WATER ACTIVITY: REACTION RATES, PHYSICAL PROPERTIES, AND SHELF LIFE

Leonard N. Bell, Ph.D. Nutrition and Food Science Auburn University, AL 36849

1

SHELF LIFE AND STABILITY

• Consumers expect quality, freshness, nutritive value, and accurate label information.

• Shelf life indicates the extent of allowable product change due to

– Time

– Environment

• Understanding stability and factors that affect stability, such as moisture, can lead to improving shelf life and shelf life predictions.

2

TYPES OF STABILITY

• Microbial

• Sensory

• Chemical

• Physical

3

FOOD PRODUCT TYPES

• Dry (powders, cereals)

• Intermediate Moisture (bars)

• Solutions (beverages)

4

THE STABILITY QUESTION

• A food has a sensitive “attribute”

– Loss of sensitive ingredient

– Formation of an undesirable product

– Undesirable change in texture

• How does this “attribute” change due to:

– Processing

– Distribution

– Storage

• Commercial

• Consumer

• Answers come from modeling kinetic data 5

REACTION KINETIC OVERVIEW

Active Compound Degradation Product(s)

6

REACTION TYPES

• Hydrolysis

• Oxidation

• Rearrangement reactions

• Amine-Carbonyl (Maillard reaction)

• Enzymatic

7

KINETIC DATA

• zero order model: [A] = [A]o - kt

• first order model: ln([A]/[A]o ) = -kt

• Terms

– k = rate constant

– t = time

– [A], [A]o = concentration

8

ZERO ORDER PLOT

70

-slope = k = conc/day 60

50

40

30 0 2 4 6 8 10 12 14 16 18 20

Time 9

FIRST ORDER PLOT

100

90

80

70

60

-slope = k = 1/day

50

40

Lee & Labuza. 1975. J. Food Sci. 40: 370.

30 0 10 20 30 40 50 60 70 80 90 100 110

Time (h) 10

FACTORS INFLUENCING STABILITY

• Temperature

• Moisture

• pH

• Ingredients

• Oxygen

11

TEMPERATURE

• Reaction rates increase with increasing temperature

• Models

– Arrhenius Equation

ln(k) = ln(A) - EA /RT

log(k) = log(A) - EA /(2.303RT)

– Q10 Approach

Q10 = (θs,T) /(θs,T+10)

12

ARRHENIUS PLOT FOR VITAMIN C DEGRADATION AT aW 0.32

0.5

0.3

Lee & Labuza. 1975. J. Food Sci. 40:370.

0.1

0.07

0.05

0.03

0.01

-slope = EA /2.303R

3.1 3.2 3.3 3.4

1/T (Kelvin-1) 13

SHELF LIFE PLOT FOR VITAMIN C DEGRADATION AT aW 0.32

70 50

-slope = log (Q10)/10 30

10

7 5

3

Lee & Labuza. 1975. J. Food Sci. 40:370.

1 20 30 40 50

Temperature (°C) 14

SHELF LIFE PLOT FOR THIAMIN DEGRADATION AT aW 0.75

100

-slope = log (Q10)/10

10

1

0.1

Arabshahi & Lund. 1988. J. Food Sci. 53:199.

25 35 45 55

Temperature (°C) 15

SHELF LIFE AND TEMPERATURE

• Define shelf life as the time for 10% thiamin loss.

• Shelf life at 25ºC is 38 days.

• Shelf life increases by 3.3 for each 10ºC decrease in temperature.

• Thus, the predicted shelf life at 20ºC is ≈80 days.

• Small temperature changes can have large effects on product shelf life.

16

TEMPERATURE FLUCTUATIONS

40

30 Tx

20

time interval x

10 0 10 20 30 40 50 60 70 80

Time 17

TEMPERATURE FLUCTUATIONS Fraction of Shelf Life Consumed (fcon)

time at temperature xi

fcon = Σ

total shelf life at temperature xi

°C Shelf Life Time f

20 150 d 33 d 0.22

30 75 d 7 d 0.093

40 38 d 2 d 0.053

fcon = 0.366 18

TIME TEMPERATURE INDICATORS

• Gives a visual indication of net exposure to time and temperature

3M Monitor Mark

INDEX/ INDICE

0 1 2 3 4 5

MonitorMark is a registered trademark of 3M. 19

TIME TEMPERATURE INDICATORS

Quality

Indicator Use or freeze before center is darker than outer ring.

