www.aqualab.com
WaterActivityNews
2006
Is the AquaLab Still the Fastest
and Most Accurate?
D
ecagon’s advertising slogan for its
AquaLab water activity meters states
that they are the fastest, most accurate,
guaranteed. This slogan is based on Decagon’s claims
that for most products, water activity tests can be
done in less than five minutes, with an accuracy of
±0.003aw, and a precision (repeatability) of
±0.001aw using an AquaLab chilled mirror
dewpoint instrument. It has been
suggested that Decagon cannot achieve
the results they claim; instead the
less-than-five-minute test is
accomplished by some type of predictor
AQUALAB
SAFETY
T
Redefining
Potentially
Hazardous
Foods
he term potentially hazardous food
(PHF) was developed by the U.S. Public
Health Service during the last half of the
twentieth century to regulate perishable food. In
1962, PHF was defined as a perishable food that
supports the rapid and progressive growth of
infectious or toxigenic microorganisms. Exemptions
for foods with an aw ≤ 0.85 or pH ≤ 4.6 were added
in 1976. Thus, foods were considered non-PHF
when their aw ≤ 0.85, which is below the water
activity for Staphylococcus aureus growth and toxin
production or the pH ≤ 4.6, which is below the pH
for proteolytic Clostridium botulinum growth and
toxin production. Potentially hazardous food means
a food that has no or inadequate hurdles to
pathogenic bacterial growth and requires
temperature control for safety (TCS) to limit
pathogenic microorganism growth or toxin
formation in the food.
continued on page 6
AQUALAB
PRECISION
mode and that the reported
accuracy actually refers to the
precision of the instrument.
What is the real story? This
article clearly details the
validity of Decagon’s claims.
AQUALAB
Water Activity QUALITY
Measurement Theory
First, an understanding of how to determine
water activity is needed to understand why the
AquaLab is fast and accurate. Water activity is
continued on page 2
See AquaLab, AquaLab Lite,
& Pawkit in action at our
2006 Tradeshows!
Visit us at the following
tradeshows and
conferences in 2006:
IFT Annual Meeting and
Food Expo
Orlando, Florida
June 24–28, 2006
National Environmental
Health Association
San Antonio, Texas
June 25–28, 2006
International Association of
Food Protection
Calgary, Albert, Canada
August 13–16, 2006
American Association of
Cereal Chemists
San Francisco, California
September 17–20, 2006
Contents
Is the AquaLab Still
the Fastest and Most
Accurate? Pg 1
Redefining
Potentially
Hazardous Foods Pg 1
Dear Anthony,
Pg 4
Teaching Students
Principles of Water
Activity Pg 7
How Decagon Tailors
Training Seminars for
Their Customers Pg 8
Moisture Sorption
Isotherms Pg 10
Water Activity
Seminar Schedule Pg 11
American Association of
Pharmaceutical Scientists
San Antonio, Texas
October 29 – November 2, 2006
Decagon Devices
Sponsors AACC
Symposium Pg 12
Eastern Analytical
Symposium
Somerset, New Jersy
November 13–16, 2006
D
Newly Redesigned
Pawkit Pg 12
DECAGON
DEVICES
1
“The AquaLab v3 is highly innovative yet extremely user friendly. It
allows for accurate results in a timely manner. Decagon’s superb
customer service is readily available and eager to assist their every
customer’s needs.” —Priscilla Rangel, Laboratory Manager, Quanta Lab
AquaLab— Still Fastest and Most Accurate?
continued from cover
Why is the AquaLab Accurate
It has been suggested that since AquaLab uses these
two temperatures to determine water activity and
each can only be measured with an accuracy in the
range of ±0.10 to ±0.15 °C, the best water activity
accuracy that can be achieved is ±0.015 based on
reference to a psychometric chart. It is critical to
remember that AquaLab uses saturated vapor
pressures to calculate water activity, not a
psychometric chart. Since dividing saturated vapor
pressures allows subtracting the sample temperature
from the dewpoint temperature, the critical
measurement is the
Table 1. The differences between final a w and initial a w for 4 saturated
difference between two
salts solutions and water when tested continuously for 90 minutes.
