Wei+Shan+Weekly+Lecture+Summary

__**Wei Shan's Weekly Lecture Summary**__
__**Week 1 to Week 5**__

= __**Chapter 1 :RHEOLOGY**__ =
 * **The study of deformation and flow of matter**
 * **Aims at measuring properties of materials that control their deformation and flow behavior when subjected to external forces**
 * **Example of external forces are pouring, stirring and spreading.**

__ **OWN EXPLANATION ON RHEOLOGY** __

Rheology is the study of flow of matter in everything around us .For example gas, liquid and solid .It means that everything will flow eventually , if we wait long enough. This happen when we apply strain or stress on the object .So what is stress and strain?? we take tomato as our example. We press the tomato with our finger, we can see that the part that we press is dented. When we press the tomato we are exerting force and that is mean by stress. Strain is the measure of the extent to which an element of material has been deformed .In this case, we measure the extent of the deform of shape we make to the tomato. For fluid, some fluid is non- newtonian for example tomato sauce .Non-newtonian are fluid that their viscosity change when we exert force to it .Viscosity is the state of being thick ,sticky and semifluid. Object with high viscosity will have resistance to flow .For example ,tomato sauce .We can see that it is hard to just pour the tomato sauce from the bottle into our plate .But when we apply force to it by shaking it hard, the tomato sauce just flow out easily compared to when we didnt apply any force to it. Shaking it mean we are acting an external forces to it. Pressing on the tomato surface using our finger also an example of acting external force



__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)


 * [[image:imk209/deformation.jpg caption="deformation.jpg"]] ||
 * deformation.jpg ||


 * Shear rate
 * Time of shearing
 * Temperature
 * Pressure

__Newtonian fluids__

 * is independent of the shear rate at which it is measured
 * 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



__ Non-newtonian fluid __

 * dependent on the shear rate
 * can be classified into time dependent and time independent

Time dependent non newtonian fluid : Time independent non newtonian fluid
 * 1) EX :Thixotropic, Rheopectic
 * 2) dependent on shear rate and time during which the shear rate is applied
 * 1) EX pseudoplastic, dilatant ,bingham ,casson plastic
 * 2) dependent on shear rate BUT independent of the time shearing

Pseudoplastic

 * know as shear thinning
 * decerease in viscosity as shear rate increases
 * flow behaviour is reversible
 * Ex. emulsion and dispersion

Dilatant

 * know as shear thickening
 * viscosity increase with increasing speed
 * Ex corn starch suspension

Thixotropic

 * decrease in apparent viscosity with time under constant shear rate or shear stress



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

\
 * [[image:imk209/yield.jpg width="219" height="178" caption="yield.jpg"]] ||
 * yield.jpg ||


 * [[image:imk209/dynamic_yield_stress.gif width="225" height="197" caption="dynamic_yield_stress.gif"]] ||
 * dynamic_yield_stress.gif ||


 * [[image:imk209/static_yield_stress.gif width="225" height="226" caption="static_yield_stress.gif"]] ||
 * static_yield_stress.gif ||

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)

MAYONNAISE CAN BE USED AS AN EXAMPLE TO EXPLAIN YIELD STRESS. THIS IS BECAUSE WHEN A MINIMUM STRESS IS ACTED ON THE MAYONNAISE, THE MAYONNAISE DEFORM. IF THERE ARE NO YIELD STRESS THAT CAUSE THE DEFORM, THEN THE MAYONNAISE WILL RETURN TO IT ORIGINAL POSITION AFTER THE GENTLE FORCE ACTING ON IT IS REMOVED

- 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

=__ Chapter 2 :FOOD COLLOIDS __=
 * dispersion of two or more immiscible of material
 * consists of dispersion phase and continuous phase called dispersion medium
 * Includes dispersions containing larger particles (>1um)
 * Forces between the colloidal particles determined by the summation of the intermolecular forces over many molecules
 * Two main interacting forces in affecting colloid system : Van der Waals attractive forces (destabilise colloids) & electrostatic repulsive force ( impart stability)
 * Ex are foam and emulsion

VDW

 * dipole dipole interaction
 * Attractive force increase, more rapidly the droplets approach

