2. Objectives
§ Define Water Activity and Moisture
Content
§ Compare and Contrast Water Activity
and Moisture Content
§ Water Activity and Moisture Content
Measurements Methods
§ Applications for Water Activity and
Moisture Content
3. Water activity vs. Moisture
content
§ Moisture Content
§ Quantitative amount of water in a sample
on a wet or dry basis.
§ An extensive property that depends on
the amount of material.
§ Water Activity
§ A measure of the energy status of the
water in a system (Qualitative).
§ A intensive property that does not depend
on the amount of material.`
4. Water Activity Defined
Correct Definition: Water Activity is a measure of the energy
status of the water in a system.
Old Definition: Water activity is the amount of “free” or
“available” water in a product as opposed to “bound” water.
aw = f/fo = p/po
Vapor pressure of water above sample @ °C
aw = ——————————————————
Vapor pressure of pure water @ same °C
aw = ERH (%) /100
5. Moisture Content Defined
§ Quantitative measure of amount of water
§ Empirical measurement with no standard
§ Loss on Drying – Fick’s Equation
gv
E = (es − ea )
Pa
§ Titration
3Z + ROH + I 2 + H 2O → 3ZH + + ROSO3− + 2 I −
6. Cheese & Cracker System
Cracker and Cheese are placed together in a
sealed container
20% Moisture 30% Moisture
Which way does water move?
Sodium Chloride .753
7. Moisture Analysis Comparison
Water Activity Moisture Content
Measures Energy of Water Amount of “Water”
No, empirical
Standard Available Yes, salt standards
measurement
Cost $1000-$10,000 $250-$25,000
Reporting
aw units Dry or wet basis %
Methods
Methods 4 methods 35 methods
Sample
Little to none Depends on method
Preparation
9. Chilled Mirror Dew Point
§ Advantages
§ Primary method of
measuring vapor pressure
(not calibrated)
Fan
§ Highest accuracy Optical Sensor
±0.003aw
Mirror
§ Rapid measurement <5 Infrared Sensor
minutes
§ Measures entire aw range
(0.03 – 1.0aw)
§ High reliability
Sample
§ Disadvantages
§ Need clean mirror
§ Readings affected by
alcohol and propylene
glycol
10. Dewpoint Moisture Content
§ Construct a moisture sorption isotherm for
product
§ Measure water activity (<5 minutes)
§ Through isotherm the water activity
measurement gives moisture content
§ Product specific
§ Water activity and Moisture Content by 1
instrument
11. Accurate Measurements
§ Calibration / Verification Standards
§ Saturated Salt Slurries
§ Unsaturated Salt Solutions
§ Temperature
§ Sample Preparation
§ Need representative sample
§ If slicing / grinding – be consistent
§ Prevent moisture exchange with
environment
13. AquaLab Series 4TE(V)
Large screen and Clam Shell lid design
easy to use menus for easier cleaning
Same accuracy and
repeatability as the
O-ring seal for tighter
Series 3TE chamber seal
Volatiles sensor
On board storage now incorporated in
to save testing data testing block and
can be used with a
menu selection, so
no need to swap out
Can be upgraded to
measure moisture a volatile block
content and water
activity Administrator settings allow user
simultaneously control and makes instrument 21
CFR Part 11 compliant when used
with AquaLink RG software
14. Applications for Water Activity
§ Water Activity Can Help You:
§ Control microbial growth
§ Formulate profitable products
§ Predict effects of temperature abuse
§ Control chemical reaction rates
§ Control moisture migration
§ Avoid caking and clumping
§ Predict packaging needs
15. Microbial Growth
§ Scott (1953 & 1957) showed that
microorganisms have a limiting water
activity level below which they will not
grow.
§ Water activity, not water content,
determines the lower limit of available
water for microbial growth.
Scott,W.J. 1953. Water relations of Staphylococcus aureus at 30ºC. Aust. J. Biol. Sci. 6:549-564.
Scott,W.J. 1957. Water relations of food spoilage microorganisms. Adv Food Res 7:83-127.
