M. Shafiur Rahman

Food Stability Determination: Challenges beyond Water Activity and Glass Transition Concepts

Mohammad Shafiur RahmanDepartment of Food Science and Nutrition, Sultan Qaboos University, P. O. Box 34, Al Khod 123Sultanate of Oman

6th Conference on Water in Food (Euro Food Water)21-23 March 2010, Reims, France Plenary Lecture

Scientific Knowledge to Make the World a Better Place

Food Preservation is One of the Keys in Food Security

Our Books and Journals

Raw Fish Uncooked Rice Grain

Microbiological Enzymatic

Chemical Physical

Mechanical

Inhibition

Avoid RecontaminationInactivation

Drying of Foods in the Sun Started during The Stone Age (10,000 years ago)

Canning (End of 1700s)14 years of experimentationNicolas Appert won award in 180950 years later Louis Pasteur

Freezing

Nicolas Appert Louis Pasteur

Microbial Growth Factors

• Water Content• Food Composition and Structure• pH• Redox Potential• Preservatives• Environmental Condition• Processing Conditions

Active Water is more Important than TotalWater

Scott (1953)

Generic Rules for Food Stability

Gaseous Phase

Food

Water Vapor

ERHaw == WaterPure of PressureVapor

Foodin Water ofPressureVapor

(At Constant Temperature)

Water is one of the main causes of food Spoilage

Water Activity Range: 0-1.0

Water Activity Limits for Microbial Growth

KarelLabuzaChirifeBuera

•Foods are most stable at itsmonolayer moisture

•No micro-organisms can notgrow below 0.6 water activity

•Critical limits exist for differenttypes of micro-organisms

K+

Turgor Cell

Normal (Turgor Cell)

Low Water Activity

K+

CS: Compatible Solutes

K+

K+

Water (if low aw environment)

Water

CS

K+K+

CSCS

Low Water Activity

Loss of TurgorCell membrane ChangePhospholipidsFatty acids

Osmotic Stress in Microbial Cell

Active Sites

Monolayer

Multilayer

Free water

Water and Deteriorative Chemical Reactions

0 1Water Activity

Rat

e or

Gro

wth

or A

ttrib

ute

Zone I Zone II Zone III

a b

c d

e

f

g

hi

j

kl

m

Mic

robi

al G

row

th

Mechanical

Properties

Chem

ical

Reacti

on

Bel

ow M

onol

ayer

Revised Food Stability Map Based on Water Activity

Solv

ent W

ater

Rahman (2009) International Journal of Food Properties

Limitations of Water Activity Concept

•Defined at Equilibrium•Shift of Critical Limits by Other Factors•Solute Selectivity or solute effect•No Indication of Mobility or Reactivity of Water•Shift of water activity with temperature is relatively very low[reaction rates are significantly affected by temperature]•Many Physical Changes Could not be Explained

Multi-phases or states (crystalline, amorphous, glass)

State of Equilibrium ????

BET-Monolayer

Solvent Water

Non-Enzymatic Browning Rate at 70oC in Gelatinized Starch MediumSource: Acevedo et al. (2008)

Nature of solute used to reduce wateractivity plays a role

aw 0.90 with sodium chloride: low growthaw 0.90 with sucrose: high growth

All these limitations could not make completely invalid the concept rather difficult to apply universally

What is Glass? It is a supercooled liquid

BrittleTransparentInertCan not handle tensionCan not flow with gravity

Glassy State Melting State

Heat

Moisture

Glassy State Melting State Flow with Gravity

α

α

α

α

β

β

β

β

β

Amorphous Solid

β

Sodium Chloride Crystal

At Glassy State•Not a regular structure

•Super-cooled liquid with viscosity 1012 and 1013 Pa s

•Flow of super-cooled liquid: 10-14 m/s (viscosity; 1014 Pa s)Typical liquid: 10 m/s

•Molecular diffusion in the order of months or years

•Molecules are kinetically immobilized

•Arrest of translational molecular motion

•Rigid and brittle

VibrateTranslate

Rotate

Glass Transition

A nature of second-order time-temperature dependenttransition, which is generally characterized by a discontinuity in physical, mechanical, electrical, thermal, and other properties of a material

Glass Transition Concept

•Foods are most stable at and below its glass transition•Higher the T-Tg, higher the deterioration or reaction rates

Fresh Dried

Freeze-Drying

•Less collapse•More Pores•Less loss of volatiles•Less biochemical deterioration

Air-Drying

•More collapse•Less pores•More loss of volatiles•More biochemical deterioration

Temperature

Temperature

Temperature

Temperature

Hea

t Flo

w (W

/g)

Tgi

Tge

Vol

ume

(m3 )

Diff

usiv

ity (m

2 /s)

Rat

e C

onst

ant (

s-1 )

Tgi

Tgi

Tgi

0 1Solute Mass Fraction

Tem

pera

ture

F

E

G

A

c

Xs′

Tgs

Tgw

Tg′

Rubber

Glass

Ice

Levine and Slade (1986) and Slade and Levine (1988)

12

3

4

Glass Line

Freezing Curve

Stable

Unstable

Higher the numberhigher the unstable

In 1960s

White and Cakebread (1966)Luyet and Rasmussen (1968)

In 1980s

Levine and Slade (1986)

Tem

pera

ture

(oC

)

