Brady Weldon Decagon Devices, Inc.

Basics of Water Activity

Objectives

Define Water Activity and Moisture Content

Compare and Contrast Water Activity and Moisture Content

Water Activity and Moisture Content Measurements Methods

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.

Water Activity Defined

aw = f/fo = p/po

aw = ——————————————————

aw = ERH (%) /100

Vapor pressure of water above sample @ °C

Vapor pressure of pure water @ same °C

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.

Moisture Content Defined

Quantitative measure of amount of water

Empirical measurement with no standard

Loss on Drying – Fick’s Equation

Titration

Cheese & Cracker System

Initial Final

Moisture Content 4% 20% Water Activity 0.30 0.75

Initial Final

Moisture Content 60% 30% Water Activity 0.90 0.75

Cracker Cheese

Cracker and Cheese is equilibrated over saturated NaCl in separate sealed containers

Cheese & Cracker System

Cracker and Cheese are placed together in a sealed container

Which way does water move? Sodium Chloride .753

20% Moisture 30% Moisture

Moisture Analysis Comparison Water Activity Moisture Content

Measures Energy of Water Amount of “Water”

Standard Available Yes, salt standards No, empirical

measurement

Cost $1000-$10,000 $250-$25,000

Reporting

Methods aw units Dry or wet basis %

Methods 4 methods 35 methods

Sample

Preparation Little to none Depends on method

Water Activity vs. Moisture

Content

Chilled Mirror Dew Point Advantages

Primary method of measuring vapor pressure (not calibrated)

Highest accuracy ±0.003aw

Rapid measurement <5 minutes

Measures entire aw range (0.03 – 1.0aw)

High reliability

Disadvantages Need clean mirror

Readings affected by alcohol and propylene glycol

Infrared Sensor Mirror

Optical Sensor Fan

Sample

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

Instruments from Decagon

Decagon Devices, Inc. Brady Weldon

International Product Manager www.aqualab.com

Decagon Devices, Inc.

Thank You!

Complete Moisture Analysis

Presented by:

Wendy Ortman

Application Engineer

Decagon Devices

Applications for Water Activity

and Moisture Content

Water Activity Can Help You:

Control microbial growth

Control chemical reaction rates

Conduct shelf life testing

Predict effects of temperature abuse

Formulate profitable products

Achieve optimal texture

Control moisture migration

Model dry ingredient mixing

Avoid caking and clumping

Predict packaging needs

Characterize products

Moisture Content Can Help You: Adjust texture at a given water

activity Control chemical reactivity and

rates

Determine ingredient concentrations

Determine nutritional content

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

Microbial Growth Pathogen aw Limits for Growth and Toxicity

Minimum aw for

Microorganism Growth Toxin Production Clostridium botulinum (E) 0.95-0.970.97

Clostridium botulinum (A) 0.93-0.95 0.94-0.95 Clostridium botulinum (B) 0.94 0.93-0.94

Staphylococcus aureus 0.86 0.87-0.90 enterotoxin A 0.97 enterotoxin B Aspergillus clavatus 0.85 0.99 patulin

A. flavus 0.78-0.80 0.83-0.87 aflatoxin A. ochraceus 0.77-0.83 0.83-0.87 ochratoxin

Pencillium cyclopium 0.82-0.87 0.97 pencillic acid P. cyclopium 0.81-0.85 0.87-0.90 ochratoxin P. martensii 0.79-0.83 0.99 pencillic acid

P. patulum 0.83-0.85 0.95 patulin

Microbial Growth Survival of Bacteria at Reduced Water Activity Bacteria can survive for long periods of time in foods at low water activity.

Examples: Salmonella in Chocolate at

aw = 0.30

Salmonella in Peanut Butter at

aw = 0.50

Salmonella and S. aureus in

powder milk at aw = 0.26

Low water activity is not a kill step, but the microorganisms will not proliferate.

Government 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>

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

redox potential

preservatives

others

Leistner, L., (1994) Further developments in the utilization of hurdle technology for food preservation., Journal of Food Engineering, 22:421-432.

