Microstructural Aspects of Glass Formation in Food Freezing and...

Yrjö H. Roos

Microstructural Aspects of Glass Formation in

Food Freezing and Dehydration

Yrjö H. Roos

Food Technology

ESPCA/São Paulo School of Advanced Science

Advances in Molecular Structuring of Food Materials

4 April 2013

Faculty of Animal Science and Food Engineering (FZEA-USP)

April 1st to 5th, 2013

Yrjö H. Roos Food Technology

Contents

• Materials Science Approach – The glass transition

• The Freezing Process – Ice formation and microstructure

– Cryoprotection

• Dehydration and Encapsulation – Structural relaxation times

– Fluidness of food solids

– Particles and microencapsulation

• Melt Extrusion


Materials Science Approach

Process

Product

Performance

Structure Microstructure (nanostructure) - Noncrystalline phases Fluidness - Heterogeneity - Water plasticisation Dynamics and kinetics

Structure formation - Component assembly Solids flow - Viscosity Dynamics and kinetics - Temperature and water

Processability - Stickiness, flow Stabilisation - Bioactives, flavor End use - Hydration, nutrient delivery

Physicochemical properties - Particle size, porosity, density… Organoleptic properties - Appearance, flavour, taste… Stability and shelf life - Component miscibility. - Hygroscopicity and aw.


The Glass Transition

• Differential scanning calorimetry

• Spectroscopic methods

• Dynamic mechanical analysis

• Dielectric analysis

TEMPERATURE

T g

Storage modulus

Loss modulus or

dielectric loss

a b relaxation

relaxation

ME

CH

AN

ICA

L O

R D

IEL

EC

TR

IC P

RO

PE

RT

Y

Increasing frequency

Glass Transition Structural Relaxation Times


Practical Applications

RE

LA

XA

TIO

N T

IME

TEMPERATURE, WATER ACTIVITY OR WATER CONTENT

Glassy State Glass Transition Years

Months

Days

Hours

Minutes

Seconds

Flow

EX

TE

NT

OF

CH

AN

GE

IN P

RO

PE

RT

Y

Hard

enin

g, C

rackin

g

Crispness

Stability Zone Critical Zone Mobility Zone

Str

uctu

ral T

ransfo

rmations

Incre

asin

g D

iffu

sio

n

‘Solid’ ‘Highly time-dependent’ ‘Instant changes’

SOLID

CRITICAL ZONE

VISCOUS FLOW

LIQUID

Food processing Food stabilization and storage


The Freezing Process

Freezing

Melting

Condensation

Evaporation

Triple point (0.0099°C, 6.104 mbar)

Condensation

Sublimation

PR

ES

SU

RE

(m

ba

r)

TEMPERATURE (°C)

SOLID

(ICE)

GASEOUS

(VAPOUR)

LIQUID

AIR AND VACUUM DEHYDRATION

FREEZE-DRYING

GASEOUS

(VAPOUR)

Vapour pressure of

Water in Food


Pressure in Freezing

Nucleation is the key in the control

of microstructure of frozen materials.

Bar 200 mm

Fernández et al Food Hydrocoll 20 510-522 (2006)


Ice Formation

TEMPERATURE

EN

DO

TH

ER

MA

L H

EA

T F

LO

W

Glass transition of unfrozen solute phase

Devitrification (ice formation exotherm)

Ice melting endotherm

(often accompanied by

an endothermal step

change)

Differential Scanning Calorimetry (DSC) in heating of rapidly cooled solutions

Tg

Td

Tm

Mobility for crystallization above the Tg


Time-dependent Freezing

Initial T of a nonannealed solution g

T'

T'

g

m

EN

DO

TH

ER

MA

L H

EA

T F

LO

W

t

t

t

t

0

1

2

3

Annealing temperature

TEMPERATURE

T m

g T after annealing time, t 1

g T after annealing time, t 2


Recrystallization above Tm’

Trehalose solution (20% w/w) freezing at -18.6ºC Trehalose solution at -19.1ºC subsequent to