Measuring Water
temperature means—
Final aw - Initial aw
Water Activity
Saturated
Activity
rather then
(90 minutes)
(25°C)
Solution
An electric hygrometer
measurement of the
0.0009
0.1130
LiCl
uses a relative humidity
temperatures
sensor to indirectly
themselves. AquaLab
-0.0002
0.4315
K 2 CO 3
determine the relative
measures this
-0.0005
0.7529
NaCl
humidity (vapor
temperature difference
-0.0006
0.8434
KCl
pressure) of the
with an accuracy of
headspace. This is an
0.01 or better. Hence,
-0.0005
1.0000
Water
indirect measurement
Decagon is able to
-0.0002
Average
because humidity
achieve the accuracy
a. Test time for each water activity test never exceeded 5 minutes and
sensors require a
they claim because the
tests were run until the cumulative test time exceeded 90 minutes.
calibration that relates
AquaLab measures
changes in electrical
water activity by
properties to relative humidity values of known
determining the difference between two
standards. Decagon’s LITE and Pawkit, and all
temperature means.
competitor instruments, are electric hygrometers.
AquaLab directly measures water activity by
Why is the AquaLab Fast
recording the dewpoint temperature of the air in
It also has been claimed that AquaLab cannot
the headspace and the temperature of the sample
complete a water activity test in less than 5
once vapor equilibrium has occurred. The
minutes because the equilibration process takes
dewpoint temperature is used to calculate the
a minimum of 20–30 minutes for all samples. In
saturated vapor pressure at the dewpoint
reality, vapor equilibration occurs very quickly
temperature. By definition, the saturated vapor
as evidenced by theoretical modeling and by
pressure at the dewpoint temperature is equal
observation using NMR techniques. So, it is
to the vapor pressure at ambient temperature.
thermal equilibration that requires the longest
The sample temperature is used to calculate the
time. When using an electric hygrometer that
saturated vapor pressure at the same
does not use the sample temperature as part of
temperature. Water activity is then determined
the calculation, it is necessary for the sample
by dividing the saturated vapor pressure at the More spark to be in complete thermal equilibration with
dewpoint temperature by the saturated vapor for your product the humidity sensor to obtain accurate
protection.
pressure at the sample temperature.
results. This process can take up to 20
defined as the partial vapor pressure of water in
equilibrium with a sample divided by the vapor
pressure of pure water at the same temperature. It is
often written as aw = p/po. All water activity
instruments operate under the same theory. A sample
is placed in a closed chamber and the water in the
sample is allowed to equilibrate with the water vapor
in the headspace. The rate limiting steps of the
process are the time it takes for vapor and
temperature equilibrium to occur. Once equilibrium
is obtained, some method is used to measure the
humidity or vapor pressure in the headspace above
the sample.
2
minutes or longer. However, if the sample
.
temperature is used as part of the calculation, it is
not necessary for the sample to be in complete
thermal equilibration with the sensor, only
Table 2. Precision and accuracy of water activity measurements of 4 saturated
that it be close so that the sample is not
salt solutions and water across 19 observations.
undergoing drastic temperature changes
Saturated
Water Activity
during the test. Consequently, since vapor
Precision
Accuracy
Solution
(25°C)
equilibration happens very quickly and
LiCl
0.1130
AquaLab measures the sample temperature,
0.0004
0.0004
tests can be completed in less than 5 minutes
K 2 CO 3
0.4315
0.0004
0.0010
for most products. It is possible that some
NaCl
0.7529
0.0003
0.0009
samples may require longer equilibration
times, such as those with coatings, high fat,
KCl
0.8434
0.0007
0.0008
high viscosity, glassy texture, or multiple
Water
1.0000
0.0005
0.0005
components.
Working together.
