Rheology in Food Testing – How a Rheometer Works and What It Can Tell You
Food processing often involves a complex flow process; therefore, the physical properties of the ingredients and the final product are vital. These properties are also important in producing a pleasant consumer experience and products that meet expectations. Rheological analysis is therefore an important tool for assessing food and its constituent ingredients at all stages of the food system, from industrial processing and production to home cooking and consumption.
What is rheology and what is a rheometer?
How does a rheometer work and what does a rheometer measure?
Rheometer vs viscometer
Common types of rheometers
- Rotational rheometer
- Capillary rheometer
- Dynamic shear rheometer
This article aims to explain what rheology is, how rheological properties are measured and how those apply to your food.
What is rheology and what is a rheometer?
Rheology is a branch of physics, specifically fluid mechanics. It describes the deformation and flow of matter: solids and fluids (liquids and gases) under the influence of stresses. In essence, rheological characterization quantifies the relationship between deformation, imposed stress, viscosity, flow behavior, elasticity and recovery of a substance.1 In food processing, rheology is essential as flow properties determine food behavior during processing or preparation. Further, rheology influences flavors and nutrients released from food during chewing and digestion. Rheological analysis mimics what happens when a material is handled.2
A rheometer is an instrument that measures how matter flows in response to applied forces and quantifies its rheological properties. An extensional rheometer applies extensional stress or strain, while a rotational rheometer controls and applies shear stress or strain.3
Rheology studies the relationship between stress (force) and deformation (strain) of a material. Professor Eugene C. Bingham coined the term in 1920 from Greek ῥέω (rhéō) “flow”, and -λoγία (-logia) “study of”. Rheology answers the question, “How does a material respond to a force?”.4, 5
How does a rheometer work and what does a rheometer measure?
Fundamentally, a rheometer applies or measures torque, angular displacement or angular velocity. However, the user is more interested in the rheological parameters, which are calculated as follows:
- stress from the torque
- strain from angular displacement
- shear rate from angular velocity
Rheological experiments are performed either by applying a small stress to the sample and measuring the strain developed or by applying a fixed amount of strain and measuring the developed stress. Small deformation measurements reveal the structure of matter at scales as small as the nanometer and micrometer level. Meanwhile, large strains and stresses can provide information on time-dependent and nonlinear viscoelastic behavior, which are relevant to food processing and eating.6
Testing with a rheometer can be conducted in either rotational (shear) or oscillatory mode, contrary to viscometers, which only measure under one flow condition. In rotational measurements, the measuring geometry rotates continuously in one direction, which provides information about the viscosity of the sample. When an oscillatory test is performed, the measuring geometry moves back and forth and measures the viscoelasticity of the matter (Figure 1).7
Figure 1: Rotational measurements (left) vs oscillatory measurements (right) made using a rheometer.
As mentioned before, rheology is concerned with the flow (characteristic of liquids) and deformation (characteristic of solids). The reality, however, is a bit more complex and some substances can exhibit a combination of these behaviors (Figure 2). In general, fluids can be classified as Newtonian (their viscosity is independent of shear rate) and non-Newtonian. Those can be further classified as time-independent; their viscosity depends on shear rate (shear thinning or thickening) or time-dependent if the deformation history also plays a role (thixotropic fluids). The third group consists of viscoelastic fluids, which exhibit a combination of solid- and fluid-like behavior. 4
Figure 2: General classification of fluids according to their rheological properties.
The particular type of behavior exhibited by a given material can be identified by applying a sinusoidal deformation (strain) and observing the value of the phase angle. A phase angle (δ) (Figure 3, in green,) is the time lag (difference) between the application of strain to the sample (blue solid line in Figure 3) and obtaining a measured result (stress, orange solid line in Figure 3). The value of δ = 0° denotes an ideal elastic solid, and the value of δ = 90° indicates an ideal viscous liquid. Viscoelastic substances have values between 0° and 90° (Figure 4). 4, 8
Figure 3: Relationship between sinusoidal stress and strain for an ideal solid – purely elastic material (left); viscoelastic material (center) and ideal liquid – purely viscous material (right).