11 cycles of temperature fluctuations to -5°C


Recrystallization in Frozen Desserts


Freezing of Water with Solutes

Ice Crystal Size

T


Freezing T and Microstructure

0

5

10

15

20

25

30

0

1

2

3

4

5

6

7

8

9

10

0 50 100 150 200 250 300 350 400

Wa

ll T

hic

kne

ss (

mm

)

log

N

Pore Diameter (mm)

M040

M100

M250

M100-Glucose

M100-Fructose

M100-Sucrose

-20°C -80°C

Number of Pores

Wall Thickness Freezing T

-40°C

Low Mw High Mw

Harnkarnsujarit et al, Carbohydr Polym 88 734-742 (2012)

Freeze-concentration

occurs to 80% solids

in unfrozen water

Hig

h F

ree V

olu

me in A

morp

hous W

alls

!

Ice Crystals

Pores

Unfrozen Water

Free Volume


Effects of Solute on Ice Formation

Tn Tg Tg

RA

TE

OF

NU

CLE

AT

ION

SIZ

E

NU

MB

ER

NUCLEATION

B A CRYSTAL SIZE

OR NUMBER

A

B

(B) (A) Tm (A) Tm (B)< Tm’(A)

Tn Tm Tg [A]

RA

TE

OF

NU

CLE

AT

ION

NU

MB

ER

AN

D S

IZE

NUCLEATION

[A’]

[A]

CRYSTAL SIZE OR NUMBER

[A]

[A’]

[A] Tm [A’] Tg [A’]

One solute – different concentrations – A higher concentration [A’] shows higher

initial Tg and a lower Tm.

– Ice crystals formed at the same nucleation

temperature, Tn, are smaller for [A’].

– Rate of nucleation and crystal growth are

reduced by viscosity and diffusion.

Two solutes – different molecular size – A higher Tg of A gives a higher rate of

nucleation.

– Ice crystals formed at the same nucleation

temperature, Tn, are smaller for A.

Roos YH, Materials science of freezing and frozen foods, In Food Materials

Science and Engineering, B Bhandari and Roos YH (eds), Springer (2012).


Cryoprotectant Systems


Stability of LGG in Cryoprotectants

Pehkonen et al, J Appl Microbiol 104, 1732-743 (2008)


Dehydration and Encapsulation

• Flavor

• Colour

• Stability

• Flow

• Rehydration


Relaxation Times in Dehydration

Stickiness in spray drying

Collapse in Freeze-Drying

Slow process

Rapid process

Experimental vs. WLF data.

1 ms

1 µs

τ T - Tg

20°C

43°C

Bellows and King, AIChE Symp. Ser.

69 (132), 33-41 (1973)

Downton et al. Ind. Eng. Chem. Fundam. 21: 447-451 (1982)

Sucrose: Fructose 3:1

25°C 0.2 ms

15°C 0.01 s

T - Tg ! τ !

Contact Time

1 to 10 s

2

4

6

8

10

12

14

16

-8

-6

-4

-2

0

2

4

6

0 10 20 30 40 50 T - Tg (°C)

log V

iscosity (

Pa

s)

log

t (

s) Contact Time

Relaxation Time Viscosity

∼10 s

∼1 s

Viscous Flow and Collapse


Enthalpy Relaxation

Haque et al, Carbohydr Res 341, 1884-1889 (2006)

60 70 80 90 100 110 120 130 140

Temperature (°C)

Heat

Flo

w (

W/g

)

Aging at 90°C

Unaged

Aging at 25°C

Aging at 60°C

Aging at 75°C

En

do

the

rmic

ΔHrelax

Tg (onset)= 102°C

Aging time-24h

Lactose


Modeling Relaxation Times

Angell Chem Rev 102 2627-2650 (2002)


a-relaxation by DEA and DMA

-4

-2

0

2

4

6

8

0.00350 0.00355 0.00360 0.00365 0.00370 0.00375 0.00380

Temperature (1/K)

lnf

DMA

DEA

0.08Hz

T g by DSC

0.05Hz

A

Structural Relaxation Times vs. DSC Tg for Xylitol


The WLF Relationship

0

2

4

6

8

10

12

0 20 40 60 80 100

A

T – Tg (K)

log

Vis

co

sity (

Pa s

)

S ckyPointViscosityCollapseViscosity

loga = logt

t g= log

h

hg=

-C1 T -Tg( )C2 + T -Tg( )