Average
How Fast is the AquaLab
Now that the theory of AquaLab has been
explained, it is time to test the theory. First, can
equilibration be achieved in less than 5 minutes?
Four saturated salts and water were evaluated
continuously in the AquaLab Series 3TE over a time
period of 90 minutes. This was accomplished using
the continuous mode setting on the AquaLab Series
3TE, which runs a test to completion, logs the data,
then immediately begins the next test. The time per
individual test never exceeded 5 minutes for any
solution. The largest difference between the initial aw
and the final aw for any solution was 0.0009, which
is less than the reported precision of the instrument.
Since there is essentially no change in water activity
from the first <5 minute test to the final test after 90
minutes, it can be concluded that for the solutions
tested, and most sample types, vapor equilibration
can be achieved in less than 5 minutes.
Accurate and Precise
The difficulty in reporting an accuracy and precision
value for a water activity instrument is that accuracy
and precision will depend on the sample being
analyzed. Standard procedure to determine the
accuracy of water activity instrumentation is to
compare its readings to literature values for
saturated salts. Since the accuracy can vary with each
solution, an average accuracy across the range of
possible water activity values may be reported.
Another difficulty lies in how to
DECAGON calculate accuracy. Precision is simply
D
0.0005
0.0006
calculated as the standard deviation across multiple
observations. However, there is no standard
calculation for determining accuracy. For this work,
accuracy was determined as the square root of the
sum of square differences between measured values
and literature values across all observations divided
by the degrees of freedom. Four saturated salts and
water representing a range of water activities were
evaluated for water activity across 19 replicated
observations per solution in an AquaLab Series 3TE.
Table 2 indicates that AquaLab’s accuracy and
precision values were very good for all 5 solutions
and were better than Decagon’s reported accuracy
(±0.003) and precision (±0.001) values. The average
accuracy across all solutions was ±0.0006 while the
average precision was ±0.0005. While the accuracy
of other types of samples may not be as good as for
these solutions since the samples themselves are not
completely uniform, the results of this study indicate
that the accuracy and precision of the AquaLab
when reading saturated salt solutions and water are
actually better than the values listed in the
specifications and certainly better than the ±0.015
that has been suggested by others.
Fastest, Most Accurate, Guaranteed
Decagon Devices claims their AquaLab water
activity instruments are the fastest, most accurate,
guaranteed. The information presented here indicates
that both in theory and in practice, they are. DEVICES
3
D
DECAGON
DEVICES
Dear Anthony,
What is the status of
Are there other uses for water activity
USP Method <1112> for
besides controlling microorganisms?
pharmaceutical products?
Yes! Water activity also influences the rate of
The Microbiology Committee at
chemical reactions, physical properties such as
USP has approved USP Method
texture, moisture migration and shelf life. Similar to
<1112> Microbiological
microbial control, the reaction rate of many chemical
Attributes of Nonsterile
reactions, that impact stability are dependent on
Pharmaceutical Products –
water
Application of Water
activity.
Activity Determination.
Water
“Thank you for such a
The method will be
may
speedy repair on our
published in the USP-NF
influence
Second Supplement in August 2006.
chemical
Pawkit meter. We
Control of the water activity in a
reactivity
appreciate everything
pharmaceutical product can be used to
by
inhibit microbial growth. Reduced water
acting
you
do
for
us.”
activity will greatly assist in the
as a
—Erin Cillessen
prevention of microbial contamination of
solvent,
Dynamic Foods
pharmaceutical products, and the
reactant,
formulation, manufacturing, and testing
or by
of nonsterile dosage forms should reflect
changing
this parameter. When formulating drug products, the the mobility of the reactants by affecting the
water activity should be evaluated so that the
viscosity of the product. Water activity influences
product may be self-preserving. To lower water
non-enzymatic browning, lipid oxidation,
activity of a food, one can remove water, add
degradation of vitamins, enzymatic reactions, protein
ingredients, or lower the temperature. Adding
denaturation, starch gelatinization, and starch
ingredients such as salt, sugars, glycols, glycerin or
retrogradation.
amino acids will result in a formulation with a lower
Since water activity describes the thermodynamic
water activity that discourages the proliferation of
energy status of the water within a system, there is a
microorganisms in the product.
close relationship between water activity and the
The new method can be used to establish a
physical stability and shelf life. Foods with high aw
microbial limits testing strategy for typical
have a texture that is described as moist, juicy, tender
pharmaceutical and over-the-counter drug products
based on water activity. Pharmaceutical drug
products with water activities well below 0.75, i.e.
