Rheology+Of+Food

By Mohamad Iswandi Bin Ishak Rheology the most sensitive method for material characterization because flow behavior is responsive to properties such as molecular weight and molecular weight distribution. Rheology measurements are also useful in following the course of a chemical reaction. Such measurements can be employed as a quality check during production or to monitor and/or control a process. Rheological measurements allow the study of chemical, mechanical, and thermal treatments, the effects of additives, or the course of a curing reaction. Isaac Newton classified fluids into two types which are Newtonian fluid and non-Newtonian fluid.
 * Rheology Of Food **

====__Newtonian fluid **: **__ ==== The fluids which obey Newton’s law of fluid flow are called Newtonian fluid. Newtonian liquids start to flow when a stress is applied and deformation stops instantly when the stress is removed**.** In Newtonian liquids the viscosity is constant with respect to the time of shearing and it does not change in the re-testing situation. Viscosity is only dependent on temperature.

====__Non- Newtonian fluid **: **__ ==== The fluids which do not obey Newton’s law of fluid flow are called non-Newtonian fluids. The relationship between shear stress and shear rate is not constant. As non-symmetrical objects pass by each other as happens during flow, their size, shape and cohesiveness will determine how much force is required to move them. At another rate of shear, the alignment of the objects may be different and more or less force may be required to maintain motion. There are several types of non-Newtonian flow behaviour that characterized by the way a fluid’s viscosity changes in response to variations in shear rate. The most common types of non-Newtonian fluids are pseudoplastic, dilatent, plastic//,// thixotropy, rheopexy, bingham and so on.

//Pseudoplastic//: This type of fluid will display a decreasing viscosity with an increasing shear rate. This type of flow behaviour is also called as shear-thinning. The example of foods that follow this flow curve are orange juice.

//Dilatent:// This type of fluid will display an increasing viscosity with an increase in shear rate characterizes the dilatant fluid. Dilatancy is also referred as shear-thickening flow behaviour. Dilatant flow may be a result of dispersions containing a high concentration (≥ 50%) of small, deflocculated particles, concentrated slurries, 52% potato starch in water and candy compounds.

//Plastic//: This type of fluid behaves as a solid under static conditions. A certain amount of force must be applied to the fluid before any flow is induced. This force is called the yield value. Below the yield value, the material behaves essentially as an elastic solid**.** Once the yield value is exceeded and flow begins, plastic fluids may display newtonian, pseudoplastic or dilatant flow characteristics.

//Thixotropy and Rheopexy//: Some fluids will display a change in viscosity with time under conditions of constant shear rate. There are two types of fluids which are thixotropic and rheopectic.

Thixotropic: A thixotropic fluid undergoes a decrease in viscosity with time, while it is subjected to constant shearing. Rheopectic: this is essentially the opposite of thixotropic behaviour, in that the fluid’s viscosity increases with time as it is sheared at a constant rate.