**Electrical double layer**
 * Same charged ion : repelled away
 * Opposite charged ions : preferentially attracted toward the surface
 * Combination of charged surface & unequal distribution of unequal distributions of coions and counterions near the surface --> electrical double layer
 * Double layer --->important for stability ( can be manipulated by adding electrolytes or change temperature) /sensitive to electrolytes & temperature

Emulsion

 * Dispersion of liquid droplets in liquid continuous phase
 * milk/cream/spread



Foam

 * Coarse dispersion of gas bubbles in a liquid continuous phase
 * beer/ice cream/bread

** DLVO THEORY **

 * electrical double layer repulsion will stabilise emulsion ( electrolyte concentration phase < certain value)
 * relates stability of emulsifed droplets to two independent potential that come into action when two droplets approach each other
 * sum of energies-> aggregation (+) / (-) repulsive forces

__ Surface tension(N/m) __
===defined as the force acting over the surface of the liquid per unit length of the surface perpendicular to the force or the amount of energy required to increase the surface area between a liquid and a gas (eg air and water) by an amount.===
 * ===The molecules exposed to air behave differently or try to contract to the smallest possible area ---> spherical shape===
 * Molecule ---> inside ( interact equally with other molecule from all sides) / at the surface ( affected by the molecule below only )
 * Property of a liquid in contact with air as it was covered with a thin membrane under tension

__ Interfacial tension __

 * Amount of energy required to increase the interfacial area between **two immiscible liquid (oil & water )**
 * Large interfacial area between he dispersed & continuous phase --->important quality of the colloids
 * Decrease size ,increase number of particles ,increase area of interfacial contact

= __Food emulsion__ =
 * Consist of two immiscible liquid > Water in oil (w/o) & Oil in water (o/w)
 * With one of the liquid dispersed as small spherical droplets in the other
 * Dispersed &/or continuous phase of many food emulsion --> partly crystalline rather than being partly liquid
 * Dispersion of one phase into small droplets ---> massive increase in interfacial area between the dispersed & continuous phase
 * Homogenization > process by which dispersed phase is broken into small droplets ( by high pressure homogenization)

Mechanism of emulsion instability

 * Ability to resist changes in physicochemical properties with time.
 * Creaming : the process in which droplets move upwards ( droplets density < density of continuous phase)
 * Sendimentation : The process in which droplets move downwards ( droplets density > density of continuous phase)
 * Flocculation : The process in which two or more droplets "stick" together to form an aggregate
 * Coalescence : The process in which two more droplets merge together to form a single larger droplets.
 * Phase inversion : The process in which o/w emulsion changes to w/o emulsion or vice versa.

__ Emulsion Stabilization __

 * Emulsifier : lower the oil/water interfacial tension & imparting short -term stability by forming a protective film around the droplets.
 * Stabilizer : compounds that are not surface active but impart long-term stability to emulsion by restricting interfacial interactions.
 * Stabilizer (hydrocolloid) = increases emulsion stability
 * Macromolecules : increasing the viscosity or partitioning into o/w interface as a physical barrier as a physical barrier to coalescence.

Emulsifier

 * Amphiphilic nature ---> absorb at the interfacial between oil & water --> form interfacial film ---> reduction of interfacial tension
 * Reduction of interfacial tension -> addition of emulsifier ---> allow emulsion formation with considerably less energy input

Liquid Crystal Stabilization

 * mixture of emulsifier & water
 * Liquid crystalline phases may form of oil droplets in o/w emulsion & reduce the rate of coalescence, even if flocculation occurs
 * Micelle : an aggregation of the emulsifier molecules oriented with the hydrophobic chains to the inside and the hydrophilic groups on the surface

Ionic Stabilzation

 * Introduction of charged groups on the surface of the emulsion droplets ---> increases the repulsive forces
 * Ionic emulsifier ---.> form an electrically charged double layer in the aqueous solution surrounding each oil droplets
 * Thickness of the electrical double layer is affected by the ionic strength
 * Ionic strength low, electrical repulsion is > Van der Waals attraction , droplets remain suspended
 * Ionic emulsifier, low salt ( enhances stability ) , high salt ( conc increases flocculation and / or coalscence.

