Nadiah_Rheology

//**RHEOLOGY OF FOODS**//

Rheology means something that "flow". It is a study about deformation and flow behavior of a food product especially when applying external forces such as pouring, spreading and scooping.
 * WHAT IS RHEOLOGY? **




 * STRESS & STRAIN **

Rheology is mainly concerned with relationship between starin, stress and time.

- a quantitative measure of the extent to which an element of material has been deformed. **STRESS:** - force per unit area.
 * STRAIN: **

In food, stress and strain impacts the food apperance, processing and texture. Pictures below show the simple rheology. When we squeeze the round ball, the shape will deform. This show when we apply stress towards the ball the appearance change.

Besides, when we squeezing an orange, actually we apply external force or stress. So, the juice will come out and the shape of orange will change. This picture show us that by squeezing an orange we are applying force or stress to it, this will allow the juice flow out, by doing such act it will also deform the orange shape

~ Thus, strain and stress are related to deformation and force, respectively.

 Viscosity means a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. For example we can compare the viscosity of honey and water. Honey is more viscous than water because it contain high concebtration of sugar. Therefore, the rate of flow of water is higher than honey or in other words water can flow easily than honey. There are various factor that affect viscosity such as:
 * VISCOSITY **
 * shear rate
 * time of shearing
 * temperature
 * pressure


 * NEWTONIAN AND NON-NEWTONIAN **

**__//Newtonian Fluids//__** The definition of Newtonian behavior are:
 * Shear viscosity does not vary with shear rate.
 * Viscosity is constant with time of shearing.
 * Shear rate in fluids falls immediately to zero when shearing is stopped.

**CHARACTERISTIC DIAGRAMS FOR NEWTONIAN FLUIDS**

Figure A show the linear relationship between Shear stress and Shear rate. Figure B show that Viscosity is independent of shear rate.

The viscosity of Newtonian fluids is influenced by the:
 * Temperature
 * Concentration


 * //Temperature and viscosity//**
 * Temperature inversely affect the viscosity
 * High temperature,low viscosity
 * //Concentration and viscosity//**
 * High concentration, high viscosity

Example of Newtonian fluids: water, milk and honey.



**__//Non-Newtonian Fluids//__** Non-Newtonian fluids is any liquid that show deviation from Newtonian behavior.

Can be devided into: // **Time Independent** :// Viscosity of fluids depend on shear rate but independent of time of sharing.
 * Pseudoplastic
 * Dilatant
 * Bingham
 * Casson plastic

// **Time Dependent** :// Viscosity of fluid is dependent on shear rate and the time during which shear rate is applied.
 * Thixotropic
 * Rheopectic

**TYPES OF FLOW**

Newtonian and Non-newtonian curve

**YIELD STRESS**

Yield stress is the minimum shear stress that need to be applied to initiate a sample to flow. Minimum shear stress is required to overcome the cohesive force that holds the structure of the material. As example: Squeezing cream in decorating the cakes.



The yield stress is the applied stress we must exceed in order to make a structured fluid flow. Yield stress can be further classified into Static and Dynamic Yield Stress.



.

**RHEOLOGICAL TESTING**

**EMPIRICAL TESTS** Examples: Bostwick consistometer, falling ball viscometer and glass capillary viscometer
 * "test and see" approach
 * measurements made without consideration of changes in product shape or control deforming forces.



Bostwick consistometer

Falling ball viscometer

Glass capillary viscometer

Example:
 * FUNDAMENTAL TESTS **
 * "Physical" based approach
 * Use well defined text fixture(geometry) and test condition.
 * Values are independent of sample geometry and data can be validated with another geometry.

Rotational viscometer