Quality

Indicator Use or freeze before center is darker than outer ring.

20

WATER &

CHEMICAL STABILTIY

21

MOISTURE DIFFUSION

MOISTURE DIFFUSION

CHEMICAL DETERIORATION

PHYSICAL DETERIORATION

22

WATER

OPEN

CHEMICAL DETERIORATION

PHYSICAL DETERIORATION

23

PERSPECTIVES ON WATER

• Moisture Content

• Water Activity (relative vapor pressure)

• Glass Transition (plasticizer effect)

• Water Mobility

24

WATER ACTIVITY

• Determines direction of moisture transfer

• Most reaction rates increase with increasing water activity

• Most rates correlate better with water activity than moisture content

• Moisture sorption isotherms are useful

25

GLASS-RUBBER TRANSITION

Amorphous Amorphous Glass Rubber Crystal

• Transition due to:

– Temperature

– Moisture

• Depicted by state diagram

• Relates to physical properties of system

– Rigid glassy matrix converts into soft rubbery matrix

• Affects reactant mobility

26

QUESTION OF THE 1990’s

Does moisture affect reactions by water activity or by increasing reactant mobility with

its plasticizing ability?

27

GENERAL EFFECT OF WATER ACTIVITY ON REACTION RATES

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0 0.0 0.2 0.4 0.6 0.8 1.0

Water Activity 28

MOISTURE SORPTION ISOTHERM FOR MALTODEXTRIN (DE 25) AT 25°C

30

Roos & Karel. 1991. Biotechnol. Prog. 7: 49.

25

20

15

10

5

0 0.0 0.2 0.4 0.6 0.8

Water Activity 29

MODELING MOISTURE SORPTION ISOTHERM DATA

• GAB equation

m =

• Terms

mokCaW

(1 - kaW)(1 - kaW + kCaW)

– mo = monolayer

– k = multilayer factor

– C = heat constant 30

GENERAL EFFECT OF WATER ACTIVITY ON REACTION RATES

1.4

1.2

1.0

0.8

0.6

0.4 mo

0.2

0.0 0.0 0.2 0.4 0.6 0.8 1.0

Water Activity 31

GLUCOSE MOVEMENT IN DRIED CARROTS MONOLAYER = 7.2%(db)

Moisture Movement % (db) Detected

5.9 No 8.1 Yes

10.2 Yes 13.9 Yes 18.1 Yes 21.7 Yes 26.3 Yes

Duckworth & Smith. 1963. Recent Advances in Food Science-3. London: Butterworths. pp 230-238. 32

GLUCOSE MOVEMENT IN DRIED CARROTS MONOLAYER = 7.2%(db)

Moisture Movement % (db) Detected

mo = 7.2

5.9 No 8.1 Yes

10.2 Yes 13.9 Yes 18.1 Yes 21.7 Yes 26.3 Yes

Duckworth & Smith. 1963. Recent Advances in Food Science-3. London: Butterworths. pp 230-238. 33

ASPARTAME DEGRADATION

• Rearrangement reaction at pH>6

• Hydrolysis reaction at pH<4

• Both reaction types between pH 4 and 6

• Catalyzed by buffer salts

• Follows pseudo-first order kinetics

34

ASPARTAME DEGRADATION AS A FUNCTION OF WATER ACTIVITY AT pH 3 AND 30ºC

8 Bell & Labuza. 1994. J. Food Eng. 22:291.

6

4

2

0

0.2 0.4 0.6 0.8 1.0

Water Activity 35

ASPARTAME DEGRADATION

• Water activity 0.3 to 0.8

– Solutes dissolving

– Mobility increasing

– Reactivity increasing

8

0 0.2 0.4 0.6 0.8 1.0

Water Activity

36

ASPARTAME DEGRADATION

• Water activity 0.8 to 1.0

– Solutes becoming diluted

– Reactivity decreasing

8

0 0.2 0.4 0.6 0.8 1.0

Water Activity

37

EFFECT OF Tg ON ASPARTAME DEGRADATION IN POLYVINYLPYRROLIDONE (PVP) AT 25ºC AND pH 7

0.15

Bell & Hageman. 1994. J. Agric. Food Chem. 42:2398.