Glass Transition Line

Water Activity

a

Source: Karel et al. 2004 and Sablani et al. 2007

Moi

stur

e C

onte

nt

IsothermBET-Monolayer

0 1Solute Mass Fraction

Tem

pera

ture

F

J

B

A

Glass

ice+glass at Tg″′

CrystalCollapseStickinessRubberSoftening zoneReaction zoneEntangle flow

Solution

C

E (Tgs)

Xs ′

Tgw

Tg′

Tm′ice+rubber (solid matrix+un-freezable water)

GTu

H

I

ice+solution (solute+free water)

LiquidL (Tms)

Solid

DM

Water vapor

Tbw

N

O

BET

-Mon

olay

erQ

ice+glass Tg″

R

K

1

24

5

6

7

8

1110

9

12

13

Tg″′

Tg″

glass

Xs ″

ice+solution (solute crystal+free water)

ice+glass Tg′

3

S

Highest Molecular Mobility

PTg

iv

U (Tds)S

tate

Dia

gram

Mac

ro-M

icro

Reg

ion

Con

cept

(Rah

man

, 200

6, T

rend

s in

Foo

d S

cien

ce a

nd T

echn

olog

y)

0 1Solute Mass Fraction

Tem

pera

ture

F

E (Tgs)

J

B

A

Glass

ice+glass at Tg″′

CrystalCollapseStickinessRubberSoftening zoneReaction zoneEntangle flow

Solution

C

Xs ′

Tgw

Tg′

Tm′ice+rubber (solid matrix+un-freezable water)

GTu

H

I

ice+solution (solute+free water)

LiquidL (Tms)

Solid

DM

Water vapor

Tbw

N

O

BET

-Mon

olay

erQ

ice+glass Tg″

R

K

1

24

5

6

7

8

1110

9

12

13

Tg″′

Tg″

glass

Xs ″

ice+solution (solute crystal+free water)

ice+glass Tg′

3

S

Highest Molecular Mobility

PTg

iv

U (Tds)

Advantages of Proposed Macro-Micro Regions

•More organized approach could be applied

•Stability Criteria could be determined in different micro-regions

•In each micro-region, it would be easy to determine how multi-factors are linked

•Knowledge base could be developed in each micro-region

Sources: Rahman 2010. Journal of Food EngineeringRahman 2009. International Journal of Food Properties

Tem

pera

ture

(oC

)

Glass

Rubber

Mass Fraction of Solids

a Room Temperature

Dry Bulb Temperature

b

c

Rahman 2009. Journal of Food Engineering (submitted)

Baking Process in the State Diagram (Source: Cuq et al. 2003)

B

Glass

Rubbe

r

Tem

pera

ture

(oC

)

Mass Fraction of Solids

A

Stability of Enzymes in the State Diagram (Source: Mazzobre et al. 2008)

Tem

pera

ture

(oC

)

Glass

Rubber

Mass Fraction of Solids

1

1

2

3

Drying Process for Rice Kernel (Perdon et al. 2000)

RR GG

Tem

pera

ture

(oC

)

Glass

Rubber

Mass Fraction of Solids

1

2

Alternative Approach for Rice Tempering (Cnossen et al. 2001)

SMCTe

mpe

ratu

re (o

C)

Glass

Rubber

Mass Fraction of Solids

SMC

A

B

A: Improved QualityB: Reduced Quality

C: CenterB: MidS: Surface

A B

C D

Data Source: Chen et al. 1999, Kouassi et al. 2001

Rate of Hydrolysis in PVP and carbohydrate

A B

C D

Rate of Hydrolysis in PVP and starch

Data Source: Schebor et al. 1995, Buera et al. 1995

Storage Time (days)

Per

oxid

e V

alue

LT

HT

MT-H

MT-L

HT: High TemperatureMT-H: Medium Temperature (high)MT-L: Medium Temperature (low)LT: Low Temperature [ ] [ ] ( ) [ ] { }{ }

Fat Oxidation Kinetics in Freeze-Dried Fish Muscle

Source: Rahman et al. 2009. Food Chemistry

⎥⎦

⎤⎢⎣

⎡−

−−+−=

⎯→⎯⎯→⎯

12

2112

)exp(exp(exp

21

kktktkAktkBB

CBA

oo

kk

Fat Oxidation in Fish Muscle (Rahman et al. 2009, Food Chemistry)

Fat Oxidation in Fish Muscle (Rahman et al. 2009. Food Chemistry)

FDA Guide Lines

Tc/Tg= Function (Xw/Xb, pH, salts, treatments)

Water activity and glass transitionconcepts could complement eachOther

Water activity and glass transition concepts overshadow the extreme complexity of food stability determination

Universality of stability could not beachieved using only one or two factors

Hur

dle

Tech

nolo

gy

Key is ????

Food Stability

Water Activity

Glass Transition

Sim

plifi

ed F

orm

of t

he R

eal C

ompl

exity

Stat

e D

iagr

am

Molecular Mobility

NMR (Nuclear Magnetic Resonance)ESR (Electron Spin Resonance)Dielectric SpectroscopyLuminescence Spectroscopy

Results indicated water activity and glass transition are the simplified nature of the real complexity

All developments arestill empirical in nature

Theoretical Development

?

Varied Characteristics of Glassy Foods

Conclusion

•State diagram combines water activity and glasstransition concepts

•Multi-factors could be linked in each micro region

•A critical temperature could be defined and then relatingit with water content and other factors

•Knowledge base is required in each macro and micro-region

•Knowledge of critical temperature and its relationshipswith other hurdle is required

Thank You