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Which Measuring Device is Better Suited for ‘Soft Solids?’
by Robert G. McGregor
Brookfield Engineering Laboratories recently conducted live interviews with industry experts via its www.TextureAnalysis.com platform to discuss how to measure “soft solid” materials for flow behavior. This category of product is typified by dairy items such as tub butter, margarine, yogurts, ice-cream, and whipped cream. The same conversation could apply to viscous beverages like smoothies and thick shakes. Chocolate could fall into both categories, depending on whether it is a sauce, a filling, or a candy. These everyday products are known as “soft solids” to food scientists. When at rest, which is most of the time, they don’t move. They hold their position. Their structure seems to be that of a solid. But once movement takes place, they may be able to flow in liquid-like fashion. So what category of flow behavior do you put these materials into?
Pondering this a bit further raises the basic question of when to use a viscometer versus when to use a texture analyzer. The viscometer assesses flow behavior. The texture analyzer measures deformation and the associated force that causes that change in appearance. Do you need to use both devices to get an integrated piscture? It’s an interesting topic and one which food industry manufacturers think about quite often.
Rotational viscometers feature standard disc-type spindles immersed in the fluid and rotated at a fixed speed. The instrument measures torque, or resistance to spindle movement, and that number is converted into viscosity in units of centipoise (cP) or milli-Pascal seconds (mPa.s) outside the Americas (fortunately 1cP = 1mPa.s). Some viscometers also come equipped with a temperature probe since viscosity is directly affected by variations in temperature. The important information to note when making a viscosity measurement includes:
- Spindle type and dimensions (geometry)
- Rotational speeds of spindle used
- to make the measurement
- Torque in percent of instrument’s full scale measurement range
- Viscosity in cP or mPa.s
- Sample temperature
- Elapsed time of spindle rotation when measurement is recorded
R&D personnel have used substitutes for the disc-type spindle to give more accurate and complete viscosity information about the flow behavior of soft-solid materials. There are several alternatives. The T-bar was invented to cut through paste-like materials while moving downward into the sample; this assures that fresh product is measured with each rotation of the spindle. Vane spindles became popular because they insure that heterogeneous materials with particulates are measured as a mixture; the traditional disc-type spindle may measure only the liquid carrier viscosity. The spiral adapter simulates an augur and, by running at different speeds, can provide viscosity versus shear rate profiles that are not possible with the T-bar, which is really a single viscosity data point tool.
One capability of viscometers, not widely used, is to analyze the viscosity data to derive the yield stress for the material. This is the amount of force required to initiate flow. Chocolate manufacturers test the melted liquid in a coaxial cylinder system that controls temperature while shearing the material at different rotational speeds. See Figure 1. The viscosity data includes shear stress and shear rate values that are plugged into a Casson math model to generate a flowability index and the yield stress for the chocolate sample. These two numbers are sufficient to characterize the chocolate for acceptability in terms of stiffness and flow behavior.
In order to penetrate into a material, texture analyzers use probes of different shapes and sizes (cylinders, cones, balls, blades, etc.). Similar to the viscometer with its controlled rate of rotation, the texture analyzer displaces or cuts into the sample material at a defined rate of penetration (mm/sec) up to a specific distance (mm) and measures the force of deformation (grams or Newtons). The instrument can also measure the pulling force exerted on the probe during retraction from the sample, which is an indication of the material’s adhesive property. Some key points to note when using a texture analyzer include:
- Shape and dimensions of the probe
- Speed of probe descent into the material
- Distance of penetration
- Force measurement in g or N
- Rate of retraction from the material
Both instruments clearly give numbers that quantify viscosity and textural properties for soft solids. Does one have an advantage over the other? The test in both cases can be relatively quick, easy to set up, and provide a valid means for QC to approve or reject a component material or finished product. Some soft solids may actually benefit from being tested by both methods in a lab that has done a thorough investigation. Most important, however, is to devise a test which mimics the way in which the end user will use the material. This is the key to assuring consumer acceptance.
Stick margarine can be solid-like when taken out of the refrigerator. A cutting test with a blade or wire makes good sense. See Figure 2 a. Tub margarine, by design, should be softer by comparison and more pliable. A spreadability test with cone fixtures will provide a good indication. See Figure 2b. The margarine is placed in a cone-shaped well and a cone probe descends into it to measure the margarine’s resistance to being sheared, which is like a spreading action. So the texture analyzer seems to be the better choice for these two soft solids as far as the consumer is concerned.
Processing the margarine before it turns into sticks and tubs poses a different challenge. The material is in a liquid state and needs to be checked for consistency. In this case, the viscometer is the correct tool because it analyzes the margarine’s flowability. Margarine, therefore, is a product which can benefit from both types of measurement.
Bottom line recommendation for proper assessment of soft solids is to review both the processing requirements on the production floor and the subsequent ways in which the end user manipulates the product. Knowing these details will lead to the most practical way to conduct testing in the QC lab. Chances are that both the viscometer and the texture analyzer have a bonafide role to play.
McGregor is the general manager, global marketing for Brookfield Engineering Laboratories, Inc. He holds MS and BS degrees in mechanical engineering from MIT in Cambridge, Mass. He can be reached at 508-946-6200 ext 7143.