Glassy Tg Rubbery

0.1

0.05

0

aw 0.33 aw 0.54 aw 0.76

-60 -40 -20 0 20 40 60

T - Tg 38

QA PLOT FOR ASPARTAME DEGRADATION IN PVP AT 25°C AND pH 7

4

ln(QA) = -slope/0.1 QA = 1.4

3

2

Bell & Hageman. 1994. J. Agric. Food Chem. 42:2398. 1

0.2 0.3 0.4 0.5 0.6 0.7 0.8

Water Activity 39

QA CONCEPT

QA =

half-life at aw

half-life at aw + 0.1

• QA = 1.4 means half-life decreases by 40% for each 0.1 aw increase

• If aw decreases by 0.1, half-life increases by 1.4 – 33 days at aw 0.33 (experimental) – 46 days at aw 0.23 (predicted) – 65 days at aw 0.13 (predicted)

40

RELATION BETWEEN ACTIVATION ENERGY OF ASPARTAME DEGRADATION AND WATER ACTIVITY

30 pH 3 pH 5

25

20

Bell & Labuza. 1991. J. Food Sci. 56:17.

15

0.2 0.4 0.6 0.8 1

Water Activity 41

WATER ACTIVITY EFFECTS ON Q10 VALUES OF ASPARTAME DEGRADATION AT pH 5

aW Q10

0.33 4.3

0.55 4.2

0.65 3.7

0.99 2.6

42

WATER ACTIVITY AND TEMPERATURE

• Activation energies often decrease as water activity increases

• Sensitivity of a reaction to temperature changes as aW changes

• A temperature increase has a larger effect (higher Q10) on a reaction in low moisture food than in a high moisture food

• However, the “dryness” adds stability, compensating for the food being more temperature sensitive

43

MAILLARD REACTION

(NONENZYMATIC BROWNING)

• Bimolecular reaction involving:

– unprotonated amines (amino acid)

– carbonyl compounds (reducing sugars)

• Reactant loss follows second order kinetics

• Brown pigment formation modeled via pseudo-zero order kinetics

44

BROWN PIGMENT FORMATION IN GLUCOSE-GLYCINE MODEL SYSTEM AT 37ºC

0.25

0.2

Eichner & Karel. 1972. J. Agric. Food Chem. 20:218.

0.15

0.1

0.05

0

0.2 0.4 0.6 0.8 1

Water Activity 45

BROWN PIGMENT FORMATION IN GLUCOSE-GLYCINE MODEL SYSTEM AT 37ºC

0.25

0.2

Eichner & Karel. 1972. J. Agric. Food Chem. 20:218.

0.15

0.1

0.05

Dissolution, Higher Mobility

0

0.2 0.4 0.6 0.8 1

Water Activity 46

BROWN PIGMENT FORMATION IN GLUCOSE-GLYCINE MODEL SYSTEM AT 37ºC

0.25

0.2

0.15

Eichner & Karel. 1972. J. Agric. Food Chem. 20:218.

Dilution

0.1

0.05

0

0.2 0.4 0.6 0.8 1

Water Activity 47

EXPERIMENTAL DESIGN TO ELIMINATE CONCENTRATION EFFECT

• Prepare each system with pre-determined reactant concentration so that after moisture sorption, all have the same reactant concentration in the aqueous phase (Bell et al. 1998. J. Food Sci. 63:625).

• For example

– 10% moisture = 10 mg added reactants/g

– 20% moisture = 20 mg added reactants/g

48

GLYCINE LOSS IN PVP-K15 AT 25ºC AS A FUNCTION OF WATER ACTIVITY

1.4

1.2

1

0.8

0.6

0.4

0.2

0

0

Bell et al. 1998. J. Food Sci. 63:625.

0.2 0.4 0.6 0.8

Water Activity 49

GLYCINE LOSS IN PVP-K15 AT 25ºC AS A FUNCTION OF GLASS TRANSITION

1.4

1.2

Bell et al. 1998. J. Food Sci. 63:625.

