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 firstname.lastname@example.org.