Figure 1 – Helipath Stand with T-bar spindle (Brookfield Engineering Laboratories, Middleboro, MA).A common request from food manufacturers is to come up with a faster and more definitive test for flowability of salad dressings. This may not be an easy task because there are many facets to the challenge of determining how salad dressings will flow. Workers at the bottling line in the manufacturing plant simply want the dressing to fill the container quickly and completely. Quality Control takes random jars from the fill line for evaluation and runs tests specified by R&D to measure viscosity and/or texture. If the test passes, the product will ship. But the challenge is not over.
During transport of the salad dressing in shipping containers and storage in warehouses until it arrives at the supermarket shelf, the consistency of the product must remain undisturbed. Suspended particles or chunks, like the ones found in blue cheese dressing, hopefully will not sink to the bottom. When inspecting the bottle of dressing on the supermarket shelf, the customer wants to behold an appealing product. If “thick and creamy” is the marketing description of the product, then the dressing had best look that way when first held up to the eye.
When used by the customer, the dressing must come out of the bottle or jar without difficulty. This may involve squeezing, spooning or application with knife or spatula. The dressing must stay in place once on the food item, whether it is bread, lettuce or whatever it coats. Should flow continue, it had best be relatively slow, so that the dressing continues to cling to the food item on which it has been placed. Finally, once in the consumer’s mouth, the dressing should give a pleasurable texture and feeling along with the expected good taste.
Figure 2 – Viscosity data from test performed with T-bar spindle.
Figure 3 – Brookfield Texture Analyzer.The call for help
Instrumentation companies that specialize in measuring flow behavior will often receive requests like the following: “We are trying to measure the ‘thickness’ of our salad dressings, which range from the consistency of mayonnaise to thick and creamy blue cheese dressing. We use different T-bar spindles with the Helipath Stand on a rotational viscometer to make a viscosity measurement (see Figure 1). Each test takes about 3 min to set up, run and clean up after. The viscosity data can vary considerably within a single test, especially for the dressings with particles and chunks; we are not sure which viscosity value to use (see Figure 2). Is there an alternative way to do this test that takes less time and involves only one spindle choice?”
The food industry has used the T-bar spindle for many years to make this type of viscosity measurement. For dressings that are creamy in consistency, the viscosity signal will not be as “choppy” as the curve shown in Figure 2. For dressings that have suspended particles, the erratic jumps in the viscosity signal are due to the T-bar hitting the particles as the spindle rotates. There is no way to avoid this because it is due to the way in which the measurement is made. T-bars are low cost and can be used with any standard rotational viscometer, so there is a practical reason for continuing the use of this measurement methodology.
Figure 4 – a) Wire mesh probe; b) ball probe.Texture analysis
A potential alternative to the rotational viscometer is the texture analyzer (see Figure 3). The principle of operation is to use a probe of defined shape and push it into the material under test. The resistance that is detected by the instrument as the probe pushes into the material is measured as a force using scientific units of grams or Newtons.
When testing for consistency of salad dressings, the texture analyzer may provide a quicker method than the viscometer. You can choose between two different types of probes: the wire mesh probe and the spherical ball probe (see Figure 4). The texture analyzer pushes the probe down into the dressing at a defined rate of speed, perhaps 1 mm/sec, and measures the load force in grams, which is the dressing’s resistance to being displaced. The load reaches a steady state value within seconds as the probe becomes fully immersed. The following example describes the test procedure using a texture analyzer to test two different products.
The salad dressings evaluated were buttermilk ranch and blue cheese. Prior to testing, the samples were refrigerated until each sample reached an average temperature of 38 °F. The ball probe was used to make the measurement. Table 1 summarizes the test method.
Test method
Table 1 – Test method using a texture analyzer to evaluate buttermilk ranch and blue cheese dressings
The probe travels down and makes contact with the sample of each salad dressing. When a trigger force of 10 g is detected at the sample surface, the instrument begins to record the measured force. The probe proceeds to penetrate the sample up to the specified distance of 15 mm. As the probe penetrates the sample, the measured force increases, as shown in Figure 5a, which is a graph of force in grams on the y-axis vs time on the x-axis. The maximum force is called “peak load” and is a measure of sample hardness; the higher the peak load value, the firmer the sample.
Data report
Software can be used to control the texture analyzer in R&D when running experimental tests on new formulations for ranch and blue cheese dressings. The advantage is that it can also analyze the data and provide additional information based on the data in the flow curve. It automatically calculates peak load values based on the data in the graph. It calculates “hardness work done” (area under the curve from start of test to peak load in Figure 5b), “adhesive force” (maximum force measured as the ball probe pulls out of the sample), and “adhesiveness” (work done to extract the probe from the sample). Multiple samples can be run if desired, and calculated values for standard deviation can be reported.
Figure 5 – a) Graph of force vs time for two salad dressings; b) graph of force vs distance for two salad dressings.Table 2 –Evaluation: blue cheese vs ranch dressing
A review of the data (Table 2) shows that the firmer sample is the blue cheese dressing. It also exhibits greater adhesiveness.
Conclusion
The test results demonstrate that a texture analyzer with ball probe provides a suitable method for testing the two salad dressings. It is a simpler test compared to a rotational viscometer in the sense that the actual test is quicker to perform, the peak force value is easy to identify, and the cleanup afterward is easy. The added value is the extra information on “work done” and “adhesiveness.” The entire test requires less than 1 min to complete.
One tradeoff in choosing an appropriate test method is that the texture analyzer is more expensive than the viscometer. However, this may not be an issue for the busy QC department, since the time savings afforded by the texture analyzer quickly pays back the added investment to purchase the equipment.
Robert McGregor is General Manager, Global Marketing/High-End Lab Instrument Sales; Eric Chiang is Product Manager, Texture Analyzers; and at the time of this writing Stephanie Shepard (summer intern) was a lab assistant, Texture Analyzers, Brookfield Engineering Laboratories, Inc., 11 Commerce Blvd., Middleboro, MA 02346, U.S.A.; tel.: 508-946-6200; e-mail: [email protected]; www.brookfieldengineering.com