“Your company is the best,
direct compression tablets, liquid-filled capsules,
nonaqueous liquid products, ointments, and others
very efficient and courteous
would be excellent candidates for reduced microbial
limit testing. This is especially true when
to work with.”
pharmaceutical products are made from ingredients
—Carol Brown, Sensus America
of good microbial quality, in manufacturing
environments that do not foster microbial
contamination, by processes that inherently reduce
the microbial content, and in manufacturing sites
and chewy. When the water activity of these
that have an established testing history of low
products is lowered, undesirable textural attributes—
bioburden associated with their products.
such as hard, dry, stale and tough—are used. Low aw
foods normally have texture attributes described as
crisp and crunchy, while at higher aw the texture
4
www.aqualab.com
changes to soggy. Also, water activity affects the
flow, caking and clumping properties of powders and
granulations.
Differences in water
activity levels between
components or a
component and the
Tune-up your
risk reduction.
environmental humidity are
a driving force for moisture
migration. Knowledge of
whether water will absorb or
desorb from a particular
component is essential to
prevent degradation, especially if
one of the substances is moisture
sensitive. For example, if equal
amounts of component A at 2% and
component B at 10% moisture
content are to be blended together, will
there be moisture exchange between the
components? The final moisture content
of the blended material would be 6%, but
did any moisture exchange between
component A and B? The answer depends on
the water activities of the two components. If the
water activities of the two components are the same,
then no moisture will exchange between the two
components. Likewise, two ingredients at the same
moisture content may not be compatible when mixed
together. If two materials of differing water activities
but the same water content are mixed together, the
Product protection is
more than luck.
water will adjust between
the materials until an
equilibrium water activity is
obtained. Therefore, water activity provides more
useful information for formulation design,
manufacturing conditions, and packaging
requirements than water content.
Finally, you can use water activity to obtain rapid
moisture content data through the moisture sorption
isotherm curve for your product. The relationship
between water activity and moisture content at a
given temperature is called the moisture sorption
isotherm. After you have developed the isotherm
curve for your product, you can use aw to obtain the
moisture content information. Thus, you will have
the complete water picture for your product in less
than 5 minutes.
Why is Decagon’s Calibration Service
important?
Decagon knows how important accurate water
activity analysis is to your company. Your AquaLab
will measure water activity of your product
accurately for years with verification of standards
and occasional cleaning. For many customers the
daily checking of water activity standards is enough
to ensure the safety and quality of their product. But
some companies need documentation to certify that
AquaLab is working within specifications. With
Decagon’s Calibration Service, you will receive
compliance documentation for either HACCP, ISO
9000, ISO Guide 25, Good Laboratory Practices, or
Good Manufacturing Practices. [ KD2 Pro
Decagon manufactures easy-to-use
instrumentation for the measurement
of thermal properties: thermal
conductivity, thermal resistivity,
specific heat (heat capacity) and
thermal diffusivity.
[ Thermal properties measurement of natural and
engineered materials.
www.thermal.decagon.com
5
continued from cover
Redefining Potentially Hazardous Foods
Growth of Infectious
Pathogens
Why was the definition of
potentially hazardous food
changed in the 2005 Food
Code? There were many
factors that went into the FDA
decision to update this
definition, including the
following reasons: the term
“rapid and progressive growth
of infectious and toxigenic
microorganisms” in the old
definition was not clearly
defined. A 0.85aw level of
safety against Staphylococcus
aureus was conservatively low.
