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Summary food physics FPH20306
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Bulk rheologyFluids
Rheology: science that studies relation between forces applied and rate of deformation
(stirring souce, chewing apple, stretching dough etc…)
Types of deformation:
- Shear
- Extension
Newtonian fluids:
Non-newtonian fluids:
Shear = extension + rotation
Rotation is for complex systems not zero
Types non-newtonian behavior:
1. Shear thinning: as it starts to flow, the shear decreases →
fluid accelerates (drains faster than Newtonian) (milk,
custard, yoghurt)
Viscosity and stress decreases if flow rate increases
Mechanisms of shear thinning in macromolecular solutions:
Resistance flow higher Resistance lower
, 2. Shear thickening: opposite of shear
thinning (peanut butter)
Viscosity and stress increases if flow rate
increases
3. Bingham and plastic flow behavior:
minimal stress needed for flow: below this
stress fluids behave like solids (dough,
margarine, ketchup, whipped cream)
Bingham: newtonian but minimal stress
needed for flow
Plastic flow: shear thinning but minimal
stress needed for flow
4. Thixotropic behavior and hysteresis: time
dependent behavior: breakdown and
recovery of structures take time
Elastic solids
When G=constant → solid = Hooken elastic solid (gelatin/ globular proteins at small deformations <1%)
, Non-hooken behavior:
1. Strain softening (almost all solid foods) 2. Strain hardening (protein/gelatin gels)
Viscoelastic materials
➔
Determine the elastic and viscous part
➔
Linear viscoelastic materials: G’ and G” are constant
But in general:
Or alternatively
Fast deformation: more elastic
Slow deformation: more viscous because the network has more time to allow for viscous relaxation
Instruments for measuring rheological properties
1. Low viscosity newtonian liquids: capillary viscometers (Ubbelhode viscometer: )
2. Non-newtonian fluids: roto-viscometers (steady shear testing)
3. Viscoelastic fluids/gels: stress controlled rheometers (steady shear, stop flow relaxation, oscillatory
shear testing)
4. Viscoelastic solids: compression/extension devices (texture analyzer)
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