Microbial Growth – Hurdle Technology

Microbial Growth – Hurdle

Technology

Chemical/Biochemical Stability

Water activity effect on reaction rate

Most reaction rates increase with

increasing water activity

Most rates correlate better with aw than

moisture content

Water may act as a:

solvent

reactant

change the mobility of reactants (viscosity)

General Effect of Water Activity on Reaction

Rates

Chemical/Biochemical Stability

Water Activity

Re

actio

n R

ate

0.0 0.2 0.4 0.6 0.8 1.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

mo

Chemical/Biochemical Stability

Maillard reactions are a

complex series of

reactions involving free amines (amino acid)

carbonyl groups (reducing

sugars)

Maximum loss of amino nitrogen occurs at 0.60-0.70 aw

Chemical/Biochemical Stability Lipid Oxidation

A minimum reaction rate at a water activity of about ~0.3 aw

Rate increase from

0-0.3aw Less available metal ions

due to hydration spheres

Reduced oxygen diffusion

Free radical quenching

Rate increase from

0.3-0.8aw

Increased dissolution of catalysts

Increased mobility of oxygen and metal ions

Nutrient Degradation

Vitamin Stability

Ascorbic acid (vitamin C)

Riboflavin

Thiamin

Aspartame

Chemical/Biochemical Stability

General Effect of Water Activity on Enzymatic

Activity

Chemical/Biochemical Stability

Determining Shelf Life

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

Controlling Water Activity

Dehydrate Product

Edible films and coatings

Keep water from migrating between the different components in a composite samples.

They are located on the surface or as thin layers between several parts within the product.

Additives

Ingredients involved in water binding

Humectants:

salt – (NaCl)

sugars – (glucose, fructose, sucrose, syrups)

glycols – (glycerol, PEG, propylene glycol)

amino acids – (glycine, alanine)

polymers – (starch, gums)

acids – (citric acid, lactic acid)

Anticaking agents

Process Control of Water

Activity

aw measurement confirms shelf life, quality,

and safety testing data on-line

aw is a more effective production specification

Control belt speed, oven temperature

Archive information for inspections, customer

complaints

The water activity concept should be

incorporated from the beginning.

If start with aw concept 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.

Product Formulation

Product Formulation Raw ingredient inspection Many companies are requiring their

suppliers to meet water activity specifications on the ingredients.

Specifying water activity levels to ingredient suppliers insures consistent product.

When product aw changes, problems can be traced to either processing or ingredients

Product Formulation Snack Cake

All 3 components have

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

0

5

10

15

20

25

30

35

40

45

50

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Water Activity

Mo

istu

re C

on

ten

t (%

d.b

.)

Icing

Creme Filling

Cake

Physical Properties

Texture, Hardness, Compaction

Dissolution

Coating Integrity

Moisture migration

Caking and Clumping of powders

Physical Properties

Texture

Hard or crisp products low water activity

break when physically stressed

are crisp and crunchy

soften when exposed to aw

Soft products intermediate to high aw

bend when stressed

are moist, juicy, tender, and chewy.

harden when exposed to aw

Moisture Migration

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.

Can lead to Excipient/Drug interactions and increased degradation of API

Causes coatings to crack or become sticky

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

Caking is water activity, time and temperature

dependent process.

Free flowing powder is transformed into

lumps and eventually an agglomerated solid.

Problem is ubiquitous in the food and

pharmaceutical industries.

Powder Flow and Caking

Solutions: To maintain the flow properties and prevent caking of powders

Establish a Critical Water Activity

Treatment of the powders below critical water activity

packaging in high moisture barrier packages

storage at low temperatures

in package desiccation

addition of anticaking agents

Powder Flow and Caking

Uses for Complete Moisture

R&D QC Storage Ingredients Semi-

Moist Dry RTE

Control Microbial Growth Formulate profitable products Control chemical reaction rates Predict effects of temperature abuse Control moisture migration Achieve optimal texture Avoid caking and clumping Predict packaging needs Determine ingredient concentrations

Complete moisture analysis is a relatively simple but powerful way to control product safety and quality.

Water Activity and moisture content can easily be measured with instrumentation

Complete moisture analysis relates to: Microbial Growth

Chemical / Biochemical Stability

Shelf Life

Product Formulation

Physical Properties

Conclusion

Fundamentals of Moisture Sorption Isotherms

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.

Moisture Sorption Isotherms The relationship between water activity and equilibrium moisture content of a

sample at a specified temperature is called the moisture sorption isotherm.

Moisture Sorption Isotherm

Each product has its

own unique moisture

sorption isotherm – due

to different interactions

(colligative, capillary,

and surface effects)

between the water and

the solid components at

different moisture

contents. 0

5

10

15

20

25

30

35

40

45

50

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Water Activity

Mo

istu

re C

on

ten

t (%

d.b

.)

