Effect of Glass Transition on Drying of Foods

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