Viscosity measurement is a universal necessity in QC labs throughout the food industry. If the item in question can flow, you can bet that there is a test method specified by R&D for viscosity. This requirement ranges from “thin” fluids, like milk and canned soups, to “average viscosity” soft solids, like salad dressings and yogurts, to “thick pasty” or “hard” materials, like peanut butter and cheese. Given the wide variety of food types that may need to be measured, choosing the proper viscometer can be daunting.
Viscometers are relatively basic instruments that measure torque using a rotating spindle immersed in the material. Resistance to spindle rotation is the physics that constitutes the basis for viscosity measurement. Continuous contact with the material is necessary to provide a steady torque signal. Mathematical calculation converts the torque reading into a viscosity value with established scientific units: “centipoise” (cP) in the U.S. and “milliPascal-seconds” in Europe and Asia.
Normally R&D will specify the viscosity test method and the recommended instrument. This is based on evaluation testing by R&D to characterize the food item for its flow behavior. The following are important questions that must be answered.
- How much of the food material is available for testing? Is there any limitation in the available quantity?
- What type of spindle is needed to test the material?
- What is the appropriate torque measurement range for the instrument?
- Is temperature measurement or control needed for the test?
- How long does the spindle rotate in the material before taking the reading?
Sample size for the viscosity test is not usually an issue for food manufacturers. There is more than enough material available in most cases. If temperature control of the sample is required, then working with a small sample size is preferable in order to minimize the time needed to achieve equilibration.
One important consideration that affects the test is the type of container holding the food item. If the test is performed in a standard 600 mililiter (mL) lab beaker, then there is no issue; if performed in the container that packages the item, then volume of material available for testing may affect the choice of spindle.
Various spindle types used for viscosity measurement appear in Figure 1. Most common are the first two which are either cylindrical in design or have a disc near the bottom of the shaft. The cone spindle is ideal for very small sample size (less than 2 mL) while the SC4 type requires 16 mL or less. T-bar is used with paste-like materials. Vane can measure mixtures with suspended solid particles as well as thick pasty substances. Spiral is appropriate for simulating processes that use augurs to move material. Fortunately, the spindles are not expensive and can connect interchangeably to any standard bench-top viscometer. Initial choice is most likely cylinder or disc, but could in future transition to one of the others for reasons indicated.
Torque range for the viscometer is chosen based on the expected viscosity range for the food material. Most common choices are “LV” for “low viscosity” or “RV” for “regular viscosity,” also referred to as “medium viscosity.” “HA” and “HB” cover the high viscosity range, but are much less frequently selected. However, the chocolate industry has elected to standardize on viscometers with “HA” torque.
The maximum torque that can be measured for each range is as follows:
LV = 673 dyne.cm
RV = 7,187 dyne.cm
HA = 14,374 dyne.cm
HB = 57,496 dyne.cm
The minimum torque recommended for use in each case is 10 percent of the maximum. Therefore, LV range goes from 67.3 to 673 dyne.cm (centimeter) and RV from 718 to 7,187 dyne.cm. The theoretical viscosity values that can be measured with each are very broad, ranging from under 10 cP to over 1 million cP. Practically speaking, LV is typically used in the range from 1 to 100,000 cP while RV is used from 100 to over 1 million cP. Because the overlap in range coverage is significant, there is another consideration that determines which one to select.
The combination of spindle and rotational speed determines the precise viscosity range that can be measured. Viscometer manufacturers provide this information in tables for easy lookup. Viscosity measurement is oftentimes targeted to fall in the middle of the torque range—around 50 percent on a scale of 0 to 100 percent. This provides flexibility for possible variance in measurements that may occur from batch to batch during production.