16. Microbial Growth
§ Growth Limit
§ Every microorganism has a water activity level
below which it cannot grow.
aw limit Microorganisms
0.91 Gram Negative Bacteria
0.86 Gram Positive Bacteria
0.88 Yeast (practical limit)
0.80 Production of mycotoxins
0.70 Molds (practical limit)
0.62 Osmophilic yeast
0.61 Xerophilic molds
0.60 Absolute limit for all growth
18. Interaction Table A
Table A. Interaction of pH and aw for control of spores in food heat-
treated to destroy vegetative cells and subsequently packaged.
aw Values pH Values
4.6 or less > 4.6 – 5.6 > 5.6
0.92 or less Non-PHF*/non- Non-PHF/non- Non-PHF/non-
TCS** TCS TCS
> 0.92 – 0.95 Non-PHF/non- Non-PHF/non- PA***
TCS TCS
> 0.95 Non-PHF/non- PA PA
TCS
* PHF means “Potentially Hazardous Food”
** TCS means “Time/Temperature Control for Safety Food”
*** PA means “Product Assessment Required”
2005 Food Code
19. Interactive Table B
Table B. Interaction of pH and aw for control of vegetative cells and spores in
food not heat-treated or heat-treated but not packaged.
aw Values pH Values
< 4.2 4.2 – 4.6 > 4.6 – 5.0 > 5.0
< 0.88 Non-PHF*/ Non-PHF/non- Non-PHD/non- Non-PHF/non-
non-TCS** TCS TCS TCS
0.88 – 0.90 Non-PHF/non- Non-PHF/non- Non-PHF/non- PA***
TCS TCS TCS
> 0.90 – 0.92 Non-PHF/non- Non-PHF/non- PA PA
TCS TCS
> 0.92 Non-PHF/non- PA PA PA
TCS
* PHF means “Potentially Hazardous Food”
** TCS means “Time/Temperature Control for Safety Food”
*** PA means “Product Assessment Required”
2005 Food Code
20. Formulating for Water Activity
§ Dehydrate Product § Additives
§ Edible films and § Ingredients involved in
coatings water binding
§ Keep water from § Humectants:
migrating between § salt – (NaCl)
the different § sugars – (glucose,
fructose, sucrose,
components in a syrups)
composite samples. § glycols – (glycerol, PEG,
§ They are located on propylene glycol)
the surface or as § amino acids – (glycine,
alanine)
thin layers between § polymers – (starch,
several parts within gums)
the product. § acids – (citric acid, lactic
acid)
§ Anticaking agents
21. Product Formulation Snack Cake
§ All 3 components have The image part with relationship ID rId5 was not found in the file.
same water activity
§ 3 components have very
different moisture
contents
§ Each component has a
unique texture
§ Icing serves as a
moisture barrier for the
cake
22. Modeling Temperature
Abuse
Granola Bar Isotherm at 3 different Temperatures
§ Water activity is
temperature 35
dependent
30
Moisture Content (% d.b.)
25
§ Most products 20
have a lower water 15
activity value at 10
5
15c
25c
lower temperature.
40c
0
§ Clausius-
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Water Activity
Clapeyron
relationship:
aw2 ΔH ⎛ 1 1 ⎞
ln =− ⎜ − ⎟
aw1 R ⎜ T1 T2 ⎟
⎝ ⎠
23. Moisture Migration
§ Moisture migration can lead
to: § Solutions to limit
§ texture changes moisture migration
§ microbial growth § make components to
§ degradation reactions same water activity
§ organoleptic changes § lower aw of high aw component
§ raise aw of low aw component
§ retard diffusion
§ Examples of Multi-domain process within
systems
§ cheese / cracker
component
§ cereal with fruit pieces
(viscosity)
§ baked dough – filling § edible barrier
§ frozen pizza crust – sauce § separate packaging
§ ice cream in cone
§ gelatin capsules
24. Example of Ingredient Mixing
DLP Combined Isotherm
w = ∑φi wi
Ingredient 1 Ingredient 2 Combined
φi = mass fraction of component i
wi = moisture content of component i. 25
Moisture Content (% w.b.)
20
3 2
wi = b3i χ + b2i χ + b1i χ + b0i 15
10
Where b3, b2, b1, and b0 are empirical constants from
the DLP isotherm model and χ is ln(-ln(aw)) 5
0
3 2
w = χ eq b3 '+ χ eq b2 '+ χ eq b1 '+b0 ' 0
0.2
0.4
0.6
Water Activity
0.8
1
Where b3’, b2‘, b1‘, and b0’ are the DLP constants for
the combined isotherm and χeq is ln(-ln(aw(eq)))
b3’ = ∑Φib3i , b2’ = ∑Φib2i , b1’ = ∑Φib1i , b0’ = ∑Φib0i
aw(eq ) = exp[− exp(− χ eq )]
25. Water Activity and Glass Transition
20
Moisture Content (% d.b.)
