Rheological Testing of FRoods

RHEOLOGICAL TESTING OF FOODS Rheological Testing of Foods Rheology is the science concerned with the deformation and flow of matter .• Most rheological tests involve applying a force to a material and measuring its flow or change in shape.• Rheology is important in a number of different areas of food science.• Many of the textural properties that human beings perceive when they consume foods are largely rheological in nature, e.g., creaminess, juiciness, smoot hness, brittleness, tenderness, hardne ss, etc.• The stability and appearance of foods often depends on the rheological characteristics of their components, e.g., emulsions, spreads and pastes.• The flow of foods through pipes or the ease at which they can be packed into containers is largely determined by their rheology. • The obvious importance of rheology in foods means that it is essential for food scientists to have analytical technique s to measure these properties.• Instruments are needed for routine analysis in quality assurance laboratories, and for fundamental studies in Research and Development laboratories.• Fundamental studies aim to better understand the complex relationship between the overall rheological properties of foods and the type and concentration of ingredients that they contain.• This type of information enables food manufacturers to optimize the ingredients and processing conditions needed to produce high quality and reliable products.• • Foods are compositionally and structurally complex systems that can exhibit a wide range of different rheological behaviors, ranging from low viscosity fluids (e.g., milk or orange juice) to hard solids ( e.g., hard candy).• One of the main objectives of food rheologists is to develop instrumentation and concepts that can be used to measure and describe these various types of rheological behavior.• De spite the diversit y and complexity of food systems it i s possible to syste matically cha racterize ma ny of their rheologica l properties in terms of a few simple models: the ideal solid, the ideal liquid , and the ideal plastic.• Complex systems can then be described by combining two or more of these simple models.• In the following sections the concepts of the ideal solid, ideal liquid and ideal plastic will be introduced, as well as some of the deviations from these models that commonly occur in foods.• •• solids In our everyday lives we come across solid materials that exhibit quite different rheological properties.• Some may be soft, others hard; some may be brittle, others rubbery; some may break easily, others may not.• Despite this range of different behavior it is still possible to chara cterize the rheological propertie s of many solid foods in terms of a fe w simple conc epts.•• Ideal solids A material that exhibits ideal elastic behavior is referred to as a Hookean solid after the scientist (Robert Hooke) who first described it.• Hooke observed experimentally that there was a linear relationship between the deformation of a solid material and the magnitude of the applied force.• In fact, he found that the force per unit area (or stress) was proporitional to the relative deformation (or strain).• Hookes law can be summarized by the following statement: ••••••••••• Stress = Modulus • Strain Most elastic materials only obey Hookes law at small deformations.• Equation 1 applies to a number of different types of deformation that a solid can experience.• The actual values of the stress, strain and constant used in the equation depend on the nature of the deformation.• For an isotropic and homogeneous solid there are three major types of deformation that are important: simple shear, simple compression (or elongation) and bulk compression.• Each of these different types of deformation can be characterized by its own stress-strain relationship.• • Simple shear:• ••••••••••••••••••••••• Stress = τ = F/A••••••••••••• ••••••••• ••••••••••••••••••••••••••••••••••••••••••••••• •••••••••••••• ••••••••••••••••••••••••••••••••• Strain = γ = ∆L/L = cosφ •••••••••••••• ••••••••••••••••••••••••••••••••• Modulus = G (shear modulus)•• •••••••••••••• ••••••••• • •••••••••••••• ••••••••••••••••••••••••••••••••••••••••••••••••••••••• Simple compression:•••••• •••••••• Stress = F/A •••••••••••••• ••••••••••••••••••••••••••••••••• Strain = ∆L/L •••••••••••••• ••••••••••••••••••••••••••••••••• Modulus = Y (Young•s modulus)•••••••••• •••••••••••••• ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• Bulk compression:•••••••••••••••••• Stress = τ = F/A = Pressure, P file:///C|/Users/ DR%20F.%20K.%20S AALIA/Desktop/ RHEOLOGICAL%20TES TING%20OF%20 FOODS.htm (1 of 8)5/7/2014 12:02:30 PM

Documents

Rheological Testing of FRoods