A pH of 4.6 may be
inappropriate as Listeria
monocytogenes grows at 4.39,
Salmonella spp. at 4.2 and
Yersinia enterocolitica at 4.2,
although a pH 4.6 is
appropriate when only spore
forming pathogens are present.
The slow growth of low
infectious dose pathogens such
as Listeria monocytogenes was
not considered. Finally,
sufficient research has been
done on food products to
construct tables that predict
the growth of pathogenic
microorganisms when the
interaction of both pH and aw
are considered (See Tables A & B).
Table A. Interaction of pH and a w for control of spores in food heat-treated
to destroy vegetative cells and subsequently packaged.
aw
pH Values
Values
< 4.6
> 4.6 – 5.6
> 5.6
< 0.92
Non-PHF*/non-TCS**
Non-PHF/non-TCS
Non-PHF/non-TCS
> 0.92 – 0.95
Non-PHF/non-TCS
Non-PHF/non-TCS
PA***
> 0.95
Non-PHF/non-TCS
PA
PA
* PHF means “Potentially Hazardous Food”
** TCS means “Time/Temperature Control for Safety Food”
*** PA means “Product Assessment Required”
Table A can be used to determine if a food which is heat-treated and packaged is
PHF, Non-PHF or Requires Product Assessment. Food must meet cooking requirements
of Food Code section 3-401.11 (no partial cooks) to eliminate vegetative pathogens.
Spore forming pathogens are the only remaining biological hazards of concern. Food
is packaged to prevent re-contamination. Therefore, higher pH & a w can be safely
tolerated.
Table B. Interaction of pH and a w for control of vegetative cells and spores
in food not heat-treated or heat-treated but not packaged.
aw
pH Values
Values
< 4.2
4.2 – 4.6
> 4.6 – 5.0
> 5.0
< 0.88
Non-PHF*/non-TCS**
Non-PHF/non-TCS
Non-PHF/non-TCS
Non-PHF/non-TCS
0.88 – 0.90
Non-PHF/non-TCS
Non-PHF/non-TCS
Non-PHF/non-TCS
PA***
> 0.90 – 0.92
Non-PHF/non-TCS
Non-PHF/non-TCS
PA
PA
>0.92
Non-PHF/non-TCS
PA
PA
PA
* PHF means “Potentially Hazardous Food”
** TCS means “Time/Temperature Control for Safety Food”
*** PA means “Product Assessment Required”
Table B can be used to determine if a food which is not heat-treated or heat-treated but not packaged is
PHF, Non-PHF or Requires Product Assessment. Food not heat-treated may contain vegetative cells and
pathogenic spores. Food that was heat-treated but not packaged may become re-contaminated. pH values
considered in Table B must include 4.2 because Staphylococcus aureus can grow at that level.
Interaction of a w and pH
The 2005 version of the Food Code updates the
definitions of PHF by using a scientific based criteria
that considers the interaction of aw and pH in
determining if a food is designated as a non-PHF/
non-TCS food. Interaction Tables A & B consider
the interaction of aw and pH under certain conditions
of heat-treatment and packaging. The hurdle effect is
used to control or eliminate pathogens that would
otherwise be ineffective when used alone. The effect
of a heat treatment which destroys vegetative cells
and the effect of packaging which prevents
re-contamination is considered.
6
Simplifying Food Safety
In the United States, most states have adopted the
Food Code definition of potentially hazardous foods.
Because of the complexity in the US food systems
and the confusion over the old definition of
potentially hazardous foods, this new science-based
definition offers a more accurate and easily defined
assessment for the safety of food products. This
definition relies solely on pH and aw as the
parameters for making decisions about the need for
time/temperature control for safety. www.aqualab.com
Teaching Students Principles of Water
Activity —the Humble Twinkie
I
f you want to get a child’s attention, start talking
about foods they like- especially sweet, sugary foods
like Twinkies. Twinkies can teach kids a lot about
food science—specifically water activity.
