How do food manufacturers of sauces and dressings distinguish their premium brand products? Taste comes first and foremost. Not far behind is the handling property—namely, visual appearance in the bottle and flow behavior when poured on salad. Consumers judge “thickness” and “creaminess” in the supermarket by holding the bottle and moving it gently from one side to another. Perception of how the dressing is likely to pour comes from this simple action. In general, careful application of dressing requires controlled flow from the bottle so that just the right amount comes out. Customer dissatisfaction arises when too much dressing gushes suddenly from the bottle or the squeezing action cannot get sufficient quantity to expel within a short time frame.
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Food scientists responsible for formulation of dressings must evaluate flow properties and then set guidelines for QC during manufacturing. Yield stress is one property of interest; this defines how much squeezing force or shaking action is needed to initiate easy flow of salad dressing. Viscosity is essentially “resistance to flow;” it quantifies the physical property that relates to flow rate of salad dressing during pouring. Creep is the property that characterizes how the salad dressing behaves after it deposits on the salad. The point of interest is whether it clings firmly to the coated items or does flow continue causing it to drain off the salad.
All three properties are important, but viscosity alone has been the traditional parameter of interest. In recent years, premium brand manufacturers have also focused on yield stress and creep for the following reasons:
- Visual inspection of salad dressing in the bottle is equivalent to making a judgement on yield stress;
- Ease of use when initiating flow requires a yield stress that can be readily overcome by shaking or squeezing; and
- Adherence to salad components like lettuce and tomato requires minimal creep flow.
Figure 1 shows a rheometer with vane spindle used by R&D to characterize the flow behavior of salad dressings. The vane is immersed into a container of salad dressing and rotated at very low speed, perhaps 1 rpm, to determine “yield stress.” Figure 2a illustrates the type of data curve that results when plotting stress on the y-axis and strain on the x-axis. The slope of the rising curve is called “modulus” and its value relates to the “stiffness” of the dressing.
The steeper the slope, the stiffer the formulation. When the peak value for stress is measured, this correlates with “yield stress” for the dressing. Figure 2b compares two salad dressing formulations for yield stress.
The upper curve shows the premium brand that has both higher modulus and yield stress. This stands to reason since dressings with more body are generally preferred by consumers who suspect that “thinner” formulations may be watered down.
Figure 3a shows the data curve that characterizes creep behavior. Low stress is applied to the vane spindle by the rheometer to simulate the action of gravity acting on dressing after it is poured on salad. The data curve shows flow movement of the dressing as a strain value on the y-axis plotted against time on the x-axis. The flatter the strain curve, the less movement of dressing after application to salad. Figure 3b compares the same two dressing formulations. Note that the premium brand has lower creep profile. This makes sense because the non-brand is more likely to not cling as readily to salad.
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.
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 firstname.lastname@example.org. McGregor is director of global marketing and high-end lab instrument sales. Reach him at email@example.com.