements and creep flow data gives a different assessment. The premium brand is more likely to have the rich creamy appearance in the bottle when evaluated in the supermarket. Its flow behavior after pouring allows it to cling to salad.
Manufacturers who strive for the higher value dressings are willing to invest in the tests that verify these differences in flow behavior. QC is now tasked with measuring not only viscosity flow curve, but also yield stress and creep. Advancements in instrumentation make it possible for rheometers to be programmed to perform all three tests at once. This allows the technician to set up the sample as before, run the test with the push of a single button, and automatically record data while tending to other tasks in the lab. Increasing use of rheometers in QC is enabling high-end manufacturers to keep pace with growing consumer demand while producing consistent high-quality dressings.
Ridley is sales manager for RS rheometer and powder flow tester at AMETEK Brookfield. Reach him at email@example.com. McGregor is director of global marketing and high-end lab instrument sales. Reach him at firstname.lastname@example.org.