Glassy Tg Rubbery

1

0.8

0.6

0.4

0.2

0

-60 -40 -20 0 20 40 60

T - Tg 50

GLYCINE LOSS VIA MAILLARD REACTION

• Concentration effects eliminated

• Water activity 0.1 to 0.4

– Mobility increasing

– Reactivity increasing

• Water activity 0.4 to 0.7

– Pass through glass transition

– Structural collapse

– Reactivity decreasing 51

GLUCOSE LOSS IN PVP AT aW 0.33 AND 25ºC AS AFFECTED BY POROSITY AND COLLAPSE

White & Bell. 1999. J. Food Sci. 64:1010. 4

Porous (Φ>0.7) Collapsed (Φ<0.2)

3

2

1

0 10 20 30 40 50 60 70 80 90 100

Time (days) 52

EFFECT OF GLASS TRANSITION ON BROWNING IN PVP AT pH 7, aW 0.54, and 25°C

3.0

rubbery glassy

2.0

1.0

0.0

Bell. 1995. Food Res. Intl. 28:591. 0 20 40 60 80

Time (days) 53

BROWNING RATES IN PVP AT 25ºC AS AFFECTED BY THE GLASS TRANSITION

Bell et al. 1998. J. Food Sci. 63:625. 15

Glassy Tg Rubbery

12

9

6

3

0

-40 -20 0 20 40 60 T - Tg

54

WATER AND THE MAILLARD REACTION

• Moisture directly impacts this reaction primarily by dissolving and/or diluting the reactants

• Moisture indirectly impacts this reaction by its effect on the glass transition.

• Various researchers have shown the glass transition affects the Maillard reaction

– Karmas et al. 1992. J. Agric. Food Chem. 40:873.

– Buera & Karel. 1995. Food Chem. 52:167.

– Lievonen et al. 1998. J. Agric. Food Chem. 46:2778.

55

VITAMIN STABILITY

• Ascorbic acid (vitamin C)

• Riboflavin

• Thiamin

56

VITAMIN C DEGRADATION AT 35ºC AS INFLUENCED BY MOISTURE

100

90

80

70

60

50

40

aW 0.32, 5%M (db)

aW 0.67, 18%M (db)

Lee & Labuza. 1975. J. Food Sci. 40:370.

30 0 10 20 30 40 50 60 70 80 90 100 110

Hours 57

QA PLOT FOR VITAMIN C DEGRADATION AT 23°C

100

QA = 1.7

10

Lee & Labuza. 1975. J. Food Sci. 40: 370.

1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Water Activity 58

VITAMIN C DEGRADATION IN DRIED TOMATO JUICE AT 20ºC

125

Riemer & Karel. 1977. J. Food Proc. Preserv. 1:293.

100

75

50

25

0

0 0.2 0.4 0.6 0.8 Water Activity 59

QA PLOT FOR VITAMIN C DEGRADATION AT 20ºC IN DRIED TOMATO JUICE

8

7 Below mo; not included in QA determination

6

5

4 QA = 1.6

3

2

1 Riemer & Karel. 1977. J. Food Proc. Preserv. 1:293.

0

0 0.2 0.4 0.6 0.8 Water Activity 60

RIBOFLAVIN DEGRADATION AT 37ºC IN MODEL SYSTEMS AS AFFECTED BY WATER ACTIVITY

7 Dennison et al. 1977. J. Food Proc. Preserv. 1:43.

6

5

4

3

2

1

0

0 0.2 0.4 0.6 0.8 Water Activity 61

QA PLOT FOR RIBOFLAVIN DEGRADATION AT 37ºC

8

Below mo; not included in QA determination

7

6 QA = 1.3

5

Dennison et al. 1977. J. Food Proc. Preserv. 1:43. 4

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Water Activity 62

THIAMIN DEGRADATION AT 45ºC IN MODEL SYSTEMS AS AFFECTED BY WATER ACTIVITY

Dennison et al. 1977. J. Food Proc. Preserv. 1:43. 12

8

4

0

0 0.2 0.4 0.6 0.8

Water Activity 63

QA PLOT FOR THIAMIN DEGRADATION IN PASTA AT 35ºC

8 Kamman et al. 1981. J. Food Sci. 46:1457.

7

6

QA = 1.6

5 0.3 0.4 0.5 0.6 0.7

Water Activity 64

EFFECT OF MOISTURE ON THE ACTIVATION ENERGY OF THIAMIN DEGRADATION

aW EA (kcal/mole)

0.44 30.8

0.54 29.8

0.65 26.6

Kamman et al. 1981. J. Food Sci. 46:1457.

