Rheology Flashcards
define texture
response of tactile senses to physical stimuli that result from contact between some part of the body and the food
what is the tactile sense?
touch
what is kinesthesics
sense of movement and position
senses used to evaluate the texture
touch
kinethesis
sight
sound
define rheology
science of dealing with deformation and flow of materials
-properties that govern a material stress/strain relationship
difference between rheology and texture
Texture is subjective
rheo is measurable, well-defined geometry and sample shape, instrument independent (mechanical properties)
why is rheo important
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)
Rheology in food context
designing machines
determining ingredient functionality
quality control
shelf life testing
investigation of sensory attributes
what is a food rheological affected by
temperature
material history
composition
microstructure
sources of force
processing plant: pump, agitator, milling
storage: weight of product stacked above gravity
applied by the consumer: breaking, chewing
lab instruments: viscometers, TPA
types of rheological tests
compression
tension
shearing
what is compression
applied stress is directed towards the material
describe tension
applied stress is directed away from the material
describe shear
applied stress is tangential
-cutting/sliding
stress equation
force/area
strain equation
change in dimension/ original dimension
when will deformation occur
when the stress applied is higehr than the force keeping it intact
what happens to the volume during compression
nothing, it stays the same for solid material
why doesn’t the volume change
the height decreases but diameter increases
what is barrelling
when a sample being compressed begins to bulge into a barrel-shape
how can barreling be avoided?
applying oil
Why is barreling unwanted
the area is used to calculate the force, barreling changes the area
3 sections of a deformation curve
linear elastic
elastic buckling
densification
what is Ff
force at fracture
what is Ff important for
understanding how a material behaves
-important to consider when stacking/transporting
describe the shape of a deformation curve
lightning bolt
Another name for the slope of the line on the deformation curve
modulus of elasticity
describe the shape of the first linear elastic part of the curve
linear
where is the limit of elasticity
the end of the linear elastic section
what is the limit of elasticity?
the point at which if stress is removed then no permanent deformation will occur
what is the fracture point of a material dependent on
composition
physical and chemical interactions taking place within the material
what happens in the elastic buckling phase of the curve
deformation is occuring
what happens during densification
the material becomes condensed
greek symbol for stress
tau
greek symbol for strain
gamma
equation to work out stress from strain
tau = E(gamma) * gamma
what is the effect of force on a solid
deform elastically unless too high in which case it breaks
key parameters for solid deformation
amount of deformation for given applied force
maximum force while still being elastic
maximum force that will cause breakage
effect force has on liquid
deform irreversibly
key parameter for force on a liquid
relationship between the rate of deformation and applied force
example of semi-solid material
cheese
a semi-solid material will have both liquid and solid properties depending on…
the magnitude of force applied
time-frame observation
what is time-frame observation
observing what is happenign at single time points (snapshots)
Type of stress used to test in material science
tension
type of stress used to test in food science
compression
what two things can happen after yeild point is reached
brittle materials fracture
plastic materials begin to flow
what kind of stress yeild do strong materials have
high
what is the modulus of elasticity a measure of
stiffness
define hardness
resistance to indentation
how is hardness measured
size of indentation left by a fixed load
force required for a fixed depth of indentation
what levels are hardness measurements performed at
well above stress of a material
define brittleness/toughness
measure of the fracture behaviour
how can toughness be estimated
integrating the area under the stress/strain curve
what happens when force is applied to brittle material
a crack propagates easily once formed and little energy input to generate a fracture
relationship increased temp has with
hardness
stiffness
brittleness
they all decrease
geometry of emprical tests
flat plates
probes
cutting wire or blades
3 point bending jig
extrusion jig
terms used to define texture
adhesiveness
bounce
chewiness
cohesiveness
gumminess
roughness
viscosity
define adhesiveness
force required to remove a material that adheres to a specific surface
define bounce
resilience rate at which a sample returns to the original shape after partial compression
define chewinesss
number of chews needed to masticate the sample to consistency suitable for swallowing
define cohesiveness
degree to which the sample deforms before rupturing when biting with molars
define gumminess
energy required to disintegrate a semi-solid food to a state ready for swallowing
define roughness
degree of abrasiveness of a product surface perceived by the tongue
define viscosity
force required to draw a liquid from a spoon over the tongue
symbol used to describe a viscous body
dashpot
symbol used to describe a elastic body
spring
three ways to characterise the viscoelasticity of a food material
dashpot, spring, sliding frictional
what is the deborah number
dimensionless number used to charaterise the fluidity of material under specific flow conditions
De value for fluids
«1
De values for solids
> > 1
what is the viscosity of newtonian fluids dependant on
temperature
what makes a fluid non-newtonian
doesnt have a linear stress/strain relationship
how are shear stress and shear rate linked
by viscosity
what is viscosity
a materials resistance to flow
shape of relationship newt fluids have with shear rate and shear stress
linear
types of fluids
bingham plastic
herschal-buckley
psuedoplastic
newtonian
dilatant
what is yield stress
a material required stress to generate a response
examples of fluids that require yeild stress
bingham plastic
herschel buckley
example of shear thinning fluids
hershel buckley
pseudoplastic
example of shear thickening fluids
dilatant
what is shear thickening
fluid thickens upon shearing
what is shear thinning
fluid thins upon shearing
examples of bingham plastic fluids
mayo
playdough
dessert sauces
example of dilatant fluids
paint, corn starch dispersion
example of pseudo plastic
most hydrocolloids
what is important when enrobbing chocolate
yield stress
What component of chocolate decreases yield stress
increasing fat
what can low yeild stress lead to in chocolate
thin layer, mishapen product, tails, poor decoration
what can high yeild stress lead to in chocolate
thick layer, incomplete coverage, prominent decoration, poor handling
other food example of plastic like materials
butter and marg, mayo, tomato paste
what is responsible for yeild stress in
-butter
-mayo
-tomaot paste
fat
oil phase emulsion (oil in gel structure)
fibre/pulp
describe the flow profile of a dilatant fluid
viscosity increases with yeild stress
describe the flow profile of a Bingham plastic and pseudoplastic with yield stress
viscosity decreases
what is thixotropy
decrease in viscosity with shearing
what is rheopexy
increase in viscosity with shearing
example of rheopexy
whipping cream
what are the simplest viscometer based on
capillary or falling ball
-time for the liquid level or ball to fall
What do the rotational viscometer measure
torque required to turn a rotor iin submerged liquid
what viscometers are favoured in theoretical studies and why
cylinder, cone and plate geometries
-they have a well-defined shear rate
how is viscoelastic behaviour normally evaluated
oscillatory rheometer
what does oscillatory rheometry provide
information about microstructure
small scale deformation
measure strain from specific stress
separate elastic and viscous components of moduli
What does oscillatory rheometry involve
small amplitudes of oscillation are applied while measuring the stress and strain waves + the phase angle (delta)
what is G’
storage modulus
-measure of the solid behaviour of the material
what is G’’
loss modulus
-measure of the liquid behaviour
the ratio between these is often _____
reported/quoted
what is the weisenburg effect
creation of a different shape around a tube when placed in a viscoelastic fluid
what is self-siphoning
the ability of a viscoelastic fluid to siphon itself out of a tube
what is jet swell
The ability of a viscoelastic fluid to remember the shape of a tube after exiting
3 levels of structure that affect rheology
molecular
microscopic
macroscopic
what is meant by molecular structure
denaturation of proteins
cross-linking between polymers
How does microscopic structure affect rheology
-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
how does the macroscopic structure affect rheology
air cells in bread
cracks and slits
