CHAPTER 2 METHODOLOGY
Rheology
The term rheology is defined as the study of flow behaviour of matters and gelation properties of materials. According to Barbosa-Canovas., et al. (1996), rheology focuses on the flow and deformation behaviour of materials in the transient between solids and fluids region. Furthermore, it also defines the relationship between the applied force/stress and the deformation of materials. Rheological measurement provides a better understanding in regards to the physics of amorphous solid, including cross-linked rubbers and polymeric glasses (Mcakenna, 2012). Rheological data is important for many aspects in the field of food manufacturing. It offers useful information about the raw materials, during processing and the final product (Steff, 1996; Dorbraszczyk & Morgenstern, 2003). Therefore the main aims of rheological measurements are:
To determine the functionality of food ingredients
To evaluate the food texture and correlate with the sensory analysis
To measure the quality of intermediate and/or final product
To evaluate and test the shelf life of the products
Rheometer is an instrument that is capable to perform the rheological measurements, such as viscosity and viscoelasticity of fluids, semi solids as well as solids. In a large deformation rheological test or flow test, the geometry turns continuously in one direction, thus, forming a uni–directional shear to the sample. The speed of shear is the controlled variable known as shear rate ( gɺ). Shear stress (σ) is measured as a function of shear rate, therefore as a consequence of the uni–directional shear, flow test is considered as destructive measurement and thus it is commonly used to analyse fluids (Rao, 1999).
According to Dorbraszczyk & Morgenstern (2003), the small deformation dynamic oscillation is one of rheological measurements that can be used to characterise the viscoelastic properties of materials as a function of time, temperature, strain or frequency. In these measurements, the geometry moves in two directions (bi–directional). The total resistance of
materials to oscillatory shear is known as complex modulus (G*) in Pascal unit. The complex modulus consists of two components, G’ and G″, where G*=(G’ 2+ G″2)1/2. G′ or elastic modulus represents the strength of network and is also known as storage modulus. On the other hand, the viscous modulus or G″ is a measure of the flow properties for the sample in the structured state and is also known as the loss modulus. Furthermore, phase angle or tan δ is a parameter associated with the degree of viscoelasticity of a sample, tan δ = (G″/G′). A high value of tan δ indicates that the sample is more viscous or liquid–like, while low value of tan δ means that the sample is more elastic or solid–like.
The measurements of linear viscoelastic region (LVR) can be done by using rheological measurements which can be conducted in the regions where the viscoelastic properties observed are independent of imposed stress and strain levels (TA Instruments, n.d- a). The materials’ LVR should be established before starting the dynamic tests. The determination of LVR of a material can be done by increasing the amplitude of oscillation and observation of the magnitude of phase lag. In dynamic oscillation test measurements, in order to verify that the results are real and not merely artifacts, it is extremely vital that all the test are carried out on amplitude within the linear viscoelastic region of the sample. The principle behind this is that if the deformation is small or applied slowly, the molecular arrangements are never far from equilibrium. The mechanical response is then just a reflection of dynamic processes at the molecular level which go on constantly, even for a system at equilibrium. Within this domain of linear viscoelasticity, the magnitudes of stress and strain are related linearly, and the behaviour for any liquid is completely described by single function of time. According to Rao (2007), viscoelastic properties of materials can be analysed using dynamic rheological tests such as:
Temperature sweep
Storage modulus (G′) and loss modulus (G″) are measured as a function of temperature at fixed frequency and strain. This test provides useful information about the gel formation (e.g protien) and gelatinisation of starch dispersion.
Time sweep
Time sweep test is used to analyse viscoelastic properties of materials as a function of time in which the strain, frequency and temperature are kept constant. The importance of a time sweep is to determine if the properties of a system changes over fixed time. This test is also known as a gel cure experiment. A curing time is usually necessary for gels in order to reach equilibrium state and it varies from gel to gel. For example, like most other biopolymer systems, the coil to helix transformation in agarose gels occur very fast which resembles a true first order phase transition. A short curing time is therefore sufficient in the case of agarose gels. However, in the case of gelatin gels, the initial phase lasts several hours thus requires a much longer curing time before a pseudo equilibrium state is achieved.
Frequency sweep
Frequency is the time required to complete one oscillation. A frequency sweep usually follows a time sweep and this test can provide information regarding the viscoelastic properties of materials as a function of frequency at a constant strain and temperature. The data obtained from frequency sweeps helps in determining under which category a sample can be classified, for example, a dilute solution, an entangled solution, a weak gel or a strong gel. Derived parameters such as complex viscosity (η*) and tan δ provide useful information about the nature of the system that being tested. In addition, data from frequency sweeps is used in time- temperature superposition in order to gauge long term properties or extremely high/low frequencies beyond the scope of the instrument or reasonable experimental time. This concept uses a direct equivalency between time (frequency of measurement) and temperature.