Icing Creme Filling Cake

Wood Pulp Granola Bar Milk Powder

Full Isotherm

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1

Water Activity

Mo

istu

re C

on

ten

t (%

d.b

.)

*Data is for Starch

Adsorption

Desorption

Working Isotherm

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1 1.2

Water Activity

Mo

istu

re C

on

ten

t (%

d.b

.)Working Desorb Working Adsorb Full

*Data is for Starch

Which Isotherm?

Full Isotherm

Product

Characterization

Transition Points

Hysteresis

Moisture Content

Prediction

Working Isotherm

Sorption from native

state

Scanning Curves

Temperature

Temperature must

be specified and

held constant.

The effect of

temperature on the

moisture sorption

isotherm follows the

Clausius-Clapeyron

equation.

Desorption isotherms of potato slices at various temperatures.

From Gorling, P. (1958) in Fundamental Aspects of the Dehydrationof Foodstuffs. Society of Chemical Industry,

London, pp 42-53.

Method 1: Desiccator Isotherm

Method

Determine equilibrium moisture contents

at several water activities.

Place product (dried or hydrated) into

controlled humidity chambers at constant

temperature

Need six to nine different aw levels (5 min)

Temperature control

Measure the weight gain or loss with time

until equilibrium.

One to three weeks for equilibrium

Desiccator Isotherm Method

Very slow

Very laborious

Lots of Space

Few data points

Usually only working

isotherm

Interpolation with Model

0

2

4

6

8

10

12

14

0 0.2 0.4 0.6 0.8 1

Water Activity

Mo

istu

re C

on

ten

t (%

d.b

.)Traditional Desiccator GAB Model Fit

Isotherm Models

BET isotherm (Brunauer, Emmet and Teller) (0-0.5aw)

GAB equation (Guggenheim, Anderson, de Boer) (0-0.85aw)

m = moisture in g/g solid at water activity aw

mo = monolayer moisture content

C, C1 and k = constants

Cm

1Ca

Cm

1

ma-1

a

o

w

ow

w

w1ww

wo1

kaCka1ka-1

akmCm

Isotherm Models

DLP (Double Log Polynomial) (0-0.95aw)

m = b3x3 + b2x

2 + b1x + b0

x = ln(-ln(aw)) (Chi Value)

Isotherm Model Comparison

6

8

10

12

14

16

18

0 0.2 0.4 0.6 0.8 1

Water Activity

Mo

istu

re C

on

ten

t (%

d.b

.)

Actual Data DLP Model GAB Model

Method 2: Dynamic Vapor

Sorption (DVS) Cycling of water activity is

automatic

Gravimetrically determines when weight change has slowed to an acceptable level

Faster than traditional methods

Facilitates evaluation of sorption kinetics

Still relies on equilibration to set water activity levels

DVS Change In Mass (ref) Plot

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

4

250 350 450 550 650 750 850

Time/mins

Ch

an

ge

In

Ma

ss

(%

) -

Re

f

0

10

20

30

40

50

60

70

80

90

100

Ta

rge

t R

H (

%)

dm - dry Target RH

© Surface Measurement Systems Ltd UK 1996-2007DVS - The Sorption Solution

Temp: 25.0 °C

Meth: polymer film.sao

MRef: 2.28063

57

Highlights of Static Isotherms

+ Decrease equilibration time, no mold growth

+ Sample not exposure to room Temp and %RH conditions

+ Can select any %RH between 0 and ~95 to 98%

+ Decreased manual labor

+ Can be used to investigate kinetics of water sorption

- Still time consuming (usual only one sample can be run at a time)

- Limited data resolution

- Difficult to capture real-time sorption events, such as Tg and

hydrate formation and loss

Method 3: Dynamic Dewpoint

Isotherm (DDI) Method

Wet air for adsorption, dry air for desorption

Water activity directly measured

Equilibration to known water activity not necessary

Fast with unequalled data resolution

Provides a more real world sorption picture

Does not provide kinetics of sorption

Infrared Sensor Mirror

Optical Sensor

Fan

Sample

Dry Air Wet Air

Precision Balance

Highlights of Dynamic Isotherms

+ Equilibration to known water activity not necessary,

making them very fast. + Incredibly high data resolution

+ Can be used to investigate sorption changes due to matrix transitions

+ Provides a more real world sorption picture, but does not provide kinetics of sorption

− Can result in non-equilibrium conditions and larger

hysteresis

Comparing Isotherm Methods

740

760

780

800

820

840

860

880

0 146.3287.9423.2 562 719.5943.3 1234 1405 1507 1615 1732 1866 2030 2270

Time (mins)