Minimize density difference

 * matching the densities of the oil & aqueous phase
 * density matching ; achieved by mixing natural oils with brominated vegetable oils

Reduce droplets size

 * increase stability of an emulsion to gravitational separation by reducing the droplet size
 * High pressure homogenization
 * velocity at which a droplet moves is proportional to r 2

Modify rheology of continuous phase

 * Increasing the viscosity of continuous phase surrounding the droplets -> decrease the velocity at which droplets moves
 * Add thickening agent ...( hydrocolloids= xanthan gum /aracbic gum )

Emulsion stability Index
> =__** Foams **__=
 * Predict long term stability of an emulsion
 * Simple test using centrifuge at a given speed and time
 * Measure the changes of particle size distribution (PSD) of an emulsion with time ( a good emulsifier will not change with time )
 * two phase system
 * gas dispersed in small amount of water (continuous phase)
 * Eg beer, whipped white egg

__ Foam structure __

 * Bubbly : formed when amount of gas incorporated is low enough for bubbles to retain roughly spherical shape.
 * Polyhedral : Formed when gas-to.liquid ratio is large enough that bubbles are pressed against one another in honey-comb type structure.

Air has to be injected into the liquid -> large air bubbles have to be broken into smaller bubbles --> small bubbles has to be prevent from fusing during the formation of a foam

Foam stability

 * surface active agent( lower the surface tension of liquid phase & allow expansion of its surface area) essential for formation of stable foam.Surfactant form a closely packed film around the dispersed gas bubbles
 * Increase protein >> increase foamability & foam stability ( mainly because viscosity effect which produces thicker lamella film)
 * adjust egg white pH to 6.5 by adding acid ingredient ---increases foam stability to heat -because the acid makes foam less prone to over-coagulation
 * Sugar increasing the viscosity of lamellar fluid which reduce the drainage rate


 * Three process in stabilization of protein foams:**
 * (a) adsorption of the protein at the gas-liquid interface**
 * (b) surface denaturation**
 * (c) coagulation of protein**

Three factors affecting it :


 * Drainage :**
 * **The draining of liquid from foam **
 * **Water drains from foam under gravity > drains along lamellae to the curved junction of thin lamellae where pressure is low---> as water leaves ,faces of film are brought closer together**


 * Disproportionation :**
 * **Diffusion of gas from small bubbles into big bubbles**
 * **occurs remarkably quickly in the absence of a stabilizing film of polymer molecules**
 * **The change in foam bubble size distribution caused by gas diffusion from small to large bubbles**

__ **Egg Foam ( essential in preparation of angel cakes ,sponges ,meringues,souffles .etc)** __

 * 1) **protein not only lower the surface tension of the egg while some are denatured at the surface**
 * 2) ** coagulation of proteins at the gas-liquid interface form a network that gives some rigidity and stability to the foam.**
 * 3) **Globulins appeared to be a good foamers, producing small bubbles & a large volume.**
 * 4) **Ovomucin ---> good foamer by itself / stabilised the foam( rapidly insolubilised at the surface)**
 * 5) **Fat ( in a small amoun t)---> egg yolk dentrimental effect on foam formation**
 * 6) **Lipids --> retard foaming because the oil molecule migrate to the oil-water interface before the protein molecule inhibiting the unfolding of the protein & thus the formation of the foam.**
 * 7) **Lecithin --> cause the antifoaming behaviour of the egg yolk -> bind to the egg white proteins /preventing them from associating at the air-water interface /competitively displacing them from the interface**


 * Silicon dioxide >>> most common chemical used in defoamers and antifoams ---can destroy and suppress processing foam.**

**__ Problem of foam __**

 * vessels of overflowing processing
 * packaging interference
 * damaging material
 * housekeeping issue

=

= =** Viscoelasticity **=
 * 1) =**Having both viscous and elastic properties**=
 * 2) ==**Consider the time frame required to measurement ( Time-dependent viscoelastic behavior)**==
 * 3) ==**Deborah number : the ratio of a characteristic relaxation time of a material to a characteristic time of the deformation process.**==