65

THIAMIN DEGRADATION IN PVP AT 20ºC AS AFFECTED BY WATER ACTIVITY

20

15

10

5

Bell & White. 2000. J. Food Sci. 65:498

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Water Activity 66

THIAMIN DEGRADATION IN PVP AT 20ºC AS AFFECTED BY Tg

0.03

Bell & White. 2000. J. Food Sci. 65:498

Glassy Tg Rubbery 0.02

PVP-LMW PVP-K30

0.01

0

-100 -75 -50 -25 0 25 50 T - Tg

67

ENZYMES AND WATER

• Enzyme activity

– Water can:

• Dissolve substrate

• Increase substrate mobility

• Be a reactant

• Enzyme stability

– Moisture can influence “denaturation”

• Hydrolysis

• Deamidation

• Oxidation 68

EXTENT OF LECITHIN HYDROLYSIS BY PHOSPHOLIPASE AFTER 12 DAYS

100

80

Acker. 1963. Recent Advances in Food Science-3. London: Butterworths. pp 239-247.

60

40

20

0 0.2 0.4 0.6 0.8 1

Water Activity 69

SUCROSE HYDROLYSIS BY INVERTASE AS A FUNCTION OF WATER ACTIVITY

0.2

0.16

0.12

0.08

Lactose/Sucrose at 24ºC (Kouassi & Roos. 2000. J. Agric. Food Chem. 48:2461.)

PVP-LMW at 30ºC (Chen et al. 1999. J. Agric. Food Chem. 47:504.)

PVP-K30 at 30ºC (Chen et al. 1999. J. Agric. Food Chem. 47:504.)

x T=Tg

0.04

0 x x

0.2 0.3 0.4 0.5 0.6 0.7 0.8

Water Activity 70

QA PLOT FOR INVERTASE STABILITY IN PVP AT 30ºC

5

Chen et al. 1999. J. Agric. Food Chem. 47:504

4

3

QA=1.3

2

0.2 0.3 0.4 0.5 0.6 0.7 0.8

Water Activity 71

INVERTASE STABILITY IN PVP AT 30ºC AS AFFECTED BY THE GLASS TRANSITION

0.07

0.06

Chen et al. 1999. J. Agric. Food Chem. 47:504

Glassy Tg Rubbery

0.05

0.04

0.03

0.02

0.01

-40 -20 0 20 40 60 T - Tg 72

RESIDUAL TYROSINASE ACTIVITY IN PVP AFTER 3 DAYS EQUILIBRATION AT 20ºC

1500

1200

Chen et al. 1999. Food Res. Intl. 32:467.

900

600

300

0

PVP-LMW PVP-K30

0.2 0.4 0.6 0.8

Water Activity 73

RESIDUAL TYROSINASE ACTIVITY IN PVP AFTER 3 DAYS EQUILIBRATION AT 20ºC

Chen et al. 1999. Food Res. Intl. 32:467. 1500

1200

Rubbery Glassy

900

600

300

0

PVP-LMW PVP-K30

-40 -20 0 20 40 60 80

Glass Transition Temperature (ºC) 74

RATE CONSTANTS FOR TYROSINASE ACTIVITY LOSS AT 20ºC AS AFFECTED BY aW

0.14

0.12

PVP-LMW PVP-K30

0.1

0.08

0.06

0.04

0.02

Chen et al. 1999. Food Res. Intl. 32:467. 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Water Activity 75

RATE CONSTANTS FOR TYROSINASE ACTIVITY LOSS AT 20ºC AS AFFECTED BY Tg

Chen et al. 1999. Food Res. Intl. 32:467. 0.14

0.12

Glassy Tg Rubbery

0.1

0.08

0.06

0.04

0.02

PVP-LMW PVP-K30

-50 -40 -30 -20 -10 0 10 20 30 40 50

T - Tg 76

LIPID OXIDATION

• Initiated by metal ions

• Measured by

– Oxygen consumption

– Peroxide values

– Hexanal formation

• Complex kinetic modeling

77

OXYGEN UPTAKE IN CELLULOSE/LINOLEATE MODEL SYSTEMS AT 37ºC

0.4

0.3

aW <0.01 aW 0.52

0.2

0.1

Labuza et al. 1971. JAOCS. 48:86. 0

0 40 80 120 Time (h) 78

RATE CONSTANT FOR OXYGEN UPTAKE BY LINOLEATE IN CELLULOSE MODEL SYSTEMS AT

37ºC AS AFFECTED BY WATER ACTIVITY

1.0 Labuza et al. 1971. JAOCS. 48:86.

0.75

0.5

0.25

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Water Activity 79

LIPID OXIDATION IN POTATO CHIPS AT 37ºC

Quast & Karel. 1972. J. Food Sci. 37:584.