K-12 Food Science Awareness
In an effort to promote K-12 education of food science,
Decagon has begun teaching teachers the importance of
water activity. Similar work is being done by the Institute
of Food Technologists (IFT). IFT is promoting food
science awareness in an effort to increase the number of
students choosing to get a degree in Food Science.
Because food is integral to life, teachers have been using
Twinkies to demonstrate shelf-life for years. One teacher
we spoke with hangs a new Twinkie from the ceiling of
her room at the beginning of each school year and they
observe it all year long to see if it will grow mold. Now
that she understands water activity, she knows why there
is no mold or microbial growth during the school year.
Interested in teaching students about
water activity?
Email to get a free water activity sample lesson or learn
more about Decagon’s lab books suitable for educators
grades 10 through collegiate. DA7200 for Water Activity
DA7200 utilizes
NIR spectroscopy to
determine the
activity of the water
in the sample.
New NIR approach to fast
water activity measurement.
Rapid
Analysis of
Water Activity
Easy
Sample Presentation
Non-Contact
and Non-Destructive Analysis
Non-Moving
Optics
Call Now.
DECAGON
DEVICES
950 NE Nelson Court
Pullman, Washington 99163
800-755-2751
fax 509/332-5158
[email protected]
www.aqualab.com
7
Your food product is only as safe as
it’s weakest link.
“We had no experience with water activity at all…”
How Decagon Tailors Training Seminars for Their Customers
A
t the Maryland Environmental Safety
Lab, they don’t measure water activity
every day. They analyze a variety of
samples for the Maryland Department of Health and
Mental Hygiene (DHMH), local health departments,
and concerned citizens, using many different lab
instruments. Doing food safety tests for the food
control program in the Health Department is just
one of their jobs. In fact, they might only use
AquaLab once a week. But they need to be confident
that their water activity readings are accurate.
Tailored to Your Needs
The seminar is tailored to the needs of the specific
customer. According to Brady Carter, Decagon
applications engineer, that can mean anything from a
three hour discussion of water activity applications
to a highly technical 8 hour short course. “Of course
we cover the basics, but we fine-tune the program to
address specific problems and issues,” he says. In the
DHMH seminar, that meant focusing on
understanding water activity as an indication of food
safety and accurately measuring the water activity of
a variety of samples.
On-site Installation and Seminar
When they started measuring water activity, they
didn’t know much about it. “It was a brand new
test,” according to Kalpana Hegde, who does much
of the analytical testing in the lab. “We had no
experience with water activity at all.” Some people
with extensive water activity experience might feel
comfortable just pulling the instrument out of the
box and getting started with measurements. DHMH
opted instead for a more complete introduction
through the on-site installation and seminar
program.
Process Standardization
Kalpana uses the seminar information
every time she tests a sample. “I use
very homogeneous samples at room
temperature. I run verification
standards to check for linearity and Plan for a
use distilled water for a blank at the rainy day.
beginning and end of the sample run.
In between, I run one or two
standards to make sure it’s running right. It gives
accurate and precise data.”
Peace of Mind
That means instead of an instrument in a box,
DHMH got on-site setup and training, an in-house
seminar tailored to their particular lab, a year’s
worth of sample cups and standards, an AquaTherm
temperature equilibration tray, three year’s worth of
verification and calibration—and peace of mind.
“It was helpful to get all the information at the same
time we got the instrument,” said Kalpana. “They
covered all the details about the meter—how to use
verification standards, how to keep the meter clean,
how to use temperature control. I use a new cup for
each test, make sure each cup is clean externally and
internally—I take so much care, I’ve never had a
problem with keeping the meter clean. When I run
my standards, it’s right on there.”
8
A Little Theory
DHMH food chemistry also found the theoretical
aspects of the seminar helpful. As Delores Willis,
program manager, notes, “We just do the testing. We
don't make the decision as to why the sample should
be tested. We’re kind of the middle person.” Like
many AquaLab purchasers, the DHMH lab staff
found both the AquaLab instrument and the concept
of water activity new.