In addition to establishing the general behavior of the substance, further information about its rheological properties can be gathered. Complex modulus G*, a measure of deformation resistance, can be estimated by performing an amplitude sweep in a stress or strain mode of operation (Figure 4, chart on the left). The deformation of the sample is increased step-wise from one measuring point to the next while keeping the frequency at a constant value. A material's rigidity, the value of the complex modulus within the linear viscoelasticity region (LVER), determines its softness/stiffness, whereas its yield stress (limit of LVER) determines its strength/weakness (Figure 4, chart on the right).
Figure 4: Complex modulus and LVER of two different materials (left) and an illustration of how to interpret material structure (right).
A frequency sweep provides further insight into the liquid's structure. This test is conducted over a range of oscillation frequencies at a constant amplitude with strain or stress values within the LVER. Frequency sweeps allow the identification of viscoelastic solids, liquids or gels (Figure 5) and observation of changes to the two components of complex modulus - viscous modulus (G”) and elastic modulus (G’). Low frequencies illustrate the material's behavior on a long time scale, and high frequencies represent the response on a short time scale. 9
Figure 5: Illustration of different material’s responses to a frequency sweep test. The storage modulus, in orange, represents the elastic component of the viscoelastic behavior; the loss modulus, in blue, represents the viscous component of the viscoelastic behavior; phase angle, in green, represents the material’s behavior.
For purely viscous liquids, flow measurements with viscosity/shear profiling can be performed as shear rate sweeps or stress sweeps. In the first mode, forced flow is simulated, such as pumping, mixing, filling and spreading. In contrast, the second mode helps to obtain data under free-flow conditions and measures zero-shear viscosity and yield stress. Capillary action, dripping, sedimentation, creaming, sagging and slumping are all examples of free flow. Figure 6 presents typical flow curves for various flow behaviors a fluid can exhibit. When any applied stress will induce flow, the curves will meet at the origin. When fluids have a yield stress, the curves intercept the stress axis at a non-zero value, meaning that only appropriate amounts of stress will induce a flow.10,11
Figure 6: Flow curves of various types of flow behavior.
The rheological properties of a material are measured in a sample- and test-appropriate geometry. Measuring geometries can be categorized into two groups: absolute or relative. The first group of geometries allows the calculation of rheological parameters in absolute units independent of the geometry. Concentric cylinders, plate-plate, cone-plate and double gap concentric cylinders are examples of absolute measuring geometries (Figure 7).12 These values can then be compared regardless of whether, for example, honey's viscosity was analyzed in a plate-plate or double cylinder system.
In the second group, relative measuring geometries deliver values specific to the geometry; therefore, results can only be compared if the same geometry is used. These include vane rotors, spindles, stirrers and geometries with sandblasted, profiled or serrated surfaces. Unlike viscometers, which usually have only relative measuring rotors, rotational and oscillatory tests with rheometers can be performed with any of the afore mentioned geometries. It is important to remember that relative measuring geometries often result in inhomogeneous fluid flow. As a result, viscosity values cannot be calculated, and test results obtained with relative measuring geometries need to be expressed as relative measurements.1
Certain samples, however, cannot be measured in absolute geometries; this is often the case with samples that separate or slip on a smooth surface (so called wall slip). In situations like these, relative measuring geometries are advised in order to avoid inaccurate results.13Spindles and vanes are used when analyzing pasty materials that do not flow homogeneously or contain large particles. Food items like yogurt and many dairy products often have a rigid three-dimensional gel structure that may be destroyed when using a double cylinder or plate-plate system. For these samples, it is usually better to select a vane as it can be immersed into shear-sensitive samples without changing their structure significantly, and additionally, wall slip can be eliminated.4
Figure 7 illustrates the most common geometries used in food science and other fields. Geometry selection is crucial for correct results and strongly depends on the sample and rheometer type. Generally, concentric cylinders are used for low- and medium-viscosity liquids, cone-plate for high-viscosity liquids, plate-plate for soft solids and vane rotors for gel-like samples and sediment-prone products.14
Figure 7: Commonly used measuring geometries for rheological testing, pale orange indicates the sample location.
