Atividade de água e sua aplicação no desenvolvimento de produtos

Prévia do material em texto

Water activity and its application in 
product development
Food developers’ challenges
1. Can we make it?
– Quality, Process‐ability, Packaging efficiency
2. Can we sell it?
– Stability, Safety, Regulation, Right to operate
3. Can we make money selling it?
– Economical feasibility
2
aw Tg
Pioneer 1953 Scott 1983 Levine/Slade
Promoters Christian/Karel/
Simatos/ 
Labuza/Chirife
Franks/Reid/
Blanshard/Roos/
Schmidt
Attributes Microbial stability
Chemical reactivity
Textural quality
Physical stability
Products Intermediate 
moisture foods
Dry foods
Frozen foods
Thermodyn‐
amic state
Equilibrium Non‐equilibrium
Water activity and glass transition are two major 
practical tools for product developers since1950s
ISOPOW books
TX553.W3
1. 1974 UK Water relations of foods (1975)
2. 1978 Japan Water activity: influences on food quality (1981)
3. 1983 France Properties of water in foods: in relation to quality 
and stability (1985)
4. 1987 Canada
5. 1992 Spain Water in foods: fundamental aspects and their 
significance in relation to processing of foods (1994)
6. 1996 US The properties of water in foods (1998)
7. 1998 Finland Water management in the design and distribution of 
quality foods. (1999)
8. 2000 Israel Water science for food, health, agriculture and 
environment (2001)
9. 2004 Argentina Water properties of food, pharmaceutical and 
biological materials (2006)
10. 2008 Thailand Water properties in food, health, pharmaceutical 
and biological systems (2010) Wiley‐Blackwell $220
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Agenda
• Water activity
– Definition & Measurement
• Moisture sorption isotherm
– Definition & Measurement
– Hysteresis & Equations
• Application to product development
– Food stability map
– Food preservation
– Moisture migration
– Models for aw prediction
– Water activity vs. glass transition
• Conclusion
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Water activity definition
At equilibrium in a closed container
Chemical potential in vapor = chemical potential in food
RT ln aw = RT ln 
p
po
aw: water activity (0~1)
p: vapor pressure of water above food
po: vapor pressure of water above pure water
p/po is easy to measure → measure %RH
µ : chemical potential of water (energy/mole)
aw = p/po
µvapor
µfood
µvaporµfood
Measurement techniques
• Hygrometer
– Chilled mirror dew point (>1970, aw 0‐1)
– Electric hygrometer (>1976, aw 0‐1)
– Fiber dimensional hygrometer (>1977, aw>0.3) 
– Thermal couple psychrometer (>1977, aw>0.55)
• Freezing point depression (>1970, aw>0.8)
• Isopiestic equilibration (>1973, aw<0.90)
• Vapor pressure manometer (>1943, aw<0.85)
Í
Chilled Mirror Dew Point Method
AQUA LAB (Decagon Devices, Inc., Pullman, WA)
•Primary method of measuring vapor pressure
•High resolution +0.001 & High accuracy +0.003
•Fast < 5 minutes
•Entire aw range: 0-1 & Highly reliable
•Readings affected by alcohol and propylene glycol
aw and water content (%) of foods
• Popped corn 0.07 0.3
• RTE cereals & Crackers 0.1‐0.3 1‐4
• Pasta 0.33 5
• Almond 0.48 3
• Raisin 0.53 10
• Fruit snacks 0.52‐0.59 9
• Marshmallows 0.63 16
• 42Hign Fructose Corn Syrup 0.74 29
• IMF dog foods 0.83 24
• Jams & Jellies 0.82‐0.94 32
• Bread & Bagel 0.93‐0.96 38
• Fruits & Vegetables 0.98‐0.99 74‐95
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Ref. 1
aw depends on solute present.
moisture(%) aw
Unsalted butter 16 0.97
Dried fruits 20 0.76
• Butter mainly hydrophobic lipids
• Fruits contains hydrophilic sugars
e.g. boiling points ‐
Sugar solution >100C
Water/Oil emulsion =100C
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aw increases with T.
o o
T dependency is less at higher moisture content.
Moisture sorption isotherm (MSI) relates aw to 
moisture content at a given T.
