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Effect of Glass Transition on Drying of Foods
• During drying, heat is applied to the solid-fluid interphase to dissociate the water layers from the surface of pores
Solid
HeatWater
Drying
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
Effect of Heated Air• Air helps to carry away the moisture
• Temperature, RH, Flow Rate (Psychrometry)
Air
Porous food
Thin air film
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
Porous Media Approach
Cell Cytoplasm forms Macropore
Micropores are present in cell walls, proteins and starch bodies
Fluid Flow Characteristics
1. Flow through complex channels and pores
2. Complex Solid-Fluid Interaction at Different Scales
Mass Exchange Momentum, Energy and Entropy
Exchange Swelling/Shrinkage
Bulk Water Micropores
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
4. Polymers May Change State
Glassy
GlassRubbery
Transition
Stor
age
Mod
ulus
(G’)
Temperature
Approaches Used to Include Effect of Glass Transition on
Fluid Flow
1.Polymer Science: Semi-Empirical
2. Hybrid Mixture Theory of Porous Media
Fick’s Law
2
2
dM d MDdt d x
Coefficient of Diffusivity (m2/s)
Analogous to thermal conductivity (K)
M: Moisture Content
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
Fickian Versus non-Fickian DryingRubbery State:Fickian
Glassy State:Fickian
Glass-Transition:Non-Fickian
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
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)
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)
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)
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)
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)
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
