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CHE2820 - Principles of Food Analysis > Rheology > Flashcards

Rheology Flashcards

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1
Q

What is Rheology?

A

Rheology is the study of deformation and flow in materials.

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2
Q

Why is Rheology an important topic in food science?

A

It allows us to analyse physical properties of food to make predictions of quality, stability and customer acceptability.

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3
Q

What do Rheological methods measure?

A

Force, deformation and flow as a function of time.

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4
Q

What type of rheological methods are there?

A

Fundamental, Imitative and Empircal.

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5
Q

What is a Fundamental rheology method? What can it measure?

A

It is a Fundamental (basic) method that tests for properties such as viscosity and elasticity. This method accounts for magnitude, force direction and deformation in samples, allowing to measure material stability, particle shape, particle size and distribution and yield stress.

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6
Q

What is a imitative rheology method?

A

It is a method that attempts to recreate the scenarios that a particular material would typically be exposed to.

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7
Q

What is a Empirical rheology method?

A

It is a limited method that tests for a single point of data, usually dependent on material variations.

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8
Q

What assumptions do we need to make when we are testing rheological properties?

A

1) Homogeneous - we need to ensure our samples are thoroughly mixed and even distributed.
2) Isotropic - That the material will display a consistent response regardless of force direction.

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9
Q

What is stress and strain?

A

Stress (Pa Pascals) is the force applied per area, and strain (no units) is the relative deformation.

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10
Q

Why is stress and strain important?

A

These two measurable qualities are important when considering rheology as we can relate them to the materials elasticity.

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11
Q

What is elasticity?

A

Elasticity is the relationship between stress and strain on a material.
Stress = E (Modulus) x Strain

It is the materials ability to recover original structures after deformation

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12
Q

What is shear flow?

A

Shear flow is the relationship between shear stress and shear strain of a liquid flowing.
t = Shear force / Area

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13
Q

What property do we use instead of shear strain if the material is a liquid?

A

Shear rate.
Shear rate = velocity / height

Shear stress = u (newtonian viscosity) x Shear rate

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14
Q

What is the difference between a newtonian and non-newtonian fluid?

A

A newtonian fluid has a constant viscosity regardless of shear stress, strain or rate. A non-newtonian fluid has an apparent viscosity which changes as those three properties do.

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15
Q

What is viscosity?

A

Viscosity provides an indication of a fluids ability to flow. Higher viscosity relate to higher resistance to flow

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16
Q

How do we distinguish between a newtonian and non-newtonian fluid?

A

We plot a graph of shear relate against shear stress. If the plot is linear then you have a newtonian fluid. If the plot does not follow a linear trend then you have a non-newtonian fluid.

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17
Q

For non-newtonian fluids, shear-rate and time are dependent on what?

A

Shear stress.

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18
Q

If viscosity thins as shear stress increases, what behaviour is present?

A

If the viscosity thins independently to time, then you have a Pseudoplastic material. If the viscosity is dependent to time then it is Thixotropic material.

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19
Q

If viscosity thickens as shear stress increases, what behaviour is present?

A

If viscosity thickens independently to time, then you have a Dilatant material, If the viscosity is dependent to time then you have a Rheopectic material.

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20
Q

If a materials’ viscosity changes with dependency to time, what happens once the stress is removed?

A

The material should return to the original viscosity.

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21
Q

What is a Bingham plastic?

A

Where there is a force required before a fluid can flow.

E.g. Tomato sauce is a gel consistency until force (agitation) is exerted on it, decreasing the viscosity.

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22
Q

What is viscoelasticity?

A

If elasticity and viscosity are two extremes on a scale, then viscoelasticity the property between these two.

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23
Q

What is an ideal elastic food?

A

It completely recovers on stress removal, recovering any energy expended during deformation.

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24
Q

What is an ideal solid food?

A

Will not recover on stress removal, complete loss of energy.

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25
Q

What is a viscoelastic food?

A

It is the partial recovery on stress removal and partial loss of energy

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26
Q

How do you measure viscoelasticity?

A

By using Oscillatory rheometry

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27
Q

What is Oscillatory rheometry?

A

It is the analysis of stress and strain as a function of time separately, and comparing the amplitudes.

28
Q

What is Creep in Oscillatory rheometry?

A

It is the rate of deformation when a constant stress is applied

29
Q

What is relaxation in Oscillatory rheometry?

A

It is the decrease of stress when a constant strain is applied.

30
Q

How do you analyse a Small Amplitude Oscillatory rheometry data?

A

By comparing the phases of stresses and strains we can determine how viscoelastic a material is.

31
Q

If a food is an ideal viscous material what would we see in Small Angle Oscillatory?

A

We would see separation of the phase by 90 degrees.

32
Q

If a food is an ideal elastic material what would we see in Small Angle Oscillatory?

A

The strain and stress would be in phase and therefor 0 degree separation.

33
Q

If a food is a viscoelastic material what would we see in Small Angle Oscillatory?

A

We would see the separation of phases 0< and >90 degrees

34
Q

When do we use the Storage and Loss Modulus?

A

The Storage modulus is used to determine energy stored and released per cycle of deformation (Elastic Component).
The Loss modulus id the energy dissipated as heat per cycle of deformation (Viscous component)

35
Q

What is a Rheometer?

A

It is a instrument for measuring viscosity and viscoelasticity.

36
Q

What does a rheometer measure?

A

It uses torque (rotational force) to measure stress and the deflection angle of the meauring geometry.

it can also use rotational speed to measure shear rate

37
Q

How does a Rheometer work?