• Useful for:
– Information on concentration 
and drying
– Formulation to avoid 
moisture transfer
– Determining packaging 
materials requirement
– Predicting microbial stability
– Predicting chemical/physical 
stability
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MSI creation: the desiccator method
(13)
(1-3 weeks)
Not enough data points from desiccator method
0
50
100
150
200
250
300
350
400
0 0.2 0.4 0.6 0.8 1
water activity
g 
H
2O
/1
00
 g
 fo
od
42 DE corn syrup
sucrose
fructose
Dynamic Dewpoint Isotherm method
Aqua‐Sorp Isotherm Generator
(new 2010 model will)
(<2 days 50points)
Isotherms differ in shape.
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Deliquescence 
point
Aqua-Sorp provides more data points in a shorter time.
Isotherm shifts right with T.
Potato slice de‐sorption curves (1958)
Some isotherms might 
crossover due to 
– Some new solutes may 
dissolve 
– Some foods hold less 
water at higher T
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Adsorption and desorption: irreversible
• Pores and capillaries fill 
and empty differently.
• Swelling of polymers
• Transition between 
glassy and rubbery 
states.
• Crystallization of some 
solute during de‐
sorption.
• The loss/gain of 
crystalline hydrate
Hysteresis in a crystalline hydrate compound
Ground sample has less hysteresis.
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Application to product development
1. Food stability map
2. Food preservation
3. Moisture migration
4. aw prediction
5. Water activity vs. glass transition
0.25 0.80
Zones of moisture
• aw < 0.25 (Zone1)
– Reactions requiring water cease in this range
– Highends is “BET” monolayer
– Typical food monolayers are 4‐8% H2O(db)
• 0.25 < aw < 0.8 (Zone2)
– Swelling
– Reaction rate accelerates
– Below 0.6, matrix osmolality stress > osmo‐regulatory 
capacity Æ no microbial growth
• aw > 0.8 (Zone3)
– “bulk phase” water & easily freezable
– Microorganisms grow well
• Zones are not “real”, but are for general discussion.
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1. Food stability map - Rate of microbial and 
chemical stability in food is predictable by aw.
(Labuza, 1970)
Application to product development
1. Food stability map
2. Food preservation
a. Microbial safety
b. Chemical stability
3. Moisture migration
4. aw prediction
5. Water activity vs. glass transition
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2a. Microbial safety
Which foods need aw control?
• No need for
– Dried foods (crackers)
– Retorted products (soups, canned meats)
– High ethanol containing products (wines)
– High salt foods (soy sauce)
– Low pH, cultured products (cheese, yogurt)
– Frozen products (frozen desserts)
• Refrigerated products
– Brownie batter 
• Shelf stable products
– IMF meats, Fruit Snacks
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Hurdle theory for food preservation
• Mini. initial count/add competitive microbe
• Lower storage temperature
• Lower oxidation‐reduction potential 
• Adding preservatives 
• Lower pH
• Lower moisture
• Lower aw
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Some IM meats
Cubes water (%) salt (%) pH aw
Chicken ala king 14.9 3.6 5.9 0.61
Corn beef 16.2 5.4 5.9 0.62
Beef stew 17.3 3.7 5.8 0.65
Barbecue beef 16.2 2.7 5.1 0.66
Barbecue chicken 19.7 4.0 5.2 0.70
Ham 19.9 4.5 5.9 0.72
Roast pork 22.4 3.6 5.7 0.74
Sausage 24.2 4.5 4.9 0.78
Chili with beans 13.9 2.6 5.7 0.79
Roast beef 22.2 3.0 5.8 0.79
range 14-24 2.6-5.4 4.9-5.9 0.61-0.79
aw control via humectants
(Safe, Soluble, Low MW, Compatible flavor, Low cost)
• Organic solvents
– ethanol, propylene glycol, glycerol, liquid flavors
• Organic acid and electrolytes
– acetic acid, lactic acid, propionic acid, ascorbic acid
• Low MW carbohydrates
– Mono and di‐sacharides, corn syrups, honey, invert sugar
– Polyols
• Low MW protein hydrolysates
• Electrolytes
– NaCl, KCl, CaCl2
– Phosphates
– Milk electrolytes
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Polyol MW 
(g/mole) 
%rel 
sweet v 
sucrose
Laxation 
threshold
(g/day) 
Solubility 
@25C 
(g/100gH2O)
Melt
Point
( C) 
Heat of 
solution 
(cal/g) 
Hygr-
oscop-
icity 
Glycerol 92 65 >125 Soluble 18 16 M 
Erythritol 122 65 125 61 121 -43 VL 
Xylitol 152 100 70 200 93 -37 H 
Sorbitol 182 60 50 235 100 -27 M 
Mannitol 182 50 20 22 167 -29 VL 
Maltitol 344 90 75 175 145 -6 L 
Lactitol 344 35 35 140 98 -14 L 
Isomalt 344 40 60 39 147 -9 VL 
Polydextrose <22000 0 90 235 N/A 9 H 
 
Polyol Comparison Chart
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2b. Chemicalstability
• Reactions can lead to off‐flavor, aftertaste, 
discoloration, toxicity.