Weissenberg effect

 * Rod-climbing phenomenon
 * manifestation of elastic component in the viscoelastic material ( depend on the ratio of elastic components to the viscous component in the viscoelastic material)
 * Some has "snap-back property of some viscoelastic fluid

Spring
 * pure elastic response
 * Hookean elastic solid ( relaxation time is infinite)

Dashpot
 * Purely viscous response
 * newtonian viscous liquid ( relaxatio time is zero)

SOLID LIKE BEHAVIOR (HIGH DE ) > 1 > LIQUID LIKE BEHAVIOR (LOW DE)

APPEAR SOLID LIKE

 * long characteristic relaxation time
 * relevant deformation process very fast

Phase angle closer to 0, more solid like behavior Phase angle closer to 90, more liquid like behavior

Complex modulus

 * measure of material overall resistance to deformation

Elastic (storage) modulus

 * measure the elasticity of the material
 * ability of material to store energy

Viscous (loss)modulus

 * ability of material to dissipate energy
 * energy lost as heat

Tan Delta

 * measure of material damping ( vibration/sound damping)

Weissenberg effect on the fluid depend on the ratio of elastic to viscous components in the viscoelastic material. Storage modulus is the ability of the material to store energy. The higher the storage modulus, the more the energy stored and the fluid will become more elastic and climb higher.

=__ Principle of texture Analysis __=

>
 * 1) Forces applied at a precise rate
 * 2) results are accurate and repeatable
 * 3) Provide an objective measure of customer acceptance
 * 4) cost-effective way to provide repeatable result quickly
 * 5) shorten new product development cycles
 * 6) provide process control feedback

Instrumental Texture Test

 * 1) Fundamental test : measure well define physical properties (Ex Uniaxial compression)
 * 2) Empirical test : developed by observation and experimental
 * 3) Imitative test : test that attempt to imitate the conditions to which the material is subjected in the mouth .( Ex Texture Profile Analysis (TPA) = Provide textural parameter which correlate well with sensory evaluation parameters)

COMPRESSION

 * : Assumes = the sample being tested has surface area =/< that the diameter of the probe in use **
 * Popular in testing = breakfast cereal / breadcrumbs / freshness of cakes or bread**

**PUNCTURE AND PENETRATION**

 * Assume = the sample being tested is of a larger surface area than the contact area of the probe used**
 * Involves small cylinder probes ( up to 10mm diameter) / needle probes / conical probes ( used for situation where the stress may vary during the use of the product e.g**
 * spreading )**
 * Cause irreversible changes in the sample**
 * involves both compressive and shear forces **
 * Popular for the testing : cheese maturity/hardness, fruit ripeness , biscuit dough consistency , gel rupture strength**
 * Multiple Chig Rig ( average forces of 10 puncture test) ... Multiple pea test Rig ( average force of 18 puncture test), Spreadability Rig ( measuring spreadability of fat**
 * and table spread.eg peanut butter)**

**CUTTING AND SHEARING**

 * forces required to cut /slice through a sample ( meat product / cheese/ vegetable)**
 * uses a notched blade instead of a flat ended blade ( gives more a cutting action during test)**
 * Final forces and force at yield indicate toughness**
 * Constant contact area with sample**
 * done by fine wire**
 * 5 or 10 blade option**
 * shear and compression of mainly multiple products or products with variable texture throughout the sample requiring "average shearing " measurement**
 * **Light knife Blade = an alternative to the knife edge blade for sample requiring limited forces to cut /break**


 * Craft knife : sharp blades allow cutting of very hard items.Blade thickness (0.5mm) allow cutting of very small samples .eg seed/nuts**

FRACTURE AND BENDING ( 3 POINT BEND RIG)

 * 1) **Measure fracture and break strength of ( biscuit / chocolate/bread sticks)**
 * 2) **Measure freshness ( vegetables )**
 * 3) **Measure fracture properties of ( crisps /snack food)**

Extrusion (forward)

 * Sample is forced through orifice in bottom of pot
 * Can stimulate real-life situation
 * Eg viscous liquid /gel/pastes/cream/toothpaste

**Extrusion (backward)**

 * **Sample is placed in a pot**
 * **Piston is forced through the sample**
 * **product extrudes around the disc**
 * **Viscous liquid/gels/pastes**