Rheometer vs viscometer
Both viscometers and rheometers are used to measure viscosity. It is often the case that viscometers are used to analyze items, processes or productions that require simple flow measurements. Meanwhile, the rheometer can be used to characterize both Newtonian and non-Newtonian materials' flow, deformation and even tackiness. A viscometer can be portable for field or remote testing, but a rheometer is much more versatile and has much wider parameters for measuring. Table 1 summarizes the differences between these two instruments.15, 16
Polymer melts, polymer solutions, emulsions, suspensions, gels, liquids, soft solids
This measurement is only applicable to liquids the viscosity of which can be expressed by a single value
Able to measure Newtonian liquids and materials that can't be defined by a single viscosity value. Able to work as a viscometer
Limited shear rate
Wide range of shear rate, shear stress and oscillation
Able to measure viscosity only if liquid follows Newton’s law of viscosity
Can perform measurements under various conditions
Used mostly to monitor quality and production consistency in an industrial setting
Used to perform full rheological assessment of a sample, research and development and quality control
Common types of rheometers
Rotational rheometry involves enclosing the sample between two surfaces of a measuring geometry, one of which is subsequently rotated. Rheometers can be classified as rate-controlled or stress-controlled depending on how the rotation is regulated. However, modern instruments can work in either of these modes. In rate-controlled mode, the velocity of rotation is controlled, while the torque is recorded. For stress-controlled mode, a specified torque is applied, and the subsequent rotation rate is recorded.17, 18
Capillary rheometers are the simplest form of a rheometer. They allow a measure of the absolute value of viscosity for Newtonian fluids and, to some extent, for liquids described by the power law equation. The amount of time required for a fixed volume of the test fluid to pass through a capillary tube is measured. Flows of fluid can be driven by gravity, pressurized gas or pistons. It is recommended to use capillary viscosimeters only for known Newtonian fluids, such as dilute solutions and vegetable oils. It is only possible to conduct limited quality control tests on other foods. Additionally, food samples should be homogeneous. Suspended solids or droplets can generate significant errors if the particle size is big enough compared to the diameter of the capillary tube. Finally, it is important to prevent suspensions from settling or separating during the test.19, 20
Dynamic shear rheometer
Dynamic rheology uses the same types of geometries as rotary rheometers in its analysis. In this case, the load is sinusoidally varying, and either shear stress or strain is controlled. Moreover, the load is small enough to prevent material destruction. As mentioned before, these tests identify a sample's viscoelastic behavior. Dynamic or rotational rheometers do not have as many constrictions as capillary rheometers. If the geometry and test sets are selected correctly, they can measure almost any food material. Most rheometers can perform both rotational and oscillatory tests.21, 22
Common problems in food rheometry
It is possible to misinterpret samples' rheological responses due to many measurement artifacts. Food items' softness and biological activity often make rheological measurements more challenging. Nonideal conditions may lead to a misinterpretation of results, such as an apparent shear thinning and thickening in Newtonian fluids.23
In general, and with food specifically, avoiding bad data is a challenging task. A good place to begin is by determining the experimental window. For soft biological systems, the minimum torque an instrument can measure is the most critical limitation regarding the measurement of rheological properties. Geometry also affects experimental limits.24
- Under transient conditions, instrument inertia can produce experimental artifacts; therefore, measured loads may not only be tied to material deformation but also to instrument acceleration. In oscillatory tests, the "material torque" should be larger than the "instrument inertia torque." It is very important to interpret high-frequency data carefully, as these errors are usually more prevalent in oscillatory tests.
