Elizabeth-Rheology


 * Lecture Summary **

= Week 1- Week 5  = = (Rheology)  =

__Rheology of foods__
The word Rheology in Greek language is rheos-->to flow, and Heraclitus say rheology is //panta rei,// meaning everything flows ( if you wait long enough)

What is Rheology of foods? -> Rheology is any study of the deformation and flow of matter if you subjected enough energy to solids or liquids food, the picture below show some of the act of applying external forces to some food to allow them to flow or change in shape.

This picture show the spreading of butter on toast, spreading is the external force causing the butter to spread on the toast, and the butter will deform and stick to the toast.

 3 of the factors that mainly related to rheology

Food material respond to stress and strain would affect their processing, appearance,and texture.
 * stress should be force intensity
 * strain should be deformation intensity

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.

__Newtonian and Non-Newtonian Fluids__
This show the different types of flow in fluid( Newtonian and Non-Newtonian)

__**Newtonian**__


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

ANY LIQUID THAT SHOW DEVIATION FROM NEWTONIAN IS A NON-NEWTONIAN

Examples of Newtonian Fluids are, water, honey, milk, and fruit juices.

__Non-Newtonian__
Can be divided into time independent and time dependent. Time independent ---> Time dependent- -->
 * Pseudoplastic
 * Dilatant
 * Bingham
 * Casson plastic
 * Thixotropic
 * Rheopectic

__Time independent__ Viscosity of fluid dependent on shear rate, but independent of time of shearing.
 * Pseudoplastic- **
 * Decrease in viscosity as shear rate increase.
 * Flow behaviour is reversible
 * Known as SHEAR THINNING
 * After exhibit shear thinning it will recover viscosity very rapidly
 * Emulsions and dispersions, ( whipped cream, ketchup)


 * Dilatant- **
 * Viscosity increases as shear rate increases.
 * Known as SHEAR THICKENING
 * Example, corn starch suspensions.


 * Bingham- **
 * Pseudoplastic material have internal forces that keep it from flowing below some value of shear stress and material begin to flow above this value--> it classifies as bingham
 * Shear stress value where bingham material begin to flow know as yield value/point.
 * Examples, ketchup,mayonnaise.

__Time dependent__ Viscosity of fluid is dependent on shear rate and time during shear rate is applied.


 * Thixotropic- **
 * Decrease in viscosity with time under constant shear rate, followed by gradual recovery when stress is removed.
 * When the shearing stopped, they regain the original viscosity but require TIME to do so.
 * Examples, carboxymethycellulose (CMC), and tomato paste.
 * Also important in chocolate industry, for chocolate coatings.( Shear reduce viscosity of chocolate,so it will flow and cover the product,and shearing stop so chocolate set up,and the coating will solidifies.

__ **Plastic fluids** __


 * Low shear stress (below critical value), fluid behave like solid, no deformation until shear stress(yield stress) reached.
 * Yield stress-minimum shear stress has to be applied to initiate flow of materials.

==== **Flow curve-application in 'stand up' characteristic of chocolate and caramel** ====



Sample A- good for 'stand up' in chocolate Sample B -good for 'coating' in chocolate

'Stand up' characteristic- how well the chocolate can retain its original structure.

This picture show the Hershey chocolate that has its fixed shape this is due to the stand up characteristic.

This picture show the Hershey chocolate that is flat due to the absence of stand up characteristic.


 * High viscosity at low shear rate- more firmly the material will retain shape and resist slumping.
 * Sample A show low shear viscosity much higher than sample B --- so it posses better stand up characteristic.

__**Factors affecting viscosity**__

 * Temperature
 * Concentration of solute
 * Molecular weight of solute
 * Pressure
 * Suspended matter


 * Temperature- ** Inverse relationship between viscosity and temperature


 * Concentration of solute- **Direct nonlinear relationship between concentration of solute and viscosity at constant temperature


 * Molecular weigh of solute- ** Nonlinear relationship between molecular weight of solute with viscosity of solution at equal concentrations.


 * Pressure- **Viscosity of most liquids essentially constant over a pressure range of 0-100 atm, so usually pressure effect be ignored for foods.


 * Suspended matter- ** Increases viscosity slightly when in low concentrations, high concentrations of suspended matter cause substantial increases because of entanglement between particles.

__Measuring viscosity__
Empirical test and Fundamental test

__Empirical test__


 * Test and see
 * Measurement made without changes in product shape, or control of deforming process
 * Bostwick consistometer, falling ball viscometer,glass capillary viscometer

__Fundamental test__
 * 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

__Viscoelasticity__
(Viscoelasticity) Viscoelastic material is any substance that having both viscous and elastic properties.

Viscoelasticity --> Weissenberg effect ( rod climbing phenomenon)

Rod climbing phenomenon...

Effect depend on ratio of elastic to viscous component in viscoelastic material.

Snap-back property --> apply in some viscoelastic fluid, example is ketchup retreating back to bottle after squeezing.

Die swelling of starch during extrusion

die swelling during extrusion...for (left)viscoelastic and (right)inelastic liquid.

**__Deborah number__**
(time dependent viscoelastic behaviour) Deborah Number, De --> ratio of characteristic relaxation time of material to characteristic time of relevant deformation process.

High De, >>1 solid like behaviour Low De<< 1 liquid like behaviour Behave like solid-like either because of a.) long time characteristic relaxation time b.) relevant deformation very fast