Large number of
water binding sites
15 become available
Caking, Clumping,
10 Crystallization, Loss
Limited Water of Texture
Binding Sites
5
RHc Critical Water Activity
0 Amorphous Metastable State
0 0.2 0.4 0.6 0.8 1
Water Activity
*Data is for Spray Dried Milk Powder
26. Package Performance
α = slope of the isotherm (g/g)
Calculations
Water activity under specific conditions
awo = initial water activity
awc = critical water activity
aw = ha − [(ha − awo ) exp(−t / τ )] pa = atmospheric pressure (kPa)
M = total mass of product inside the
Time Constant
package (g)
α pa M es = saturation water vapor pressure at
τ=
es Ag v package temperature (kPa)
Shelf life prediction of packaging A = package surface area (m2)
gv = package conductance (g m-2 s-1)
⎛ h − a wc ⎞
t shelf = −τ ln⎜ a
⎜ h − a ⎟ ⎟ ha= Humidity of air,
⎝ a wo ⎠
WVTR = Water Vapor Transmission Rate
Determine Package Conductance t = Time in package,
α pa M paWVTR τ = Time constant
gv = =
es Aτ ha es
27. Conclusion
§ Water Activity is a measure of the energy status
of water.
§ Decagon provides a range of products for
measuring water activity
§ Decagon is always happy to provide application
and technical support
§ Water Activity is important for
§ Microbial Safety
§ Chemical Stability
§ Physical Stability
28. Who uses Aw?
WHO ARE OUR CUSTOMERS?
In the U.S. alone, over 120 universities, 55 analytical labs, 20
government agencies, and hundreds of companies use water
activity
29. Applications for Water Activity
Ø Microbial Growth
Ø Hurdle Technology
Ø Chemical/Biochemical Stability
Ø Physical Properties
Ø Moisture Migration Control
Ø Compliance with Government Agencies
Ø Shelf Life Testing
Ø Product Formulation
Ø Process Control
Ø Quality Control
Ø Inspection of Incoming Ingredients
Decagon Devices, Inc.
30. Microbial Growth
• Microorganisms have a limiting water activity level below which they will not
grow:
v Bacteria ~0.91
v Yeast ~0.88
v Molds ~0.70
• Water activity, not water content, determines the lower limit of available
water for microbial growth.
Decagon Devices, Inc.
32. Hurdle Technology
Hurdle technology deliberately combines existing and new preservation
techniques to establish a series of preservative factors (hurdles) that the
microorganisms in question are unable to overcome (jump over).
These hurdles may be temperature, water activity, acidity, preservatives, and
others.
Decagon Devices, Inc.
35. Moisture Migration
Multi-Domain Systems
• Two distinct regions at different aw
• Water moves from areas of high water activity to
areas of low water activity.
• Driving force for water migration directly related
to aw difference.
• Rate of migration depends on structure/diffusion properties.
Decagon Devices, Inc.
36. Moisture Migration
Moisture migration can lead to:
• texture changes
• microbial growth
• degradation reactions
Examples of Multi-domain systems
• cheese / cracker
• cereal with fruit pieces
• baked dough – filling
• frozen pizza crust – sauce
• ice cream in cone
• gelatin capsules
Decagon Devices, Inc.
37. Moisture Migration
Solutions
• make components to same water activity
lower aw of high aw component and keep soft
raise aw of low aw component and keep crisp
• retard diffusion process within component (↑ viscosity)
• edible barrier
• separate packaging
Decagon Devices, Inc.
38. Moisture Migration
Traditional Fruit Cake
• make components to same water
activity
(flour, sugar, candied fruits, raisins,
butter, eggs, nuts)
Water activity Moisture Content
(% db)
Dough 0.857 24.5
Fruits (mixed) 0.862 52.2
Decagon Devices, Inc.
39. Compliance
Water activity is a critical parameter for compliance with:
• FDA’s Good Manufacturing Practices (21CFR)
• HACCP – critical control point
• ANSI/NSF Standard 75 for shelf stable baked
goods
• USP Method <1112>
Decagon Devices, Inc.
40. Food Safety Concerns
?
In the United States alone:
• 76 million illnesses
• 325,000 hospitalizations
• 5,000 deaths annually
due to foodborne disease
Source: Centers for Disease Control (CDC)
Decagon Devices, Inc.
41. Product Recalls
The consequence of a microbiological failure, as it relates to product
recalls, can be very costly.
• Recalls can cost millions of dollars in product
losses and operational delays
• Sales may suffer as a result from:
- loss of consumer confidence
- consumers relating the recall to other products
• Company reputation damage.
Decagon Devices, Inc.
