By Richard Crowley
Appearance, flavor, nutrition and texture are the four principal quality factors in food, and each is an essential component of the pleasure people derive from food. Achieving and maintaining the proper texture and being able to quantify it for quality control purposes is crucial. These properties are also critical in the design of production processes and equipment. Although some people draw the distinction that texture applies to solid foods and viscosity applies to fluid foods, many products exhibit both solid and liquid properties, making it increasingly difficult to delineate between texture and viscosity. This is especially true when stress is applied to a product.
Viscosity is the internal friction of a fluid, or its tendency to resist flow—in other words, the resistance to pouring and changes in form. Although the name Isaac Newton is rarely connected to the discipline of food science, this eminent scientist was the first to study the flow of fluids and hypothesized that “the resistance which arises from the lack of slipperiness of the parts of the liquid, other things being equal, is proportional to the velocity with which the parts of the liquid are separated from one another.”
He subsequently developed a mathematical description of the resistance of a fluid to deform or flow when a stress is applied. This principle was used to define the class of liquids known as Newtonian fluids (e.g., water). In the 1800s, other scientists advanced this knowledge and developed methodologies to study the flow of fluids in capillary tubes and other orifices. These advances led to the development of the first efflux and rotational viscometers in the late 1800s. These instruments measured viscosity using stress-driven (gravity) flow and are quite similar to some of the instruments, techniques and principles still in use today.
A viscosity primerThe resistance to flow is mathematically defined as the shear stress divided by the rate of shear strain. Shear stress is the force acting in the plane of the fluid, and shear rate is the velocity gradient of the fluid between the plates. The shear rate takes into account the distance between the plates. It is defined in terms of the force required to move one plane surface continuously past another under specified steady-state conditions when the space between is filled by a specific liquid. Although absolute viscosity can be measured directly if accurate dimensions of the measuring instruments are known, it is more common to calibrate the instrument with a liquid of known viscosity (i.e., standard) and to determine the viscosity of the unknown fluid by comparison with that of the known.
Foods exhibit different types of flow. In Newtonian materials, viscosity is not affected by changes in shear rate and remains constant. However, changes in shear rate do affect the viscosity of non- Newtonian materials. Most foods fall into this category and exhibit specific behaviors:
- Pseudoplastic: as the shear rate increases, the viscosity decreases.
- Plastic (aka viscoplastic): a yield point must be reached before flow begins and the fluid exhibits psuedoplastic behavior with decreasing viscosity and increasing shear.
- Dilatant: As the shear rate increases, the viscosity increases and time dependent or directional viscosity characteristics occur.
- Thixotropic: Viscosity decreases at a constant shear rate over time.
- Rheopectic: Viscosity increases with a constant shear rate over time.
- Bingham plastic: Shear stress (i.e., yield point) must be applied to initiate flow and the flow is not affected by changes in shear rate
Non-Newtonian flow is based upon the cohesiveness of the mixture of molecules with different shapes and sizes, and how much force is required to move them. At each rate of shear, the alignment may be different, and more or less force may be required to maintain motion. The flow behavior is characterized by the way a fluid’s viscosity changes in response to variations in shear rate.
In addition to shear rate and time, several other factors must be controlled to ensure accurate data. Temperature is critical because viscosity decreases as temperature increases. The most obvious example is the change in flow from ice to fluid water (e.g., 1 cP at 20°C to 0.2838 cP at 100°C). The sample must be representative of the whole, and homogeneity will ensure phase separation as well as nonuniform air and particle dispersion. In most cases, physical or chemical changes (e.g., hardening or gelling) should be avoided unless the intention of the data is to determine a viscosity/time relationship. Turbulence can impact the shear stress, so samples should not be agitated during measurement. The type of instrument being used and its parameters must be specified in order to have a relationship between viscosity readings, especially in relative measurements. In most cases, changes in pressure do not affect food products, although extremely high pressures tend to increase viscosities.
Many substances have variable viscosity and are less resistant to flow at higher flow rates. In such cases, a given set of conditions is selected for measurement, and the measurement obtained is considered to be an apparent viscosity. Since a change in the conditions of measurement would yield a different value for the apparent viscosity of such substances, the instrument parameters and conditions for measurement must be closely followed.
Strike a poiseThe basic unit of absolute viscosity is the “poise,” which is 1 gm/cmsecond or dyne-second per square centimeter. However, viscosities generally represent fractions of the poise, so the common unit for expressing absolute viscosity is the “centipoise” (1/100of a poise). Measurement of viscosity is done using the equation: viscosity (ß) = shear stress (t) divided by shear rate (D). The units of viscosity are expressed either as Pascal seconds (Pas) and millipascal seconds (mPas) or as poise (P) and centipoise (cP). One mPas equals one cP. The viscosity of food products is usually expressed in cP or mPas. Water at 20°C has an absolute viscosity of one mPas or one centipoise.