1.6

1.2

0.8

0.4

0 0 0.2 0.4 0.6 0.8

Water Activity 80

WATER AND LIPID OXIDATION

• Increase aW from 0 to 0.3 – Less available metal ions

due to hydration spheres

– Reduced oxygen diffusion

– Free radical quenching

• Rate decreases

0 0.2 0.4 0.6 0.8

Water Activity

81

WATER AND LIPID OXIDATION

• Increase aW from 0.3 to 0.8 – Increased dissolution of

catalysts

– Increased mobility of oxygen and metal ions

• Rate increases

0 0.2 0.4 0.6 0.8

Water Activity

82

LIPID OXIDATION AND GLASS TRANSITION

• Glassy encapsulants decrease oxidation by reducing oxygen diffusion

• Conversion to a rubbery matrix can increase oxygen diffusion and thus oxidation

• Structural collapse can:

– Reduce oxidation of entrapped lipid by eliminating pores

– Enhance oxidation if lipid becomes exuded

• For additional information about water and lipid oxidation:

– Nelson & Labuza. 1992. Lipid Oxidation in Food. Washington, D.C.: ACS, pp. 93-103. 83

pH EFFECTS

• Reactions affected by pH

– Aspartame degradation

– Maillard reaction

• non-enzymatic browning

• amino acid destruction

– Vitamin degradation

• The importance of pH is not limited to solutions

– Reducing water activity/moisture content can result in a pH change

– pH changes can affect food chemical stability 84

pH ESTIMATION OF REDUCED-MOISTURE SOLIDS USING A CHEMICAL MARKER

• Bell & Labuza. 1991. Cryo-Letters. 12:235.

– At aW 0.99, 0.1 M phosphate buffer in agar/MCC has a pH of 6.5

– After lyophilization and equilibration to aW 0.34, system behaves as pH 5

– Generally attributed to increased concentration of buffer salts and their selective precipitation

• Results supported by other methods of estimating reduced-moisture pH

– Bell & Labuza. 1992. J. Food Proc. Preserv. 16:289. 85

ASPARTAME DEGRADATION IN 0.1 m PHOSPHATE BUFFER AT 25°C AS AFFECTED BY pH

Bell & Chuy. 2001. IFT Poster.

0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.01

0

Control Added 2 m sucrose

4.5 4.7 4.9 5.1 5.3 5.5 5.7 5.9

Initial pH 86

ASPARTAME DEGRADATION IN 0.1 m PHOSPHATE BUFFER AT 25°C AS AFFECTED BY pH

Bell & Chuy. 2001. IFT Poster. 0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.01

0

Control Added 2 m sucrose

4.5 4.7 4.9 5.1 5.3 5.5 5.7 5.9

Actual pH 87

STABILITY TESTING

• System as close to commercial product as possible

– formulation

– processing

– packaging

• Method of analysis

• Number of data points/extent of reaction

– 8 points minimum

– 50% change in reactant concentration

• Three temperatures

• Three water activities

• Extreme conditions may be used to accelerate testing 88

SHELF LIFE TESTING EXAMPLE

Given the following aspartame degradation data, estimate the shelf life at 20ºC and aW 0.15

Conditions Rate Constant (d-1) t10% loss (days)

aW 0.34, 30ºC 0.00548 19.2 aW 0.35, 37ºC 0.01538 6.9 aW 0.42, 45ºC 0.04828 2.2

aW 0.34, 30ºC 0.00548 19.2 aW 0.57, 30ºC 0.01574 6.7 aW 0.66, 30ºC 0.02224 4.7

Bell. 1989. M.S. Thesis. University of Minnesota. 89

SHELF LIFE PLOT FOR ASPARTAME DEGRADATION AT aW 0.34

4

3

2

1 Q10 = 4.25

0

20 25 30 35 40 45 50

Temperature (ºC) 90

QA PLOT FOR ASPARTAME DEGRADATION AT 30ºC

3

2

QA = 1.55

1

0.2 0.4 0.6 0.8

Water Activity 91

SHELF LIFE CALCULATION

• From kinetic data:

− At aW 0.34 and 30ºC, t10% = 19.2 days

− Q10 = 4.25

− QA = 1.55

• Want to know shelf life at aW 0.15 and 20ºC T/10 x ( QA )Δa/0.1− (t10%, a w 0.34, 30ºC ) x ( Q10 )Δ

− ΔT = change in temperature from 30ºC

− Δa = change in water activity from 0.34

− (19.2) x (4.25)10/10 x (1.55)0.19/0.1 = 188 days

92

PROBLEMS OF ACCELERATED SHELF LIFE TESTING AT EXTREME TEMPERATURES

• Water activity changes with temperature

• pH changes with temperature

• Solubility of reactants change

• Phase changes

– Glass transition

– Crystallization

• Competing chemical reactions

93

SIMULTANEOUS DEGRADATIVE REACTIONS WITHIN A FOOD

-2

Reaction 1

-3 Reaction 2

Crossover at 25ºC

-4 3.0

3.1

3.2

3.3

3.4

3.5

3.6

3.7

1000/Temperature (K-1) 94

WATER &

PHYSICAL STABILITY

95

PHYSICAL STABILITY

• Solutions

– separation

– crystallization

• Low and intermediate moisture systems

– hardening or softening*

– caking and sticking of powders*

– crystallization*

*Influenced by conversion of a glassy matrix into a rubbery matrix

96

SENSORY CRISPNESS INTENSITY OF SALTINE CRACKERS AS A FUNCTION OF aW

Katz & Labuza. 1981. J. Food Sci. 46:403 1.4

1.2

Very crisp

1

0.8

0.6

0.4

0.2

0

aW,c

Moderately Crisp

Minimum acceptability

Slightly crisp

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Water Activity 97

TEXTURAL ISSUES

• Staling (softening) of cereal products

– Crispness, crunchiness lost if aW > 0.4 – Aim to keep aW < 0.4

• Hardening of fruit pieces

– Softness, chewiness lost if aW < 0.5-0.6 – Aim to keep aW > 0.5-0.6

• Contradictory goals for a fruit-containing cereal product

98

CAKING AND STICKING OF POWDERS

• Powders in the glassy state become rubbery from:

– Moisture sorption

– Temperature abuse

• Rubbery powders flow together under the force of gravity

• Over long periods of time, the net result is formation of a caked mass

• Supporting literature

– Aguilera et al. 1993. Biotech. Prog. 9:651.

– Chuy & Labuza. 1994. J. Food Sci. 59:43.

– Lloyd et al. 1996. J. Food Eng. 31:305.

– Netto et al. 1998. Int. J. Food Prop. 1:141. 99

MOISTURE SORPTION ISOTHERM OF FREEZE-DRIED LACTOSE AT 34ºC

8 Berlin et al. 1970. J. Dairy Sci. 53:146.

7

6

5

4

3

2

1

0 0 0.2 0.4 0.6 0.8 1

Water Activity

100

EFFECT OF EXPOSURE TIME AT aW 0.3 AND 24ºC ON MOISTURE CONTENT AND CRYSTALLINITY OF

AMORPHOUS SUCROSE

8 Palmer et al. 1956. J. Agric. Food Chem. 4: 77.

7

6

5

4

3

2

1

0 0 5 10 15 20 25 30

Time (days)

120

100

80

60

40

20

0

101

EFFECTS OF CRYSTALLIZATION

• Amorphous ingredients may crystallize from:

– Moisture gain

– Temperature abuse

– Storage (time)

• Crystalline materials hold less moisture so upon crystallization water is released.

• This water is redistributed in the food, causing:

– Water activity to increase

– Glass transition temperature of remaining amorphous regions to decrease

– Further chemical and physical deterioration!

• Literature

– Roos & Karel. 1990. Biotech. Prog. 6:159.

102

EFFECTS OF CRYSTALLIZATION

• Amorphous ingredients may crystallize from:

– Moisture gain

– Temperature abuse

– Storage

• Crystalline materials hold less moisture so upon crystallization water is released.

• This water is redistributed in the food, causing:

– Water activity to increase

– Glass transition temperature of remaining amorphous regions to decrease

– Further chemical and physical deterioration!

• Literature

– Roos & Karel. 1990. Biotech. Prog. 6:159.

103

USING MOISTURE SORPTION ISOTHERMS AND PHASE DIAGRAMS TO ESTIMATE CRITICAL WATER ACTIVITY VALUES

104

APPLICATION OF PHASE TRANSITION TO A MALTODEXTRIN POWDER

• Caking of powders occurs due to the conversion of glassy particles into rubbery particles which stick together.