Weig

ht

(mg

)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Wate

r A

cti

vit

y

Weight Water ActivityDVS Change In Mass (ref) Plot

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

4

250 350 450 550 650 750 850

Time/mins

Ch

an

ge In

Mass (

%)

- R

ef

0

10

20

30

40

50

60

70

80

90

100

Targ

et

RH

(%

)

dm - dry Target RH

© Surface M easurement Systems Ltd UK 1996-2007DVS - The Sorpt ion Solut ion

Temp: 25.0 °C

Meth: polymer f ilm.sao

MRef: 2.28063

DDI DVS

Comparing Isotherm Methods

0

5

10

15

20

25

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

Water Activity

Mo

istu

re C

on

ten

t (%

d.b

.)

DVS Dessicator DVS2 PEC DDI Method

Milk Powder Adsorption Isotherm

0

2

4

6

8

1012

14

16

18

20

0 0.2 0.4 0.6 0.8 1

Water Activity

Mo

istu

re C

on

ten

t (%

d.b

.)

Traditional Desiccator GAB Model Fit DDI Method

AquaLab Vapor Sorption Analyzer Dynamic and Static in One Instrument Dynamic and static methods

can be analyzed on 1 sample

Dynamic isotherms for matrix transitions

Static isotherms for kinetic research

Run a sample initially with Dynamic isotherm to determine regions of interest

Investigate the time dependency in the regions of interest using a Static isotherm

VSA is a Stand Alone

Instrument

External computer only needed for downloading data

No external gas is needed (but optional)

No special power needed.

Only requires desiccant tubes and a full

water chamber

VSA Can be Controlled by a

Computer

USB connection to a computer

VSA Downloader software

Downloads data from instrument

Allows viewing in table or chart format

Allows viewing of multiple isotherm tests

Can also be used to setup new tests (interface

easier than on the instrument)

VSA Temperature Range is 15-60°C

* Data is for Adsorption of Microcrystalline Cellulose

VSA is Easy to Maintain

Instrument performance verified with water activity and weight standards

Chamber is easily accessible for cleaning

Unique actuator system eliminates

balance drift

Diagnostics screen provides information about current instrument performance

VSA is Also a Water Activity

Instrument

Uses same chilled mirror technology as the AquaLab water activity meters.

Accuracy of +/- 0.005

VSA Can Generate DDI

Isotherms Dynamic Dewpoint

Isotherm Controls neither the water

content nor the water activity

Water activity is determined using a standard chilled-mirror dewpoint sensor

Weight tracked by magnetic force balance

Isotherm data in approx. 24 hrs

Infrared Sensor Mirror

Optical Sensor

Fan

Sample

Dry Air Wet Air

Precision Balance

VSA Can Generate Equilibrium

Isotherms Cycling of water activity is

automatic

Gravimetrically determines when weight change has slowed to an acceptable level

Faster than traditional methods

Facilitates evaluation of sorption kinetics

Still relies on equilibration to set water activity levels

DVS Change In Mass (ref) Plot

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

4

250 350 450 550 650 750 850

Time/mins

Ch

an

ge

In

Ma

ss

(%

) -

Re

f

0

10

20

30

40

50

60

70

80

90

100

Ta

rge

t R

H (

%)

dm - dry Target RH

© Surface Measurement Systems Ltd UK 1996-2007DVS - The Sorption Solution

Temp: 25.0 °C

Meth: polymer film.sao

MRef: 2.28063

VSA Can Generate Both Types of

Isotherms on One Sample

* Data is for Microcrystalline Cellulose at 25C

0.00%

2.00%

4.00%

6.00%

8.00%

10.00%

12.00%

0.0000 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 0.7000 0.8000 0.9000

% M

ois

ture

Co

nte

nt

Water Activity

Static Method Dynamic Method

Why have both Dynamic and

Equilibrium Isotherms?

Dynamic Isotherm Only

0

5

10

15

20

0 0.5 1

Mo

istu

re C

on

ten

t (%

d

.b.)