- 10

- 8

- 6

- 4

- 2

0

2

0 20 40 60 80 100

B

T – Tg (K) lo

g R

ela

xa

tio

n T

ime (

s)

S" cky&&Point&τ&

Collapse&τ&

The onset temperature of the calorimetric glass transition taken as the Tg

Significant viscous flow

and microstructural

transformations


“Fragility” by Angell

h = h0exp

DT0

T -T0

æ

èçç

ö

ø÷÷

F½

Trehalose

Glucose

Glycerol

Water

384 K

304 K

191 K

138 K

Tg

0

2

4

6

8

10

12

log

η P

a s

-2

-4

-6 0 0.2 0.4 0.6 0.8 1.0

Tg/T

SiO2 1473 K

(2Tg/T½)-1 A

0 0.2 0.4 0.6 0.8 1

log

τ (

s)

Tg/T

- 12

- 10

- 8

- 6

- 4

- 2

0

2

- 14

- 16

FWLF

Trehalose

Fructose

Glycerol

Propanol

384 K

278 K

191 K

98 K

Tg

SiO2 1473 K

B

100 s

1 ms

WLF model with universal constants: ➩ The same temperature dependence. ➩ Significantly different fragilities.

VTF (Fragility) D (fragility parameter) can be defined only at T = Tg Limited value only when Tg values are the

same (Arrhenius reference is global T = 0 K).


WLF vs. VTF

- 4

- 2

0

2

4

6

8

10

12

0 0.2 0.4 0.6 0.8 1

SiO2

Glycerol

WLF

Glucose

Water

log V

ISC

OS

ITY

(P

a s

)

1/(T – Tg) (1/K)

- 4

- 2

0

2

4

6

8

10

0 0.01 0.02 0.03 0.04 0.05

SiO2

Glycerol

Glucose

Water lo

g V

ISC

OS

ITY

(P

a s

)

1/(T – Tg) (1/K)

Collapse Zone

Stickiness Zone

STRONG

FRAGILE

h =h0 expB

T -T0

æ

èç

ö

ø÷

Glass transition

Viscous flow

Liquid flow

WLF

T – Tg (°C) 2 5 10

T – Tg (°C) 25 50 100 20


The Fluidness Concept

Maltodextrins (DE10 – DE25) with 5% Glucose at aw 0.23 – 0.76

-5

-4

-3

-2

-1

0

1

2

0 20 40 60 80 100 120

log

t

T - Tg

Fructose

Glucose

WLF

“universal”

DMA (dynamic loss modulus)

DEA (isothermal dielectric loss)

C1 = 8; C2 = 25

C1 = 8; C2 = 20 Fluidness - WLF constants

- Solids composition

- Water plasticization

loga = logt

t g= log

h

hg=

-C1 T -Tg( )C2 + T -Tg( )

Note: C1 is number of log

decades for the change in t C2 is measure of steepness


Fluidness in Dehydration

-8

-7

-6

-5

-4

-3

-2

-1

0

1

2

0 10 20 30 40 50

log

t (

s)

T – Tg (K)

WLF equation:

Universal values: C1=17.44

C2=51.6

Lactose

Lactose – MD23 (70:30)

Lactose – MD9 (70:30)

Glucose

(viscosity)

DMA

DEA

Tg Tg+10 K Tg+15 K

logt

t s=

-C1 T - Ts( )C2 + T - Ts( )

Lo

ss M

odu

lus (

x1

08 P

a)

Die

lectr

ic L

oss

DMA

DEA

Potes&et#al.&Carbohydr.&Polym.&89,&105071059&(2012)&&

A metal pocket

Parallel plate capacitors


Microstructural Diversity

CFLM Cryo-SEM

Some parameters:

- Inlet/Outlet air T

- Atomizer - Feed T

- Feed solids

- Glass former

- Particle size

- Composition

- Viscosity


Microstructure by Freezing

Volatiles entrapment and retention in microregions formed during freezing in

freeze-dried materials. Flink JM and Karel M, J Agric Food Chem 18, 295-297 (1970)

• Microstructure formation in freezing.

