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FOSC301 > Rheology > Flashcards

Rheology Flashcards

1
Q

define texture

A

response of tactile senses to physical stimuli that result from contact between some part of the body and the food

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2
Q

what is the tactile sense?

A

touch

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3
Q

what is kinesthesics

A

sense of movement and position

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4
Q

senses used to evaluate the texture

A

touch
kinethesis
sight
sound

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5
Q

define rheology

A

science of dealing with deformation and flow of materials
-properties that govern a material stress/strain relationship

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6
Q

difference between rheology and texture

A

Texture is subjective
rheo is measurable, well-defined geometry and sample shape, instrument independent (mechanical properties)

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7
Q

why is rheo important

A

Design, operation, and optimization of processes (pumping, mixing, spraying, emulsifying, filling)
behaviour of packaging material under load
consumer perception of consistency or texture of foods (flow, gel strength, crunchiness, tenderness)

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8
Q

Rheology in food context

A

designing machines
determining ingredient functionality
quality control
shelf life testing
investigation of sensory attributes

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9
Q

what is a food rheological affected by

A

temperature
material history
composition
microstructure

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10
Q

sources of force

A

processing plant: pump, agitator, milling
storage: weight of product stacked above gravity
applied by the consumer: breaking, chewing
lab instruments: viscometers, TPA

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11
Q

types of rheological tests

A

compression
tension
shearing

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12
Q

what is compression

A

applied stress is directed towards the material

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13
Q

describe tension

A

applied stress is directed away from the material

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14
Q

describe shear

A

applied stress is tangential
-cutting/sliding

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15
Q

stress equation

A

force/area

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16
Q

strain equation

A

change in dimension/ original dimension

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17
Q

when will deformation occur

A

when the stress applied is higehr than the force keeping it intact

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18
Q

what happens to the volume during compression

A

nothing, it stays the same for solid material

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19
Q

why doesn’t the volume change

A

the height decreases but diameter increases

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20
Q

what is barrelling

A

when a sample being compressed begins to bulge into a barrel-shape

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21
Q

how can barreling be avoided?

A

applying oil

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22
Q

Why is barreling unwanted

A

the area is used to calculate the force, barreling changes the area

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23
Q

3 sections of a deformation curve

A

linear elastic
elastic buckling
densification

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24
Q

what is Ff

A

force at fracture

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25
Q

what is Ff important for

A

understanding how a material behaves
-important to consider when stacking/transporting

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26
Q

describe the shape of a deformation curve

A

lightning bolt

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27
Q

Another name for the slope of the line on the deformation curve

A

modulus of elasticity

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28
Q

describe the shape of the first linear elastic part of the curve

A

linear

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29
Q

where is the limit of elasticity

A

the end of the linear elastic section

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30
Q

what is the limit of elasticity?

A

the point at which if stress is removed then no permanent deformation will occur

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31
Q

what is the fracture point of a material dependent on

A

composition
physical and chemical interactions taking place within the material

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32
Q

what happens in the elastic buckling phase of the curve

A

deformation is occuring

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33
Q

what happens during densification

A

the material becomes condensed

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34
Q

greek symbol for stress

A

tau

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35
Q

greek symbol for strain

A

gamma

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36
Q

equation to work out stress from strain

A

tau = E(gamma) * gamma

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37
Q

what is the effect of force on a solid

A

deform elastically unless too high in which case it breaks

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38
Q

key parameters for solid deformation

A

amount of deformation for given applied force
maximum force while still being elastic
maximum force that will cause breakage

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39
Q

effect force has on liquid

A

deform irreversibly

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40
Q

key parameter for force on a liquid

A

relationship between the rate of deformation and applied force

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41
Q

example of semi-solid material

A

cheese

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42
Q

a semi-solid material will have both liquid and solid properties depending on…

A

the magnitude of force applied
time-frame observation

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43
Q

what is time-frame observation

A

observing what is happenign at single time points (snapshots)

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44
Q

Type of stress used to test in material science

A

tension

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45
Q

type of stress used to test in food science

A

compression

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46
Q

what two things can happen after yeild point is reached

A

brittle materials fracture
plastic materials begin to flow

47
Q

what kind of stress yeild do strong materials have

A

high

48
Q

what is the modulus of elasticity a measure of

A

stiffness

49
Q

define hardness

A

resistance to indentation

50
Q

how is hardness measured

A

size of indentation left by a fixed load
force required for a fixed depth of indentation

51
Q

what levels are hardness measurements performed at

A

well above stress of a material

52
Q

define brittleness/toughness

A

measure of the fracture behaviour

53
Q

how can toughness be estimated

A

integrating the area under the stress/strain curve

54
Q

what happens when force is applied to brittle material

A

a crack propagates easily once formed and little energy input to generate a fracture

55
Q

relationship increased temp has with
hardness
stiffness
brittleness

A

they all decrease

56
Q

geometry of emprical tests

A

flat plates
probes
cutting wire or blades
3 point bending jig
extrusion jig

57
Q

terms used to define texture

A

adhesiveness
bounce
chewiness
cohesiveness
gumminess
roughness
viscosity

58
Q

define adhesiveness

A

force required to remove a material that adheres to a specific surface

59
Q

define bounce

A

resilience rate at which a sample returns to the original shape after partial compression