Water Activity

DDI Method

Glass Transition Inflection Point

No Crystallization or Kinetics

* Data is for Spray Dried Milk Powder at 25C

Static Isotherm Only

* Data is for Spray Dried Milk Powder at 25C

0.0000

0.1000

0.2000

0.3000

0.4000

0.5000

0.6000

0.7000

0.8000

0.00%

1.00%

2.00%

3.00%

4.00%

5.00%

6.00%

7.00%

8.00%

9.00%

10.00%

0.0 2000.0 4000.0 6000.0

Wa

ter

Ac

tiv

ity

% M

ois

ture

Co

nte

nt

Time (min)

0.00%

1.00%

2.00%

3.00%

4.00%

5.00%

6.00%

7.00%

8.00%

9.00%

10.00%

0.0000 0.2000 0.4000 0.6000 0.8000

% M

ois

ture

Co

nte

nt

Water Activity

No inflection point visible to indicate glass transition

Crystallization

Kinetics of Sorption and Diffusion

Static and Dynamic on 1 sample

0

5

10

15

20

25

0.0000 0.2000 0.4000 0.6000 0.8000 1.0000

% M

ois

ture

Co

nte

nt

Water Activity

Static Isotherm Dynamic Isotherm

* Data is for Spray Dried Milk Powder at 25C

Glass Transition Inflection Point

Crystallization

VSA’s Stage Interface Maximizes

Utility

A test can consist of 1 to 20 stages

All stages performed on 1 sample

Each stage has unique settings

One test could have a DDI stage, a DVS

stage, each with different starting and

stopping points and each at a different temperature

VSA Review

Stand Alone Instrument

Can be Controlled by a Computer

Temperature Range is 15-60 °C

Easy to Maintain

Also a Water Activity Instrument

Can Generate DDI Isotherms

Can Generate Equilibrium Isotherms

Can Generate Both Types of Isotherms on One Sample

Stage Interface Maximizes Utility

Who Uses Isotherms and for

What? Companies Uses

Kraft Ingredient mixing, powder flow,

product formulation, DUO

General Mills Ingredient mixing, product

formulation, deliquescence

Glaxo-Smith Kline Excipient stability, glass transition,

moisture migration, API stability

Quaker Ingredient mixing, product

formulation, deliquescence

Meade Johnson Powder flow, caking, chemical

stability, glass transition, DUO

Nestle Pet Care Powder flow, caking, chemical

stability, glass transition,

Sorption Isotherm Applications

Chemical Reaction Rate Stability

monolayer moisture content

Calculate Shelf Life Estimate

Determine Levels of Hysteresis

Wetting/Drying Curve

Product Formulation

Dry Ingredient Mixing

Temperature Shifts

Moisture Content Prediction

Physical Changes

Glass Transition

Crystallization

Stickiness

Crispness

Product Characterization

Amorphous vs. Crystalline

Moisture Sensitivity

Coatings

Packaging Requirements

Monolayer Stability Value

For most dry foods at or below the monolayer value

(from the BET or GAB isotherm) the rates of quality

loss are negligible. The monolayer moisture content

corresponds to an aw of 0.2 – 0.4.

Shelf Life – critical aw value

The higher the aw the faster the reaction rate,

thus, the shorter the shelf life.

For most dry foods,

an increase in aw

by 0.1 unit decreases

shelf life two to

three times.

Bell, L. N. and Labuza, T. P. (1994).

Influence of the low-moisture state on pH and its implication for reaction kinetics.

Journal of Food Engineering. 22:291-312.

Hysteresis

Adsorption

starts from the dry state, aw = 0

Desorption

starts from the initial wet state, aw ~ 1.

Drying/Wetting Curve

0

1

2

3

4

5

6

7

8

9

0 0.2 0.4 0.6 0.8 1

Water Activity

Mo

istu

re C

on

ten

t (%

d.b

.)

Product Safe

Product

Could Mold

Product Formulation

Moisture sorption

isotherms of cellulose

model systems

containing various

amounts of glycerol at

37°C.

Labuza, T. P., Heidelbaugh, N. D., Silver, M., and Karel, M. (1971). Oxidation at intermediate

moisture contents. Journal of American Oil Chemists Society. 48:86-90.

Determine Relative Hygroscopisity

At low aw, Croscarmellose is most hygroscopic

At aw > than 0.70, Sorbitol and Sucrose experience phase changes and become hygroscopic

Mannitol is non-hygroscopic over the entire water activity range

Dry Ingredient Mixing

When two ingredients

of different water

activities are mixed

together in a sealed

package they will

exchange moisture

until an equilibrium aw

(chemical potential) is

reached.

aeq

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1 Mo

istu

re C

on

ten

t (%

w.b

.)