• Explains volatiles and flavor retention in freeze-drying. – Freeze-dried berries, fruits, etc.

– Freeze-dried coffee.

Pores Microregions of encapsulated volatiles

Sugar glass

Microencapsulation in freeze-drying

Freeze-drying


Microencapsulation

• Volatiles retention in liquid particles by selective diffusion.

– Thijssen HAC, J Appl Chem 21, 372–377 (1971)

• Volatiles microencapsulation in microregions during vitrification of glass forming encapsulant matrix.

– Flink JM and Karel M, J Agric Food Chem 18, 295-297 (1970)

• Encapsulation of dispersed particles in emulsions.

Particle Temperature

Glassy Lactose (Glassy Skim Milk)

Lactose Syrup

Stickiness and Caking Zone

Surface properties of particles are strongly dependent on composition

and state of solids

0 5 10 15 20 25 -50

0

50

100

Gla

ss T

ran

sitio

n (

°C)

Water Content (%, w/w)

Concentrate Volatiles retention (Selective diffusion)

Powder

Volatiles retention (Microencapsulation)

Spray drying

Several mechanisms:


Spray Dried Materials

S-D Lactose S-D Lactose/Na-Caseinate (3-1)


Microencapsulation in Spray Drying

Preferential migration of proteins

to air-liquid interphase Adhikari et al J Food Eng 94 144-153 (2009)

Encapsulation of

dispersed particles

Microencapsulation of

dispersed particles

(S. Drusch 2013) Voids

Protein on particles reduce stickiness and improve powder recovery.

Drusch S (2013)


Emulsion Destabilization

Mun et al, J Food Eng 86, 508–518 (2008)

• Primary and secondary emulsions were unstable in freezing and freeze-drying.

• Maltodextrin (glass former) was required to stabilize emulsions.


Surface Characteristics and Stability

Zinoviadou et al, Food Funct 3, 312-319(2012)

Sodium stearoyl lactylate (SSL)

Chitosan (CH) Surface tension

Particle shape Serfert et al, J Microencapsul, Early Online (2012)

Drusch and Berg, Food Chem 109, 17–24 (2008)


Colloids and Dispersions

<1nm <1μm >1μm

Gasmix(air)

Aerosol(fog)

Aerosol(rain)

Solidaerosol(smoke)

Par culate(dust)

Solu on(CO2so drinks)

Foam(cappucinofoam)

Foam(breadcrumb)

Solu on(ethanolinwater)

Emulsion(homogenizedmilk)

Emulsion(cream)

Solu on(polymerinliquid)

Sol(caseinmicellesinliquid)

Suspension(fruitpulp)

Solu on(dissolvedgasincandy)

SolidFoam(airinmilkpowderpar cles)

SolidFoam(meringue)

Solu on(dissolvedflavorincandy)

Gel(fruitjelly)

Solidemulsion(bu er)

Solu on(alloys)

Solidsol(milkfatinicecream)

Solidsuspension(icecrystalsinicecream)

Gas

G

L

S

Liquid

Solid

G

L

S

G

L

S

Bubblesingas

DISPERSIONS COLLOIDSGas

Liquid

Solid

COARSEDISPERSIONS

Freeze-dried and spray

dried structures with

microencapsulated and

encapsulated

components. – Gas (air) in voids.

– Volatiles in microregions.

– Encapsulated dispersed

particles (oil droplets).

– Dissolved, miscible

components in glass formers.

– Phase separated

components (proteins and

sugars)


Oxidation and Microstructure

Shchukina and Shchukin, Curr Opin Coll Interface Sci 17, 281–289 (2012)

Serfert et al, Food Hydrocoll 31, 438-445 (2013)


Melt Extrusion

Sucrose-gum arabic-gelatin (1:1:1) film

Trehalose-Na caseinate (1:1) film


Summary

• Materials science has advanced understanding of food systems in freezing and dehydration.

• Knowledge of freezing and frozen state microstructure is not well developed.

• Glass formers are of crucial importance to microstructure formation in freezing, extrusion and dehydration.

• Interface engineering coupled with fluidness characterization can be used to improve processability, storage stability and nutrient delivery.

Microstructural Aspects of Glass Formation in Food Freezing and...

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