Module 1 - Viscous behaviour and texture analysis

Content of module 1 - Viscous behaviour and Texture analysis

1. Introduction to rheology

2. Viscous behaviour

3. Texture analysis

1. Introduction to rheology - Definition rheology

study of the relationship between the deformation of a material and the corresponding forces as a function of the time during which the material is subjected to these forces

amount of deformation depends on

• timescale

• intensity of acting force

1. Introduction to rheology - deformation

= change in sample dimensions under an acting force

1. Introduction to rheology - flow

= continuous deformation under the acting force

1. Introduction to rheology - rheological behaviour

Type of rheological behaviour	Type of deformation	interaction forces
ideally viscous	irreversible deformation	hydrogen bonds = weak electrostatic bonds highest moisture content
viscoelastic	partial deformation behaviour	disulfide bonds = strong covalent bonds → network is deformable, but is also elastic
ideally elastic	reversible deformation	amorphous glass lowest moisture content

1. Introduction to rheology - deformation behaviour

1. Compression

2. Extension

3. Shear = force applied parallel of the top plane

1. Introduction to rheology - deformation behaviour: compression (1)

• all side compression → volume of sample will decrease in function of the compression force

1. Introduction to rheology - deformation behaviour: compression (1) of ideal elastic products

$$P=-K\epsilon_{V}$$

• P = compression stress

• K = compression modulus

$$\epsilon=\frac{\Delta V}{V}$$

1. Introduction to rheology - deformation behaviour: extension (2)

• forces clamped on 2 sides —> move in opposite way

$$\sigma_{L}=\frac{F}{A}$$

$$\epsilon_{L}=\frac{\Delta L}{L}$$

• $\epsilon_{L}$ = extension strain

1. Introduction to rheology - deformation behaviour: shear deformation (3)

• A = surface area (m²)

• F = shear force (N)

• shear stress = $\tau=\frac{F}{A}$

• shear deformation = shear strain = $\gamma=\frac{du}{h}=\tan\left(\alpha\right)$

→ instrument = rheometry

1. Introduction to rheology - deformation behaviour: shear deformation (3) of elastic solid

→ deformation is reversible = elastic

1. Introduction to rheology - deformation behaviour: shear deformation (3) of liquid

→ deformation is irreversible

strain rate = shear rate = \gamma^{\cdot}=\frac{d\gamma}{\differentialD t}\left\lbrack\frac{1}{s}\right\rbrack

1. Introduction to rheology - test classified according to deformation

2. Viscous behaviour

• Fluids

• studied under shear in a viscometer or rheometer

• deformation behaviour = flow behaviour

2. Viscous behaviour - General content

1. Terminology: shear stress, shear rate, shear viscosity

2. Newtonian behaviour

3. Rotational tests

   1. flow curve

   2. time-dependent behaviour

2. Viscous behaviour - Terminology (1)

• shear stress = $\tau=\frac{F}{A}\left\lbrack\frac{N}{m^2}\right\rbrack=\left\lbrack Pa\right\rbrack$

• shear strain = shear deformation = $\gamma=\frac{du}{h}\left\lbrack-\right\rbrack$ → !! not constant

• strain rate = shear rate =\frac{d\gamma}{\differentialD t}=\frac{dv}{dh}\left\lbrack\frac{1}{s}\right\rbrack

• shear viscosity = $\eta=\frac{\tau}{\gamma^{\cdot}}\left\lbrack Pa\cdot s\right\rbrack$

  • $\eta$: temperature dependant

2. Viscous behaviour - Newtonian behaviour (2)

• Deformation is irreversible

• ex. water, solvents, oils

General: deformation energy → frictional heating → fluid heating + environment heating → deformation energy is lost → deformation is irreversible

2. Viscous behaviour - Rotational tests (3) - Flow curve: newtonian behaviour

• viscosity fluid 1 < viscosity fluid 2

• tau 1 < tau 2 → more force is needed at fluid 2 to get the fluid flowing at a certain shear rate

2. Viscous behaviour - Rotational tests (3) - Flow curve: non-newtonian behaviour

• T-dependant !!

$\eta$ ^*= apparent shear viscosity

• shear-thinning

  • shear stress ↑ → viscosity ↓

  • reasons: disintegration, orientation, deformation

• shear-thickening

  • shear stress ↑ → viscosity ↑

  • reasons: particles come more and more into contact with one another → increased flow resistance

2. Viscous behaviour - Rotational tests (3) - Flow curve: yield point

$\tau_{y}$ = yield point = minimal shear stress inducing sufficient deformation for the material to flow

• below yield point

  • elastic behaviour

  • acts like solid

• beyond yield point

  • sample flows

  • irreverisble deformation

Methods to find yield stress

• dynamic yield stress = $\tau_{y}$ = intersection at $\gamma^{\cdot}=0$

• static yield stress

  • increasing shear stress from 0 shear till it starts to flow → $\tau_{y}$ = crossover point of low deformation and high deformation region

Dependant on

• time

• sample manipulation

• test conditions

2. Viscous behaviour - Rotational tests (3) - Flow curve: rheological models

2. Viscous behaviour - Rotational tests (3) - Flow curve:time-dependent behaviour

First method

possible responses of a fluid put under constant shear rate

• no change in shear stress and viscosity → time independent (ex. oil)

• decreasing shear stress and viscosity over time → thixothropic (ex. ketchup)

• increasing shear stress and viscosity over time → rheopectic (ex. starch solution)

rheomalaxis

• weak particle gel network → easily irreversibly damaged → loss initial structure

Alternative method

• time-dependent → hysteresis

  • thixothropic → curve will shift to lower shear stress

  • rheopectic → curve will shift to higher shear stress

3. Texture analysis

• Large deformation

• Compression or extension

• Recording force during deformation

• Texture parameters can be derived

• Texture profile analysis

3. Texture analysis - different tests

• puncture test

• compression-extrusion test

• cutting shear test

• compression

• texture profile analysis

• tensile/extension test

3. Texture analysis - different tests: puncture test (1)

• action: push a probe into the sample

• preset: penetration depth and speed are constant

• result: force as a function of time

Measured force depends on

• food texture

• size and shape of the probe

• penetration depth

• penetration speed

Graph

• initial rapid rise = deformation under load

• at sudden change of slope → penetration starts

  • irreversible damage

  • bio-yield point

3. Texture analysis - different tests: compression-extrusion test (2)

Steps

• food is packed closely

• liquid is pressed out

• food ruptures and flows up through annulus

3. Texture analysis - different tests: cutting shear test (3)

cutting action divides the product in 2 pieces

3. Texture analysis - different tests: compression test (4)

sample is compressed between 2 plates

3. Texture analysis - different tests: texture profile analysis (5)

two subsequent compressions imitate two bites (‘of consumer’)

Parameters

• Fracturability

  • load suddenly drops

  • does not occur for all foods

• Hardness

  • maximum force recorded during first and second cycle

• Adhesiveness

  • adhesion of probe to sample when moving upwards

• Cohesiveness

  • = area 2/area 1

  • measures how the sample stays cohesive during compression

• Springiness

  • distance 2/distance 1

  • to determine how much sample recovers in height after the first compression

• Gumminess = hardness*cohesiveness

• Chewiness = gumminess*springiness

3. Texture analysis - different tests: tensile/extension-test (6)

• sample is stretched out by hook