Hui+Qin+'s Weekly+Lecture+Summary

__**Week 1 to Week 5**__
 * __ Hui Qin's Weekly Lecture Summary __**

= __**RHEOLOGY**__  = = =
 * the study of fluid and structural properties of raw material, the flow and deformation of materials experiencing an applied force (pouring, sucking).
 * eg: pouring tomato ketchup, spreading jam, scooping ice cream and squeezing toothpaste. These action require external force to let the fluid or material to flow out.



__ Stress and Strain __
a) Stress-Simply defined as force per unit area. b) Strain--Simply a quantitative measure of the extent to which ann element of material has been deformed. The deformation implies the change of shape. (dimensional change)

deformation

__ Newtonian and Non-Newtonian fluids __
variables that affect viscosity :
 * Shear rate
 * Time of shearing
 * Temperature
 * Pressure

__Newtonian fluids__

 * viscosity does not vary with shear rate
 * viscosity is constant with time
 * stress in fluids fall immediately to 0 when stop shearing
 * eg: water, milk

no matter how we stir water, its viscosity is still same



__Non-Newtonian fluids__
-shear thining (pseudoplastic-viscosity low ,shear rate high ) -shear thickening (dilatant - viscosity high ,shear rate high ) || thixotropy ||
 * **Time independent** || **Time dependent** ||
 * Viscosity depend on shear rate || Viscosity depend on shear rate ||
 * Viscosity independent on time || Viscosity dependent on time ||
 * Apparent viscosity





__eg: shear thickening egg__

A thixotropic fluid tends to look like a solid until you stir it, and then it behaves like a runny liquid. eg: yoghurt


 * Yield stress ** is the minimum shear stress that has to be applied to initiate a sample flow.



Yield stress can be further classified into Static and Dynamic Yield Stress. The intersection on the stress axis is then taken as the yield stress, the assumption being that any stress below this is insufficient to cause the sample to flow. Rheologists call this a dynamic yield stress. (refer to figure 1 and figure 2)

- Empirical test

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

- Fundamental test

 * 'Physics based approach". Use well defined text fixture(geometry) and test condition.
 * units/values are independent of sample geometry and data can be validated with another geometry.
 * Examples : Rotational viscometer



Yield stress can be further classified into Static and Dynamic Yield Stress. The intersection on the stress axis is then taken as the yield stress, the assumption being that any stress below this is insufficient to cause the sample to flow. Rheologists call this a dynamic yield stress. (refer to figure 1 and figure 2)

- Empirical test

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

- Fundamental test

 * 'Physics based approach". Use well defined text fixture(geometry) and test condition.
 * units/values are independent of sample geometry and data can be validated with another geometry.
 * Examples : Rotational viscometer

**__Emulsion and Foam__**

**Texture**- encompasses all the rheological and structural attributes of a food product perceptible by means of mechanical, tactile, visual and auditory receptor.
 * __Principles of texture analysis__ **

**__INSTRUMENTAL TEXTURE TEST__**
 * Fundamental test- physical properties (young's modulus, uniaxial compression)**


 * Empirical test- observation, experimentation (compression, punture & penetration, cutting & shearing, fracture & bending, extrusion)**


 * Immitative test- conditions in mouth (texture profile analysis TPA, two bite test)**


 * __ Food Colloids : emulsion and foams __**

__Dispersed phase__ __Continuous phase__ liquid liquid **emulsion**

gas liquid **foam**

__**Interaction between dispersed droplets**__ 1. **Van Der Waals attractive forces** - originate from dipole-dipole interaction, closer=stronger 2. **electrostatic repulsive forces** - electrical double layer- the combination of the charged surface and the unequal distribution and counterions near the surface. conc. of electrolytes ↑, more ions will be attracted to the surface droplets = increase or decrease the thickness of the layer. temp ↑, more energy, double layer more dynamic temp ↓, emulsion more stable temp ↑, electrolytes energy ↑ , not form a thick double layer, droplets cluster together.
 * thickness of double layer ↑, more stable emulsion



-when two charged surfaces approach so that their electrical double-layers begin to overlap, a repulsive force is induced which tends to oppose further approach. -electrical double layer repulsion will stablilise emulsion, when the electrolyte conc. phase is less than a certain value.
 * DLVO Theory** (describe 2 types of interactive force- attractive and repulsive force)

sum of the two energies = -ve(attractive force) = aggregation sum of the two energies = +ve(repulsive force) = stable

__**Surface and interfacial tension**__ **Surface tension (γs)**---defined as the amount of energy (ΔE) required to increase the surface area between a liquid and a gas (e.g. air and water) by an amount ΔA. ΔE = γsΔA
 * Surface tension**-the property of a liquid in contact with air makes it behave as if it is covered with a thin membrane under tension.
 * Surface tension (N/m)**---force acting over the surface of liquid per unit length of the surface perpendicular to the force.
 * Interfacial tension ( γ ** **i)**---defined as the amount of energy required to increase the interfacial area between two immiscible liquids (e.g. oil & water).

