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Summary Food Physics (FPH2o306)
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Summary of all the lectures of Food Physics (FPH20306).
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Food physicsChapter 1 Introduction
- What is food physics
o Soft matter physics applied to food products
o Structure-function relations: ingredients (0.1-10 nm) → macroscopic properties (0.01-100 mm)
- Multiscale approach
o Study how ingredients form colloidal structures (solutions, dispersions, emulsions, gels, foam) because
of interactions
o Study how colloidal structures (0.01-10 um) affect macroscopic properties of products
- Examples
o Milk (viscous fluid) has casein micelles, milk fat globules and whey proteins.
o Gelatin desserts (elastic solid) has triple helix structures formed by gelatin molecules.
o Beer foam has protein network
Chapter 2 Bulk Rheology
- Rheology = Science that studies the relation between forces applied on a material, and its (rate of) deformation.
o Applied force F or stress σ (F/area) → Deformation γ (or rate) dγ/dt
→ e.g. stirring sauce, chewing food, stretching dough
- Can be complex because of complex structures.
Viscous liquids: Newtonian fluids
- 2 types of deformations
o Shear
o Extension
- Simple shear flow
o t = 0 : vx (L) = u → vx is velocity of flow in x-direction (independent of x-coordinate)
o Encountered in flow of liquids between parallel plates of heat exchanger, or in pipes or channels.
o Layer attached to upper plate moves with velocity of plate. Bottom later has velocity of zero. There are
friction forces between the different moving elements.
o There is a linear velocity profile vx(y)=(u/L)y. Measure for how much deformation.
Shear rate:
▪ When γxy is the only nonzero shear rate, we speak of simple shear flow.
▪ Vxy when vx changes in the y-direction (shear), vxx when vx only changes in x-direction
(extensional)
, o You can have parabolic profile in pipe. Slope and shear rate are still defined the same. Shear rate is slope
to the parabolic line.
- Extensional flow
→ e. g. extrusion process in piping bag
o Velocity gets faster along the piping bag or other thing.
o Extensional rate:
▪ It is γxx because it’s changing in the x direction.
o Uni-axial extensional flow
- Shear and extensional flows are related to eachother
o Every shear flow can be decomposed in an extensional and rotational contribution.
Shear = extension + rotation
▪ The reason for the arrows at the bottom directing to the left: The lower part is moving slower, so
it is dragged to the other direction.
o Shear stress:
η = shear viscosity (viscosity: resistance to flow)
x denotes direction of the stress, y the direction of the velocity gradient
In extensional flow:
ηE = extensional viscosity
o When and both are constant and independent of time or deformation rate,
the fluid is Newtonian.
▪ For simple molecular fluids the rotational viscosity is neglible and this is indeed found.
▪ → e.g. water, glycerol, maple syrup
, - So for Newtonian fluids
o Linear relation between stress and deformation rate
o Shear viscosity and extensional viscosities are constant
Viscous liquids: non-Newtonian fluids
- Most liquid foods are non-Newtonian viscous liquids
- Rotational resistance / viscosity not zero, so for most dispersions and macromolecular solutions
Clustered particles and polymers.
- Effect of non-constant viscosity in dispersions and polymer solutions.
Left Newtonian and right polymeric.
o At rest you prepare them that they have identical viscosity. As soon as it starts to flow, there are changes
in the polymeric solution that reduce the viscosity, so it runs out faster.
▪ Shear thinning
- Types of non-Newtonian (nonlinear) behavior
o Shear thinning (most common)
o Shear thickening
o Bingham and plastic flow behavior
o Thixotropic behavior
, - Shear thinning behavior
o In macromolecules, dispersions or emulsions.
Log-log scale
η = σxy / γxy
o Macromolecular solutions: Highly entangled state gives high viscosity at rest. As force is increased it gets
disentangled, stretch in direction of flow, decreases friction.
If Δtγ > Δte entanglements are formed, and if Δtγ < Δte no entanglements are formed.
(Δte is characteristic time to form an entanglement)
Because rate of disentanglement > rate of formation of new entanglements number of entanglements
decreases.
▪ 1 / γxy = Δtγ
o Shear thinning in dispersions and emulsions:
When applying forces on cluster they are being rotated and dragged (increase of shear rate), so they
break up. The individual particles organize themselves in layers.
- Shear thickening behavior
o Often they first have had thinning behavior, so start is layered structure.
Then layered structure becomes unstable, collisions happen and big clusters are formed.
o In monodisperse systems with concentrations close the max. packing density the increase in viscosity can
be as high as a factor 1000.
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