A

By using pre-set motor current that correlates to a to a defined torque, the resistance put on the set torque, can calculate the shear stress.

The deflection angle can be converted in to deformation (strain)

the speed can be converted into shear rate.

38
Q

What is Food texture?

A

Food texture is the rheological and structural attributes of a food product, which are perceptible by mechanical, tactile, visual and auditory receptors.

39
Q

Why is it important to study the texture of foods?

A

To help preserve and improve customer acceptability.

40
Q

What are the three common texture parameters?

A

Mechanical, Geometry and Other

41
Q

What parameters does mechanical texture involve?

A

Hardness, Cohesiveness, Viscosity, Elasticity and Adhesiveness.

42
Q

What parameters does Geometry texture involve?

A

‘Particle size and shape’ (Gritty, Grainy and Coarse) and ‘Particle shape and orientation -such as crystallisation or fibrous

43
Q

What parameters does ‘Other’ texture involve?

A

Moisture content and Fat content.

44
Q

What method of analysis do we use for texture analysis and why?

A

We use Empirical methods typically, we do this because texture is widely varied and so we have to test for those.

45
Q

What do we use to analyse texture?

A

Either instrumentally (Objective) or through People (Subjective)

46
Q

What are the Pros and Cons of using Fundamental methods for analysis?

A

Fundamental methods allow you to know exactly what is being measured, however they have a poor correlation with sensory methods.

47
Q

What are the Pros and Cons of using Imitative methods for analysis?

A

The Pros are that you get good correlation with sensory methods and you can closely duplicate mastication. However it is not suitable for routine work and there is no equivalent physical measure to be compared against.

48
Q

What are the Pros and Cons for using Empirical methods for analysis?

A

They are simple and rapid which makes them good for routine quality analysis. They are also good at large scale analysis to produce averages.
The cons however are there is no fundamental understanding.

49
Q

What is the axial compression test?

A

This is the test used to determine viscoelasticity of a food which can be correlated to the hardness of a food.

50
Q

What properties does a firm food exhibit in a axial compression test?

A

If an increase of stress has little effect on the strain, then you have a firm food.

51
Q

What properties does a soft food exhibit in a axial compression test?

A

If an increase in stress has a significant effect on strain, then you have a soft food.

52
Q

What does Poissons ratio account for?

A

That in an ideal material volume and density remain constant when exposed to stress and strain (axial compression test), however in food materials this is rarely the case, so if poissons ratio = 0.5 then there is no volume change.

53
Q

What are some reasons for a change in volume in a axial compression test?

A

There could be bubbles which could get destroyed or removed. there could be moisture that could get exuded when exposed to stress.

54
Q

What is a uniaxial tension test? What does it measure?

A

it is the reverse of a compression test, it is used to test the adhesiveness property of a food such as dough or pudding. It can also be used to determine the strength of mozzarella cheese and noodles.

Two plates pull apart the material in a dumbbell shape with the structure failure to occur in the middle.

55
Q

What is a puncture test? What does it measure?

A

This test measures the force required to puncture or probe into a food. This can be used to determine when fruit is ready to harvest but testing firmness. The material is irreparably damaged after this test. The point at which the material is punctured is called the bio-yield point, for materials that cannot be punctured this test will not work.

56
Q

What is a cone puncture test?

A

This puncture test examines ‘spreadability’ for things like butter by determining the ‘flow’ between the cone and material.

57
Q

What is a torsion test? What does it measure?

A

This test rotates one part of the material on an axis in respect to the other part. (Like twisting a oreo apart). It works like a rheometer, where it looks at the effect the material has on torque.

58
Q

What is the Bending and Snapping test?

A

A bar or sheet is exposed to stresses at certain parts to determine the force required to snap/bend it. There are two main setups, the triple beam which applies the force to the centre. and the cantilever where the force is applied to the end.

59
Q

What is a compression-extrusion test?

A

This tests the force required to disrupt a liquid and extrudes through a annular medium or outlet. this measure viscosity properties

60
Q

What characteristics affect the compression-extrusion test?

A

The annular spaces, the viscoelastic properties of the material, sample size, temperature, cellular formation (plant material), homogeneity

61
Q

What is a Warner-Bratzler Shear test?

A

Testing cutting shear resistance - this measures tenderness

62
Q

What is a ultrasound test in food analysis?

A

It is an indirect non-destructive test that measures changes of ultrasound between a medium.

63
Q

What is a acoustic emission test?

A

Analysing the sounds emitted from a food when it undergoes deformation. It is subject to variance with moisture content.

64
Q

What is TPA?

A

Texture Profile Analysis - It is a double compression test that determines the texture of a food by using a texture analyser.

65
Q

What are some concerns with TPA?

A

TPA provides highly empirical data (tests one thing), which means you need to ensure that the data you are testing for is entirely necessary such as the tenderness of a steak, and not meaningless like a boiled lolly and chewiness. If the operator does not understand what they are testing for then the data is pointless. Standardised methods are key, alterations or modifications can cause issues.
If you are measuring a crunch, then this is pointless as products typically only crunch once.

66
Q

How does TPA measure?

A

it uses a cycle of two compression tests to plot force and distance. This graph can be used to calculate other characteristics provided standardised methods are used.

67
Q

What characteristics are measured in TPA?

A

Fracturability, hardness, chewiness, cohesiveness, gumminess, resilience and springiness.

CHE2820 - Principles of Food Analysis (15 decks)

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