Food Engineering

Lesson 3. RHEOLOGICAL METHODS

Module 1. Rheology of foods

Lesson 3

RHEOLOGICAL METHODS

3.1 Introduction

Generally rheological properties are judged by sensory panel, it has its advantages and disadvantages depending on the person selected for judging the products. To have unbiased scores as well as reproducibility of the values of rheological attributes, it is necessary to go for instrumental measurement. There are many instrumental methods are developed based on fundamental principle as well as experimental data. There are certain mathematical models developed by different scientist based on empirical methods, which are widely used for measurement of rheological properties of most of the food products.

3.2 Tests for Measurement of Rheological Properties

Instrumental methods for measurement of rheological properties are classified into two broad categories as follow:

• Fundamental tests which measure the properties that are inherent to the material and do not depend on geometry and shape of the sample, conditions of loading or type of apparatus used, e.g. relaxation time, Poisson’s ratio, shear modulus and bulk modulus;
• Empirical tests (because data are based on comparison with sensory) or imitative tests (because these imitate the chewing in mouth). e.g. properties like puncture force, extrusion energy, cutting force required, pressing/compression force required for juice extraction, etc. – where mass of sample, geometry and speed of test will decide the magnitude of parameter estimated.

Generally fundamental tests are applied on solid foods and these are further classified into quasi-static and dynamic tests

The tests conducted under conditions of static/quasi-static loading are known as quasi-static tests while those conducted under dynamic loading conditions are called dynamic tests.

The use of Instron in determining the modulus of elasticity under compression is an example of quasi-static test while if the determination is done using a vibrating device of certain frequency (generally 200 Hertz), then the test is dynamic. You can say that rate of loading can be used to determine whether test is dynamic/quasistatic.

3.3 Quasi-Static Testing of Solid Food Products

Two types of behaviour can be studied– elastic behaviour of solid and another is pure viscous flow in case of liquids. Pure elastic behaviour is defined such that when force is applied to the material, it will instantaneously and finitely deform and when the force is released, the material will instantaneously come to the original form. Such materials are called ‘Hooken solids’ i.e. which follow Hook’s law. The amount of deformation is proportional to the magnitude of the force. Rheological representation of this type of solids is a spring. The material of this nature can be given a rheological constant modulus of elasticity is ratio of stress/strain, where stress = force/area, and strain = deformation due to force applied/original dimension. There are 3 types of moduli depending on type of force applied.

• If force is applied perpendicular to area defined by stress and it is calculated as – modulus of elasticity(E)
• If modulus is calculated by applying force parallel to area defined by stress i.e. a shearing stress, then it is called a shear modulus or modulus of rigidity(G or n) and
• If force is applied from all directions (isotropic force) then change in volume over original volume is obtained that can be calculated by bulk modulus(B or K)

Creep: In an experiment if a constant stress is applied to sample and corresponding strain is followed as a function of time and results are expressed in terms of a parameter of compliance (J=strain/stress). The change in the strain of material can be measured, when stress is removed it known as creep curve. In short we can say that creep curve shows strain as a function of time at constant stress. Visco-elastic materials can often be characterized by a modulus and relaxation time, which can be determined by an analysis of strain curve with time.

Relaxation curve (stress relaxation) – It is the curve obtained when stress is applied as a function of time at a constant strain. That means that instead of applying constant force and measuring the change in strain with time, it is also possible to apply a constant strain and measure change in stress with time. This type of experiment is called relaxation stress and the curve is known as relaxation curve.

These relaxation and creep experiments are known as Transient experiments in which a constant force is applied to the material and resulting strain is measured as a function of time and vice-versa.