**__ Food Emulsion __** **Emulsion**-consists of two immiscible liquids with one of the liquids dispersed as small spherical droplets in the other. - the dispersion of one phase into small droplets, result in a massive increases in interfacial area between the dispersed and continuous phase.


 * Homogenization**- the process by which the dispersed phase is broken into small droplets.


 * Emulsion stability** - ability to resist changes in its physiochemical properties with time.



**__Mechanism of emulsion breakdown (cause emulsion unstable)__** -**creaming** -droplets move upwards -**sedimentation** -droplets move downwards -**flocculation** -aggregate (without breakdown of interfacial film) -**coalescence** -two or more droplets merge together to form a single larger droplets (breakdown of interfacial film) -**phase inversion** -O/W emulsion change to W/O emulsion, vice versa.

**__Emulsion Stabilization__** protective film around the droplets.
 * Emulsifier (surfactant)** - compound that facilitate the formation of emulsion by lowering the oil/water interfacial tension and imparting short-term stability by forming a


 * Stabilizers** - compounds that are not surface active but impart long-term stability to emulsion by restricting interfacial interations.

__**Functions:**__ amphiphilic, adsorb at the interface between oil and water and form an interfacial film= reduction of interfacial tension.
 * 1. Adsorption at interface**

mixture of emulsifier and water - liquid crystals form on the surface of oil droplets in o/w emulsion
 * 2. liquid crystal stabilization (arrange in specific order manner)**

ionic emulsifier will form an electrically charge double layer in the aqueous solution surrounding each oil droplet. ionic strength ↓, electrical repulsion › VDW attractive forces = the droplets remain suspended. salt conc ↑, flocculation/coalescence ↑ , stability ↓
 * 3. ionic stabilization (electrostatic)**

hydrocolloids - increase the viscosity - partition into the o/w interface as a physical barrier to coalescence.
 * 4. Stabilization via steric hindrance**

droplets in an emulsion have a different density to that of liquid which surrounds them - a net gravitational force acts upon them- cause creaming or sedimentation.
 * 5. Gravitational separation**

creaming rate --- Stoke's equation

**__Methods of controlling gravitational separation__** 1. minimize density difference 2. reduce droplets size (high pressure homogenization) 3. modify rheology of continuous phase ↑ viscosity of continuous phase, ↓ velocity at which droplet moves. add thickening agent

**__ FOAMS __**- a two-phase system in which the gas phase is dispersed in a small amount of liquid continuous phase. **Bubbly foam** -amount of gas incorporated is low enough for bubbles to retain roughly spherical shape. **Polyhedral foam** -the gas-to-liquid ratio is so large that bubbles are pressed against one another in a honeycomb-type structure.

__**Foam Formation:**__ 1. air injected into liquid2. large air bubbles broken---smaller bubbles3. smaller bubbles prevented from fusing

**__Surface active foaming agent__** -lower the surface tension of liquid phase-allows expansion of its surface area-form a closely packed film-essential for formation of stable foam. **__Protein as surface active agent__** 1. adsorption of protein at the gas-liquid interface.2. surface denaturation3. coagulation of protein (form a network that gives rigidity and stability to the foam)

**__3 factors affecting foam stability:__** 1. **drainage**: the draining of liquid from foam. (under gravity) - as water leaves, faces of film are brought closer together. 2. **disproportionation**: the diffusion of gas from small bubbles into big bubbles 3. **coaslescence**: the fusion of foam bubbles

**__EGG FOAMS:__** -**protein**-surface active agent-**globulins-**help to foam gas bubbles-**ovomucin**- help to stabilize the foam (rapidly denatured at the bubbles surface)



protein conc. ↑, foamability and foam stability ↑ pH 6.5 (adding acid ingredient) ↑ foam stability to heat sugar increase viscosity of lamellar fluid, drainage rate decrease, foam stability increase. .
 * presence of fat -ve effect on foam formation