water activity: reaction rates, physical properties… · water activity: reaction rates, physical...
TRANSCRIPT
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!
上海市徐汇区肇嘉浜路 269 号云福大厦 2607 室 200032 Tel:0086-21-64161745/64227532 Fax:0086-21-64161759
上 海 博 鑫 科 技 有 限 公 司
112