Rheology in Food Testing

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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?

Rheology definition

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

Common problems in food rheometry

Measuring rheological properties in the food and beverage industry

This article aims to explain what rheology is, how rheological properties are measured and how those apply to your food.

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

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:

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

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

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

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).

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

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

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.13 Spindles 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.

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

Table 1: Differences between a viscometer and rheometer. 15, 16

Measurement Type

Viscometer

Rheometer

Viscometry

Flow curves

Single shear

Stress relaxation

Flow curves

Yield stress

Thixotropy

Single shear

Stress relaxation

Creep

Oscillation

Not applicable

Amplitude sweep

Frequency sweep

Single frequency

Sample types

Liquids

Polymer melts, polymer solutions, emulsions, suspensions, gels, liquids, soft solids

Functionality

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

Range

Limited shear rate

Wide range of shear rate, shear stress and oscillation

Requirements

Able to measure viscosity only if liquid follows Newton's law of viscosity

Can perform measurements under various conditions

Application

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

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 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

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

Here are some of the most common problems that can lead to incorrect measurements and conclusions:

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.

References

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