Kinetic viscosity is sometimes used as the measurement unit for Newtonian liquids measured via capillary tube. The kinetic viscosity (v) = viscosity (ß) divided by the density of the fluid. The units of kinetic viscosity are millimeters squared per second, stokes or centistokes. To obtain the kinematic viscosity from the absolute viscosity, the latter is divided by the density of the liquid at the same temperature (i.e., kinematic viscosity = absolute viscosity/density).
Instrument and method selectionThe choice of method and instrumentation is based on the properties of the matrix and degree of accuracy required. The USP method for measurement of viscosity involves the determination of the time required for a given volume of liquid to flow through a capillary tube. Several types of capillary-tube viscometers are available. Capillary tubes measure the time required for a standard volume of fluid to pass through a specified length of tubing. However, an on-line instrument used for QC purposes may not require the same degree of precision as one for a laboratory experiment. Repeatability within a certain range is always a requirement.
The rotational viscometer is an extremely efficient instrument widely used in the food industry. This viscometer utilizes a bob or spindle immersed in the test specimen and measures the resistance to movement of the rotating part. Different spindles are available for given viscosity ranges and several rotational speeds are available. A cylinder or other circular geometric configuration is used to create a defined shear rate in the sample and measure resistance to flow by the torque generated.
For example, testing for a tomato extract is done using a rotational viscometer equipped with a spindle having a cylinder 1.47 cm in diameter and 0.16 cm high attached to a shaft 0.32 cm in diameter. The distance between the top of the cylinder to the lower tip of the shaft is 3.02 cm. The spindle rotates at the appropriate speed and immersion depth to obtain a scale reading between 10% and 90% of full scale. The viscosity is calculated by multiplying the scale reading by the constant for the spindle and speed used.
These instruments produce precise measurements of absolute viscosity for a wide range of matrices. Because the shear rate can be varied, it is possible to plot the flow curves of non-Newtonian fluids, and time effects can be studied either manually or automatically.
These instruments measure shear rate against a constant torque or measure torque with a defined shear rate. Calibration of viscometers is done by determining the viscometer constant, k, for each viscometer by the use of an oil of known viscosity. The constant (k) is calculated the viscometer using the equation: k = v / d t in which v is the known viscosity of the liquid in centipoises, d is the specific gravity of the liquid tested, and t is the time in seconds for the liquid to pass from the upper mark to the lower mark.
Other methodologies are also available. For example, empirical consisto-meters measure the distance of flow of a specified volume of product during a specified time. With the Ostwald viscometer, a tube is filled with an exact amount of sample and the meniscus of the column of liquid in the capillary tube is adjusted to the level of the top graduation line with the aid of either pressure or suction. Both the filling and capillary tubes are opened to permit the liquid to flow into the reservoir against atmospheric pressure. The time for liquid to flow from the upper mark to the lower mark in the capillary tube is recorded in seconds. Cellulose derivatives are commonly used as texturizers, and the measurement of the viscosity of solutions of the high-viscosity types of methylcellulose are too viscous for the commonly available viscosimeters. The Ubbelohde viscometer may be adapted to the measurement of the ranges of viscosity encountered in methylcellulose solutions.
With falling-ball viscometers, a standardized tube is filled with the product to be measured and the time for a ball to pass between two specified points is measured. The time for a given viscosity depends on the density of the ball, as well as the diameter of the tube and the ball. Oscillating viscometers use a vibrating rod, ball or plate oscillating at a controlled amplitude. Orifice viscometers consist of a dripping cup with a specific size hole at the bottom. The Zahn viscometer which is commonly used in the food industry is often referred to as a “cup viscometer.” Many of these measures are relative and unrelated to units of absolute viscosity.
The whole truthViscosity analysis is an essential component of quality control for many products. However, it must be remembered that sensory and organoleptic properties are largely a matter of personal perception and cannot be measured with analytical instrumentation. Advances in instruments will continue to provide increased monitoring of textural properties using special devices for quantifying viscosity.
Richard Crowley is a senior science writer with Covance Laboratories in Madison, WI. He is the editor of the Covance Food Science Newsletter and the author of numerous articles in the field of analytical chemistry. He has a B.S. in agricultural journalism from the University of Wisconsin-Madison and is a senior member of the Society for Technical Communication. For more information, firstname.lastname@example.org, or visit www.covance.com/analytical.