• Oxidation
• Hydrolysis
– Enzymatic hydrolysis (lipase, esterase, invertase..)
– pH induced hydrolysis
• Browning
– Enzymatic browning (PPO)
– Maillard browning
• Degradation
– Nutrients
– High intensity sweeteners
– Antimicrobial agents
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3.
39
0
10
20
30
40
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Moisture migrates from raisins to corn flakes 
aw
MC(%) Raisin too hard
Flakes not crunchy
4. Models for aw prediction
• For all solutions:
– Norrish equation (needs Binary Interaction Constants)
– Grover equation (needs Grover Constants)
– Money & Born equation (sucrose based, for candy)
– Ross equation (most often used)
aw = aw1*aw2*aw3*aw4
• For electrolyte solutions:
– Pitzer equation (needs Pitzer constants)
– Bromley equation (needs Bromley constants)
Ross equation is used most often.
aw = aw1*aw2*aw3*aw4…
Rault’s law
awi =ri*nw/(nw+ni)
nw?total number of moles of water
ni : number of moles of solute i
ri : activity coefficient. assumed to be 1.
Good for aw 0.6‐1 with <10% relative error.
aw Tg
Pioneer 1953 Scott 1983 Levine/Slade
Promoters Christian/Karel/
Simatos/ 
Labuza/Chirife
Franks/Reid/
Blanshard/Roos/
Schmidt
Attributes Microbial stability
Chemical reactivity
Textural quality
Physical stability
Products Intermediate 
moisture foods
Dry foods
Frozen foods
Difference Thermodynamics
equilibrium
Kinetics
non‐equilibrium
5. aw vs. Tg comparison
Sucrose-Water State Diagram 
-140
-100
-60
-20
20
60
100
140
180
0 20 40 60 80 100
% Sucrose
Te
m
pe
ra
tu
re
 (C
)
-220
-120
-20
80
180
280
Te
m
pe
ra
tu
re
 (F
)
Boiling Line
Freezing 
Line
Ice & 
Solution
Glassy State
Solubility 
Line
Vapor
Sucrose in 
Solution Rubbery 
State
Tg 
Line
Tm
Sucrose
Tg
Sucros
Tg
Water
Tm
Water
Conclusion
• aw is critical for designing microbial and chemical 
stabilities into products.
• Limits on controlling aw for stability exist
– Process‐ability
– Legal regulation 
– Sensory properties
– Profitability
• Tg concept is more powerful for designing physical 
stability and textural quality.
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Recent discovery on water properties:
Liquid water has structure (Pollack, 2006)
References
1. Water activity in foods: Fundamentals and 
Applications. Ed. Barbosa‐Canovas, G.V., 
Fontana,A.J., Schmidt,S.J., Labuza,T.P. Blackwell 
Publishing, 2007. 
2. Water relations of Staphyococcus aureus at 30C. 
Scott,W.J. Australian J. Biol. Sci. 6, 549‐564, 1953.
3. Water and the cell / edited by Gerald H. Pollack, 
Ivan L.Cameron, Denys N. 
Wheatley.Published/Created: Dordrecht: Springer, 
c2006.
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