- The sample will always have finite inertia even if the instrument inertia is eliminated. Consequently, soft gels, low-viscosity and low-moduli fluids are more likely to experience sample inertia effects at high frequencies.
- Sample inertia through secondary flow can incorrectly increase the apparent viscosity of the liquid. Low-viscosity fluids are more prone, and small gaps are advised to eliminate this phenomenon. At high rotational velocity for cone-plate and plate-plate geometries, secondary flow is always present and can be used to set criteria and experimental limits. In general, lower viscosity fluids have a smaller experimental window with limitations at a high shear rate due to secondary flow.
- In rotational rheometers, surface tension leads to torques that shouldn't occur in rotationally symmetric geometry. In most cases, it is ignored, but in some cases, it may exceed the instrument's low-torque limit by orders of magnitude. Consequently, Newtonian fluids, including water, appear as shear-thinning fluids with finite elastic moduli. In order to minimize the effect of surface tension, matched plate geometries, solvent traps and precise sample loading can be utilized. It is necessary to eliminate the surface tension torque for low viscosities and soft materials, when measuring subdominant viscoelastic components or intrinsic viscosity, when using small gaps and in any circumstance where the low-torque limit is important.
- A biological fluid may contain proteins and other surface-active components. Surface films may form at liquid-air interfaces due to the accumulation of these components. Increased viscosity, higher shear thinning and apparent yield stress are primary signs of a free surface film. Repeated measurements with different geometries could be carried out to test for the artifact of surface film rheology.
- Samples are typically assumed to stick to contacting boundaries, known as no-slip conditions. However, food items can easily slip, which may cause significant artifacts in the data. This is characterized by decreased flow stress and inconsistent apparent stress and strain rates depending on the geometry gap. In most cases, sandpaper is sufficient, but sometimes serrated or grooved plates or vane rotors are necessary.
- The torque in cone-plate and plate-plate geometries is proportional to the contact radius. The loss of rotational symmetry caused by underfilling and overfilling can introduce additional torque due to surface tension forces. Evaporation can also lead to underfilling. By using a micropipette and paying close attention to the fill level, the fundamental problem of incorrect sample volume can be eliminated.
- Edge fractures is another issue relevant to biological gels. It is important to monitor the edge of the sample visually. In the data, it manifests as a decrease in apparent stress. Edge fracture artifacts can be minimized by moving the sample’s free surface away from the measuring surface.
- High viscosity fluids and low thermal conductivity can experience viscous heating, which reduces the apparent viscosity with shear rate. Smaller gaps help minimize that effect.25, 26
Measuring rheological properties in the food and beverage industry
Many food products are simple liquids or solids, but others may be suspensions, emulsions, foams, biopolymer gels or mixtures. The use of rheological measurements is particularly important when developing new products or alternative ingredients, such as analogs to meat or milk.
The mouthfeel, texture, taste and flavor of meat analogs still differ from real meat despite advances in making plant-based fibers. Research and development specialists can utilize the rheological properties of plant-based protein to improve the acceptability of such products.27, 28 Another valuable piece of information about animal and plant-based food items is fat's behavior at various temperatures.29 One method for exploring this behavior is measuring the change in phase angle that can reveal reversible and irreversible changes cheese (or cheese analog) will undergo when heated.30, 31
Furthermore, an in-depth analysis of rheological characteristics can provide insight into the stability and appearance of starch-based products.32 "Coatability" and drainage behavior are crucial for the visual and sensory appeal of food glazes, sauces and dressings. A material's cling (ability to hold onto food) results from a combination of three rheological factors: yield stress, zero-shear viscosity and viscoelasticity.33, 34
Despite an increase in demand for milk alternatives, consumer acceptance is low due to differences in appearance, mouthfeel and storage behavior. Similarly, zero-fat yogurts are also expected to have a creamy, silky texture.35, 36, 37
In conclusion, rheology is a powerful tool that can be used to develop new food items or improve or control existing products.
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