Water Activity

Ingredient 1 Ingredient 2

Example of Ingredient Mixing

)]exp(exp[ eqeqwa

iiww DLP Combined Isotherm

i = mass f raction of component i

wi = moisture content of component i.

Where b3, b2, b1, and b0 are empirical constants from

the DLP isotherm model and χ is ln(-ln(aw))

0

5

10

15

20

25

0 0.5 1

Mo

istu

re C

on

ten

t (%

w

.b.)

Water Activity

Ingredient 1 Ingredient 2

Combined

iiiii bbbbw 01

2

2

3

3

'''' 012

2

3

3bbbbw eqeqeq

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

Modeling Temperature Abuse

y = -0.4634x + 1.1543

R2 = 0.9599

-0.5

-0.45

-0.4

-0.35

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

3.15 3.2 3.25 3.3 3.35 3.4 3.45 3.5

1000/Temp (K-1)

ln(a

w)

80.0353

1

298

1463exp680.0

11exp

21

12

TTR

Haa ww

What is the water activity of Granola Bar with 25% m.c. at 80C (353K)?

Isotherms at Different Temperatures

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1

Water Activity

Mo

istu

re C

on

ten

t (%

d.b

.)

15c 20c 25c 30c 35c 40c

*Data is for Spray Dried Milk Powder

Isotherm and Phase Transitions

0

5

10

15

20

0 0.2 0.4 0.6 0.8 1

Water Activity

Mo

istu

re C

on

ten

t (%

d.b

.)

Critical Water Activity

Amorphous Glassy State

Rubbery State

Surface and Bulk

Adsorption

Surface

Adsorption

Particles are wetting and

going into solution

*Data is for Spray Dried Milk Powder

Savitsky/Golay 2nd Derivative

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0 0.2 0.4 0.6 0.8 1

Glass Transition at

0.437

Dissolution Onset at

0.731

*Data is for Spray Dried Milk Powder

DDI Method for Phase Transitions

Low %Moisture Content or aw High

Low

High

Constant T

Scan aw

Critical %MC or aw

Tg

Glassy State

Rubbery State

Amorphous vs. Crystalline

0

1

2

3

4

5

6

7

0 0.2 0.4 0.6 0.8 1

Water Activity

Mo

istu

re C

on

ten

t (%

d.b

.)Amorphous Crystalline

*Data is for Sucrose

Determining Deliquescence

Point

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 0.2 0.4 0.6 0.8 1

Water Activity

Mo

istu

re C

on

ten

t (%

d.b

.)

*Data is for Sucrose

Deliquescence

Point

Whole Vs. Ground

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1

Water Activity

Mo

istu

re C

on

ten

t (%

d.b

.)

Ground Corn Whole Corn

Water Activity as a CPP for Gel

Coating Integrity

Coatings start to stick at the point when water

penetrates the coating as indicated by the moisture

sorption isotherm.

0

2

4

6

8

10

12

14

16

18

20

0 0.2 0.4 0.6 0.8 % W

eig

ht

Ch

an

ge

fro

m I

nit

ial

Water Activity

Package Performance

vs

a

Age

Mp

= slope of the isotherm (g/g)

awo = initial water activity

awc = critical water activity

pa = atmospheric pressure (kPa)

M = total mass of product inside the package (g)

es = saturation water vapor pressure at package temperature (kPa)

A = package surface area (m2)

gv = package conductance (g m-2 s-1)

ha= Humidity of air,

WVTR = Water Vapor Transmission Rate

t = Time in package,

= Time constant

woa

wca

shelfah

aht ln

Shelf life prediction of packaging

Time Constant

Package Performance Example

y = 2.59x + 2.89

R2 = 0.97

0

2

4

6

8

10

12

14

0 0.2 0.4 0.6 0.8

Water Activity

Mo

istu

re C

on

ten

t (%

wb

)

Example: A milk powder producer wants to know more information

about his packaging material, especially under abuse conditions.

Critical aw = 0.43

Packaging Calculations

Predict shelf life in a given type of packaging

Determine package conductance

Determine required package conductance

Conclusions

Isotherms characterize the relationship between

water activity and moisture content

Different types of isotherms

DDI method is a different method for measuring

isotherms

Isotherms have many applications

VSA is a versatile instrument for isotherm

generation

Thank you

Wendy Ortman

Decagon Devices, Inc.

2365 NE Hopkins Court

Pullman, Washington 99163

Phone: (509) 332-2756 / (800) 755-2751

Fax: (509) 332-5158

Email: wendy@aqualab.com

Web: www.aqualab.com