42. Product Recalls
The annual medical costs, productivity losses, and costs of premature deaths
due to five major foodborne pathogens are estimated to be $ 6.9 billion.
(Crutchfield and Roberts, 2000 FoodReview 23(3):44-49)
FDA posts recalls and safety alerts on website
http://www.fda.gov
Many food, cosmetic, and pharmaceutical recalls are due to bacterial
(Listeria, Salmonella) or mold contamination.
Decagon Devices, Inc.
43. Shelf Life Testing
• The presence of water influences the shelf
life.
• Water solubilizes reactants and increases
their mobility, both of which can lead to faster
deterioration.
• Define unacceptable product conditions:
microbial growth, chemical status (loss of
nutrient), physical properties, sensory.
Decagon Devices, Inc.
44. Shelf Life
Case study:
• Bakery company has re-formulated products along with
packaging changes to more than double the shelf life of its
products. Now at 10 to 17 days
• Quality remains constant in terms of texture, flavor, color,
etc. Ingredients are sampled directly off delivery trucks to
ensure they meet specifications.
• The added shelf life is an
advantage selling to stores
– managers have more time
to sell the product
– opened new markets.
Decagon Devices, Inc.
45. Product Formulation (R&D)
The water activity concept should be incorporated
from the beginning.
If you start with the concept of Aw then new
products will be safe, have a maximized shelf life,
and of the highest quality.
Water activity data can be used to predict
potential hazards for new products.
Decagon Devices, Inc.
46. Product Formulation
Case Study
A pet food manufacturer produced to 8% moisture content.
Why? Because at that level he had never had spoilage
problems or shelf life concerns. He had a safe product.
Then, he used water activity measurements. The aw of his
product was 0.50aw. After a little research he understood that
he needed only to stay below 0.65aw.
47. Product Formulation
0.50aw (8% moisture): 0.65aw (10% moisture)
1000 kg/hour 1020 kg/hour
16 hrs/ day 16hrs/ day
5 days/ week 5 days/ week
= 3,840 tons/year = 3,916 tons/year
It means 76,800 kilos more every year!
Now, let’s assume that this company sells each kilo of their PetFood at US$0.84.
Then 76,800 kilos x 0.84 = $64,512 extra in revenues every year!
The Results:
ü Increase revenues and profits
ü Low-cost solution (water is cheap)
ü Reduce electricity, heat, etc.
ü Same labor, man hours, etc.
ü Better product!
48. Process Control
• aw measurement confirms shelf
life, quality, and safety testing
data on-line
• Control belt speed,
oven temperature
• Archive information for
inspections, customer complaints
Decagon Devices, Inc.
49. Ingredients Inspection
Many larger food companies are requiring
their suppliers to meet water activity
specifications on the ingredients.
Rather than just buying raisins from a
supplier, a food producer can purchase raisins
at any aw level depending on the application.
The aw of the raisin will be much lower if the
application is for a breakfast cereal versus a
higher aw raisin that is to be baked into a
muffin.
Decagon Devices, Inc.
50. Review: Why use aw?
Ø Microbial Growth
Ø Hurdle Technology
Ø Chemical/Biochemical Stability
Ø Physical Properties
Ø Moisture Migration Control
Ø Compliance with Government
Agencies
Ø Shelf Life Testing
Ø Product Formulation
Ø Process Control
Ø Quality Control
Ø Inspection of Incoming Ingredients
Decagon Devices, Inc.
51. EminTech (http://emintech.com/)
Independent Water Activity Study
AquaLab
+Very good reproducibility
even after 11 days
+ reproducible results
+ stable calibration
+ quick measurements
+ nice design
52. Primary Method VS Secondary
Method
• Aqualab uses Dewpoint Method
which is a Primary Method.
• All other water activity meters
on the Market use secondary
method.
53. Testimonials
The Vapor Sorption Analyzer has become an invaluable
instrument to our R&D lab. The Dynamic Dewpoint Isotherm
(DDI) method allows us to generate an isotherm in 12-24
hours, at a wide range of temperatures. The Dynamic Vapor
Sorption (DVS) method provides a fully automated process of
obtaining kinetics of moisture uptake in our products. The
“Test Wizard” function in the software allows for quick and
easy test set-up, and exporting the results is done with one
click of a button. The Vapor Sorption Analyzer has provided
us with very reliable and repeatable results that have been
instrumental to the development of our products.
Thanks!
Kara Grant
Process Development Intern
GMCR Specialty Coffee Business Unit
54. Thank You!
Decagon Devices, Inc.
Brady Weldon
International Product Manager
www.aqualab.com
Decagon Devices, Inc.
