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Go With the Flow
Advances in Flow Behavior Test Methods Improve Quality Control
by Bob McGregor
Customer perception of food quality depends not only on taste, but also on physical properties, one of which is commonly referred to as “flow behavior.” Consider the following examples. Ketchup must pour out of a squeeze bottle easily and coat the hot French fries without running off. Cream filling in a doughnut needs to hold in place when you bite into it and not squirt out. Salad dressing is better appreciated when it coats the lettuce and vegetables and doesn’t run off into the bottom of the bowl. The icing on a cake must hold its shape – especially on the sides where sag can become a problem or on top where decorative styling is important. These instances show why good test methods in the food quality control (QC) lab are important to identify batches of product that do not meet the desired performance and appearance standards expected by customers.
Two instruments have become workhorses in the food industry for making quick physical property measurements in quality control (see Figure 1.) Rotational viscometers are used to characterize the general flow behavior of the food items mentioned above. Texture analyzers provide complementary information on flow behavior for fluids and offer added capability for testing the compressibility of solid food items such as bread and rolls. Both instruments are easy to use, perform tests in a short time frame (under one minute in many cases) and can be purchased and maintained within a reasonable budget.
The Versatile Vane Spindle
QC labs typically establish test methods with rotational viscometers that perform “single point” checks. This means that a specific instrument with a defined torque range (see Figure 2) will use a designated spindle running at an established speed to measure the viscosity in units of centipoises (or milliPascal seconds, which is the European equivalent unit).The viscosity value obtained must fall within the maximum or minimum limits set by the QC lab or by R&D. Otherwise, the test fails and the batch will require reworking to meet specification.
New opportunities to improve this standard viscosity test method have materialized on two fronts. Traditional spindle types – the disc and T-bar for fluids and paste-like materials respectively (see Figure 3) – are being supplemented by the vane spindle (see Figure 4.) Food sauces, yogurts with fruit pieces and fillings with suspended particles are measured more directly with vane spindles. All of the mixture captured between the vanes rotates with the spindle, which gives a more accurate determination of the overall viscosity. One generally observed phenomenon is that viscosity values can be higher due to the increased resistance of the fluid flow around the suspended particles. This important information has bearing on process design, such as pipe size and pump horsepower.
Another major opportunity does not relate to vane spindles per se, but is the consequence of realizing that viscosity is not a single number for most fluid mixtures. The faster the fluid is moved, the lower the resistance (viscosity). This is a fortunate situation because it means fluids can move faster, if need be.
The flow curve is simply a graph of viscosity versus rotational speed; most food materials show a decrease in viscosity with increasing rotational speed (see Figure 5.) The signature curve generated by this approach allows the manufacturer to better compare questionable batches of product against an established standard viscosity curve to ensure the best-case product, which performs exactly the way the manufacturer wants.
One shortcut is to select two rotational speeds an order of magnitude apart and record the corresponding viscosity values. This gives a quick relative indication of the flow curve behavior without necessitating the time to measure all the data points. The low speed relates to how the food material flows when disturbed slightly, whereas the high speed corresponds to pouring or spreading, for example. The two viscosity values must fall within defined limits or the ratio of the two must do the same. For better results, choose a third rotational speed, measure the viscosity and note the shape of the flow curve with three data points.
Vane spindles have provided a new opportunity for the food industry to measure “yield stress,” the amount of force required to cause a material to flow initially. The ability of foods, such as gravies with meat or yogurt with fruit, to hold particles in suspension depends on a gel structure within the carrier liquid that has sufficient holding capacity. One way to measure this property is to use vane spindles with a viscometer or rheometer that runs at a low speed (see Figure 6.); as the motor rotates, the vane spindle imparts increasing force on the fluid until a peak torque value is achieved and viscous flow occurs (see Figure 7.) The peak torque value is equivalent to a yield stress value, which must be within defined limits for the material to pass inspection.
Measures for Flow in Semi-solids
Complementing the above instruments in the QC lab is the texture analyzer (see Figure 8.), which can also measure flow curve behavior for semi-solid foods like yogurts, spreadable butters, icings and surimi – to name a few. Methodology involves the choice of an appropriate probe (see Figure 9.), which is used to penetrate the food material, moving into the food sample a defined distance at a controlled rate. Gelatin is the one major example where a defined test procedure, called the gelatin Bloom test, is used throughout industry to measure compressive behavior (see Figure 10.)
A more elaborate test method for food items is called texture profile analysis, which involves a two-cycle compression test and produces a flow curve similar to (see Figure 11.) From this data, it is possible to analyze various behavioral characteristics such as stiffness and yield stress for flowable food materials.
This summarizes some of the recent food industry developments that are improving QC methodology for evaluating flow behavior of fluids and paste-like materials.
Bob McGregor is the marketing manager for Brookfield Engineering Laboratories in Middleboro, Mass. Reach him at email@example.com.