105

APPLICATION OF PHASE TRANSITION TO A MALTODEXTRIN POWDER

• Caking of powders occurs due to the conversion of glassy particles into rubbery particles which stick together.

• From state diagram, moisture content for Tg to be at room temperature (20-25°C) is 8% (wb).

106

GLASS TRANSITION STATE DIAGRAM FOR MALTODEXTRIN (DE 25)

150

100

Roos & Karel, 1991, Biotech. Prog., 7: 49.

50

0

-50

-100

-150

0 10 20 30 40 50 60 70 80 90 100

Moisture (%wb)

107

APPLICATION OF PHASE TRANSITION TO A MALTODEXTRIN POWDER

• Caking of powders occurs due to the conversion of glassy particles into rubbery particles which stick together.

• From state diagram, moisture content for Tg to be at room temperature (20-25°C) is 8% (wb).

• From moisture sorption isotherm, maltodextrin holds

8% water at aW 0.5.

108

MOISTURE SORPTION ISOTHERM FOR MALTODEXTRIN (DE 25) AT 25°C

30

Roos & Karel, 1991, Biotechnol. Prog., 7: 49.

25

20

15

10

5

0 0.0 0.2 0.4 0.6 0.8

Water Activity

109

APPLICATION OF PHASE TRANSITION TO A MALTODEXTRIN POWDER

• Caking of powders occurs due to the conversion of glassy particles into rubbery particles which stick together.

• From state diagram, moisture content for Tg to be at room temperature (20-25°C) is 8% (wb).

• From moisture sorption isotherm, maltodextrin holds

8% water at aW 0.5.

• Moisture gain from an environment when aW > 0.5 will promote caking of the maltodextrin.

110

SUGGESTIONS FOR CONTROLLING MOISTURE TRANSFER (GAIN OR LOSS)

• Understand moisture sorption isotherms

• Formulation approaches

– Humectants

– Anticaking agents (e.g. calcium silicate)

• Packaging approaches

– Select to minimize water permeation

– Resealable packaging

• Handling instructions

111

OVERVIEW: WATER AND STABILITY

• Chemical Stability

– No universal physicochemical parameter describes the effects of water

– Water activity and plasticization by water affects chemical stability differently depending upon:

• Reaction type

• Matrix

• Physical Stability

– Water affects physical stability primarily by plasticizing glassy systems into rubbery systems

本文通过翔实的实验数据、图表、直观的电镜照片对食品化学上的反应速率、物理上的表观性状以及食品的货架保存时间和水活度的相关性做出了详细的 论证，由本文数据可见：水活度的高低对食品的化学稳定性、物理稳定性和保存时间直接关联。 事实上水分活度aw能直接影响食品的“保质期、色泽、香味、风味和质感”。是食品安全，食品研究，设计，开发，品质控制非常重要的指标！ 化学及生物化学反应（Chemical / Biochemical Reactivity） 水分活度除了能影响食品中的微生物繁殖，把食品变坏；还会影响化学及酶素的反应速度。对食品保质期、色泽、香味和组织结构均有影响。 Non-enzymatic browning 食品褐变（非酶褐变） Lipid oxidation 脂肪氧化 Degradation of vitamins 维生素破坏 Enzymatic reactions 酶素的活力 Protein denaturation 蛋白质变质 Starch gelatinization / reno gradation 淀粉变质 物理特性（Physical Properties） 水分活度除了能预测化学及生物化学反应速度，水分活度也影响食品组织结构。高aw的食品结构通常比较湿润，多汁，鲜嫩及富有弹性。若把这些食品的aw降低，会产生不理想食品结构变化，例如坚硬，干燥无味。低aw的食品结构通常比较松脆，但当aw提高后，这些食品就变得潮湿乏味；水分活度aw还改变粉状及颗粒的流动性，产生结块现象等。 食品水分活度aw，温度、酸值（pH）等均影响微生物的生长；但水分活度对食品包装后的保质期有着至关重要的影响。水分活度（不是水分含量）是直接影响食品中细菌、霉菌及酵母菌等繁殖的重要指标。对食品安全非常重要。“传统测定食品中水分含量是食品的总水分含量，不能提供以上的重要数据”。 水活度对微生物生长影响的论证请参考本公司其他的技术文章！

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