60
Q

define chewinesss

A

number of chews needed to masticate the sample to consistency suitable for swallowing

61
Q

define cohesiveness

A

degree to which the sample deforms before rupturing when biting with molars

62
Q

define gumminess

A

energy required to disintegrate a semi-solid food to a state ready for swallowing

63
Q

define roughness

A

degree of abrasiveness of a product surface perceived by the tongue

64
Q

define viscosity

A

force required to draw a liquid from a spoon over the tongue

65
Q

symbol used to describe a viscous body

A

dashpot

66
Q

symbol used to describe a elastic body

A

spring

67
Q

three ways to characterise the viscoelasticity of a food material

A

dashpot, spring, sliding frictional

68
Q

what is the deborah number

A

dimensionless number used to charaterise the fluidity of material under specific flow conditions

69
Q

De value for fluids

A

«1

70
Q

De values for solids

A

> > 1

71
Q

what is the viscosity of newtonian fluids dependant on

A

temperature

72
Q

what makes a fluid non-newtonian

A

doesnt have a linear stress/strain relationship

73
Q

how are shear stress and shear rate linked

A

by viscosity

74
Q

what is viscosity

A

a materials resistance to flow

75
Q

shape of relationship newt fluids have with shear rate and shear stress

A

linear

76
Q

types of fluids

A

bingham plastic
herschal-buckley
psuedoplastic
newtonian
dilatant

77
Q

what is yield stress

A

a material required stress to generate a response

78
Q

examples of fluids that require yeild stress

A

bingham plastic
herschel buckley

79
Q

example of shear thinning fluids

A

hershel buckley
pseudoplastic

80
Q

example of shear thickening fluids

A

dilatant

81
Q

what is shear thickening

A

fluid thickens upon shearing

82
Q

what is shear thinning

A

fluid thins upon shearing

83
Q

examples of bingham plastic fluids

A

mayo
playdough
dessert sauces

84
Q

example of dilatant fluids

A

paint, corn starch dispersion

85
Q

example of pseudo plastic

A

most hydrocolloids

86
Q

what is important when enrobbing chocolate

A

yield stress

87
Q

What component of chocolate decreases yield stress

A

increasing fat

88
Q

what can low yeild stress lead to in chocolate

A

thin layer, mishapen product, tails, poor decoration

89
Q

what can high yeild stress lead to in chocolate

A

thick layer, incomplete coverage, prominent decoration, poor handling

90
Q

other food example of plastic like materials

A

butter and marg, mayo, tomato paste

91
Q

what is responsible for yeild stress in
-butter
-mayo
-tomaot paste

A

fat
oil phase emulsion (oil in gel structure)
fibre/pulp

92
Q

describe the flow profile of a dilatant fluid

A

viscosity increases with yeild stress

93
Q

describe the flow profile of a Bingham plastic and pseudoplastic with yield stress

A

viscosity decreases

94
Q

what is thixotropy

A

decrease in viscosity with shearing

95
Q

what is rheopexy

A

increase in viscosity with shearing

96
Q

example of rheopexy

A

whipping cream

97
Q

what are the simplest viscometer based on

A

capillary or falling ball
-time for the liquid level or ball to fall

98
Q

What do the rotational viscometer measure

A

torque required to turn a rotor iin submerged liquid

99
Q

what viscometers are favoured in theoretical studies and why

A

cylinder, cone and plate geometries
-they have a well-defined shear rate

100
Q

how is viscoelastic behaviour normally evaluated

A

oscillatory rheometer

101
Q

what does oscillatory rheometry provide

A

information about microstructure
small scale deformation
measure strain from specific stress
separate elastic and viscous components of moduli

102
Q

What does oscillatory rheometry involve

A

small amplitudes of oscillation are applied while measuring the stress and strain waves + the phase angle (delta)

103
Q

what is G’

A

storage modulus
-measure of the solid behaviour of the material

104
Q

what is G’’

A

loss modulus
-measure of the liquid behaviour

105
Q

the ratio between these is often _____

A

reported/quoted

106
Q

what is the weisenburg effect

A

creation of a different shape around a tube when placed in a viscoelastic fluid

107
Q

what is self-siphoning

A

the ability of a viscoelastic fluid to siphon itself out of a tube

108
Q

what is jet swell

A

The ability of a viscoelastic fluid to remember the shape of a tube after exiting

109
Q

3 levels of structure that affect rheology

A

molecular
microscopic
macroscopic

110
Q

what is meant by molecular structure

A

denaturation of proteins
cross-linking between polymers

111
Q

How does microscopic structure affect rheology

A

-Fat crystals in networks spread- only part of the structure returns when disrupted, hence work softening behavior
-a network of aggregated particles - time constant associated with particles re-aggregating

112
Q

how does the macroscopic structure affect rheology

A

air cells in bread
cracks and slits

113
Q
A

FOSC301 (17 decks)

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