Strain sweep
A strain sweep helps determine the extent to which a sample undergoes deformation and is mostly used to determine the LVR of the system. In this test, the material response to increasing amplitude at a constant frequency and temperature is measured. Sample is assumed to be stable before performing a strain sweep. An unstable sample is subjected to time sweep prior to strain sweep to determine the stability.
3.2.1.1 Large deformation in shear
Large deformation in shear is most suitable for studying of fluid flow process or disruption of food products during storage measured using a viscometer or rheometer. Uni-directional shear is applied on the samples in a large deformation rheological test or flow test and, in this, the geometry turns continuously in one direction. As a consequence the sample can be destroyed and thus it is now commonly used to analyse fluids (Rao, 2007). An application of a shear stress on a fluid produces a laminar flow between two parallel surfaces creating a velocity gradient. This gradient is defined as a change in the applied strain or shear rate. In case of Newtonian fluids, the shear stress is directly proportional to shear rate and a constant viscosity and for non-Newtonian fluids the apparent viscosity (h) value varies in the ratio between a shear stress applied and the shear rate (Borwankar, 1992).
Fluid and semisolid foods exhibit a wide variety of rheological behavior such as Newtonian, shear-thinning, shear-thickening and time-dependent behavior (Figure 3.1). Newtonian behavior exhibits a linear relationship between shear rate and shear stress and the plot begins at origin. Examples of Newtonian materials include water, sugar syrups, most honeys, filtered juices and milk. Non-Newtonian material exhibits time-dependent rheological behavior as a result of structural changes, which indicates the shear stress shear rate plot is not linear and/or the plot does not begin at the origin. With shear thinning fluids, increasing shear rate gives less than proportional increase in shear stress.
Shear-thinning fluids also called pseudoplastic. Many salad dressings and some concentrated fruit juices exhibit shear-thinning behavior. Some materials may not commence to flow until a threshold of stress, the yield stress (so), is exceeded. Examples of foods with this type of behavior include tomato ketchup, mustard, and mayonnaise. Bingham plastic materials exhibit linear relationship between shear rate and shear stress with yield stress. Shear-thickening behavior can be observed in partially gelatinized starch dispersions where an increase in shear stress gives less than proportional increase in shear rate. Time-dependent behavior can be categorized into two types, time dependent shear-thinning (thixotropic) and time-dependent shear-thickening (antithixotropic) behavior (Rao, 2007).
Small deformation in shear
One of the most common techniques widely used to study the viscoelastic behaviour and gelation properties of materials involves small deformation studies using dynamic oscillation experiments on a rheometer. Bi-directional movement is provided by the geometry while rheological measurements are being taken. Complex modulus (G*) in Pascal units is the total resistance of the materials to the oscillatory shear. It consists of two components storage modulus (G′), viscous modulus (G”) and these are related by the following equation:
G* = (G’2 + G”2)1/2
The storage and viscous moduli are of prime importance to characterise viscoelastic systems including gels. As the name suggests, storage energy in a structure is described by the storage modulus or it defines how “solid-like” a material is. Its magnitude depends on the number of interactions between different constituents in a sample under study. The value of the storage modulus is directly proportional to the number of interactions and their strength. On the other hand, the viscous or loss modulus, describes the part of energy lost as viscous dissipation. It is related only to the number of interactions and is independent of their strength. Furthermore, the degree of viscoelasticity of a sample can be evaluated by phase angle or tan δ which is the ratio of loss modulus to storage modulus (G”/G′). Hence, a sample is more viscous or liquid-like if it has a high value of tan δ while the sample with low tan δ would be more elastic or solid-like.
3.2.2 Micro DSC
During food processing heat is involved at different steps. The food undergoes different types of transformations, including melting, crystallization, gelation, gelatinization, denaturation and oxidation during heating, cooling or freezing. All these transformations occur in a certain range of temperature and are associated with heat variations. Thermal analysis techniques, particularly DSC, are used as a primary approach for investigating these properties of foods. However, food processing involves mixtures of food constituents and not just simple systems; these may be mixed or diluted with a liquid (water, milk, oil) or with a powder (sugar, fibre). For simulation of such transformations and interactions, the limited volume and the lack of in situ mixing, constitute the major drawbacks of techniques involving DSC. Micro-calorimetry provides an ideal solution for such investigations because it has the capacity to work on bulk materials and diluted solutions with a very high sensitivity.
Principally, it uses the heat flux calorimetric principle for food characterization and the calorimeter consists of a measurement chamber surrounded by a detector (thermocouples, resistance wires, thermisters, and thermopiles) to integrate the heat flux exchanged by the sample contained in an adapted vessel. The chamber is insulated in a surrounding heat sink made of a material having high thermal conduc.
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