effect of glass transition on drying of foods
TRANSCRIPT
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Effect of Glass Transition on Drying of Foods
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• During drying, heat is applied to the solid-fluid interphase to dissociate the water layers from the surface of pores
Solid
HeatWater
Drying
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What happens during drying?• Foods are heated
• Moisture is removed
• Chemical properties may change
• Change in Physical properties- Food may change from rubbery to glassy state
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Effect of Heated Air• Air helps to carry away the moisture
• Temperature, RH, Flow Rate (Psychrometry)
Air
Porous food
Thin air film
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Starch
Protein
Lipid
Cell Wall (Micropores)
Porous Media Approach
Cell Membrane (Micropores)
Cell Cytoplasm (Macropore)
Water Cell wall and membrane pervious to water, impervious to lipids
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Porous Media Approach
Cell Cytoplasm forms Macropore
Micropores are present in cell walls, proteins and starch bodies
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Fluid Flow Characteristics
1. Flow through complex channels and pores
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2. Complex Solid-Fluid Interaction at Different Scales
Mass Exchange Momentum, Energy and Entropy
Exchange Swelling/Shrinkage
Bulk Water Micropores
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3. Viscoelastic Nature of Polymers
Long polymer chains at the molecular scale, make polymeric matrix viscoelastic at the microscale
Reference: Dynamics of Polymeric Liquids (1977). Bird, Armstrong and Hassager. John Wiley and Sons. pp: 63.
Energy Storage +Dissipation
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4. Polymers May Change State
Glassy
GlassRubbery
Transition
Stor
age
Mod
ulus
(G’)
Temperature
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Approaches Used to Include Effect of Glass Transition on
Fluid Flow
1.Polymer Science: Semi-Empirical
2. Hybrid Mixture Theory of Porous Media
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Fick’s Law
2
2
dM d MDdt d x
Coefficient of Diffusivity (m2/s)
Analogous to thermal conductivity (K)
M: Moisture Content
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Simplified form of General Fluid Transport Equation
2 2
2 20
( )( )t
cdM d M d dMD B G t ddt d x d x d
Fickian Part Non-Fickian PartHas memory
Stress Relaxation Function (Similar to Coefficient of Elasticity)
Units: Pascals (N/m2)
Coefficient
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Fickian Versus non-Fickian DryingRubbery State:Fickian
Glassy State:Fickian
Glass-Transition:Non-Fickian
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Fluid Transport Equation for Viscoelastic Systems
, ,0 ,
( 1) ( ) ( ) 0t
f f f fk v k
k
D B t d
Fickian Part Non-Fickian Part
Has memory
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Fickian and Non Fickian
00.020.040.060.08
0.10.120.140.160.18
0.2
0 0.5 1 1.5 2 2.5 3
Radial Position (mm)
Moi
stur
e C
onte
nt (d
.b.) 0
0.10.30.612468
25 oC
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
0 0.5 1 1.5 2 2.5 3
Radial Position (mm)
Moi
stur
e C
onte
nt (d
.b.)
0
0.1
0.3
0.6
1
2
4
6
8
50 oC
time (hrs)
00.020.040.060.080.1
0.120.140.160.180.2
0 0.5 1 1.5 2 2.5 3
Radial Position (mm)
Moi
stur
e C
onte
nt (d
.b.) 0
0.1
0.3
0.6
1
2
4
6
8
70 oC
time (hrs)
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
0 0.5 1 1.5 2 2.5 3
Radial Position (mm)
Moi
stur
e C
onte
nt (d
.b.)
00.10.30.61246890 oC
time (hrs)
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Comparison with experimental data of Misra and Young (1980)
Temp 35 oC, RH 30.3%
00.05
0.10.15
0.20.25
0.30.35
0.4
0 5 10 15 20 25 30Time (hrs)
Moi
stur
e C
onte
nt (d
.b.) 14 Nodes
22 Nodes
Experimental
Avg. Abs. Difference 8.4%
(a)
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Temp. 55 oC, RH 14.8%
0
0.050.1
0.15
0.2
0.250.3
0.35
0.4
0 5 10 15 20 25 30Time (hrs)
Moi
stur
e C
onte
nt
(d.b
.)
Predicted
Experimental
Avg. Abs. Difference 13%
(b)
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Temp 75 oC, RH 6%
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 5 10 15 20 25 30
Time (hrs)
Moi
stur
e C
onte
nt (
d.b.
)
Predicted
Experimental
Avg. Abs. Difference 14%
(c)
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Temp 95 oC, RH 1.45%
00.05
0.10.15
0.20.25
0.30.35
0.4
0 5 10 15 20 25 30Time (hrs)
Moi
stur
e co
nten
t (d
.b.)
Predicted
Experimental
Avg. Abs. Difference 6%
(d)
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Summary• Drying is Fickian in rubbery and
glassy state when significantly far from the glass-transition region
• In the vicinity of glass transition, drying is non-Fickian