Everyone needs it but why?
According to Brady Carter, most people involved
with food safety know they need to measure water
activity—frequently they’ll find it built into product
specifications—but they don’t know exactly how or
why. During the seminar, they'll realize all the
“I’ve never had a
problem with
keeping the meter
clean.”
applications for water activity—everything from
predicting moisture migration to controlling quality
factors like clumping, caking, and texture.
Product Discovery
“People realize, ‘Oh! We could be doing this!’,”
Carter says. “One cheese manufacturer discovered
that he could be predicting the water activity of a
finished product based on the water activities of the
ingredients he was adding. He was thrilled to think
that he could tweak the recipe to hit a certain water
activity without ever mixing up a batch of product.”
Understanding Safety Monitoring
The food lab at DHMH is not involved with quality
issues. “We use AquaLab to analyze food samples—
both raw and finished products—to check for shelf
life. It’s routine safety monitoring,” explains Delores.
Most of their samples come from the food control
inspectors in the Health Department. They are asked
to analyze everything from home-canned jams and
jellies sold at a local farmer's market to hams
smoked and sold by a fire station as a fund raiser.
But Kalpana still found it helpful to understand the
concepts of water activity.
“It makes a big difference to understand what you're
actually looking for,” she says. “It gives me
confidence in the testing.”
Learn More
The seminar option is also available to customers
who already own the instrument. If you
are interested in finding out more
about an in-house seminar,
contact Decagon’s
application engineers at
1-509-332-2756.
Get a different
perspective on
water activity
and food
protection.
From the Sales Desk:
T
he sales department at Decagon
is dedicated to the instruments
we sell and the customers who
buy them. Occasionally, we hear
someone say something that
gives us pause. Recently a customer said
that our instrument was “delicate”. This term
is not one we generally hear when
discussing the AquaLab. After further
investigation, we found out that this user
was measuring a product that is very sticky
and causes a lot of problems if the user is
not cautious when presenting the sample to
the instrument. They felt it was delicate
because of the maintenance required to keep
the block in working order. To this problem
the sales department presents a simple
solution; never fill the cup over half full and
wipe the rim of the cup before sampling.
These two quick, easy steps can save a lot of
time in cleaning. A misnomer exists that
suggests the more product, the faster or
more accurate the reading; however, the
AquaLab only requires enough sample to
cover the bottom of the cup.
Another customer sent in a meter that they
had affectionately named the “boat anchor”.
Best guess suggested that the instrument
was not reading correctly, took too long to
read, or was unstable. We tell all of our
customers that cleaning will solve over 90%
of the problems that they may have. If you
are unsure how to clean it, give us a call. We
can walk you through the cleaning process or
we can send you a CD-ROM that contains
videos on proper cleaning procedure.
Your AquaLab is a highly accurate piece of
scientific instrumentation that will provide
years of accurate and reliable service if you
care for it properly. As cautious consumers
we baby our televisions, coddle our cars and
do our best to protect our personal items.
Your AquaLab should receive the same care
and attention that you provide your other
investments.
9
Moisture Sorption Isotherms
he relationship between water activity and
moisture content at a given temperature is
called the moisture sorption isotherm. This
relationship is complex and unique for each
product due to different interactions (colligative,
capillary, and surface effects) between the water and
the solid components at different moisture contents.
An increase in aw is almost always accompanied by
an increase in the water content, but in a nonlinear
fashion. Moisture sorption isotherms are sigmoidal
in shape for most foods, although foods that contain
large amounts of sugar or small soluble molecules
have a J-type isotherm curve shape. There is a
difference (called hysteresis) in the adsorption
(wetting) and desorption (drying) isotherm curves.
T
vs. dry to a moisture content. The moisture content
associated with a safe water activity on the drying
curve will not be associated with the same water
activity on the wetting curve due to hysteresis. If the
production technician controlling drying of a
product over-dries beyond the specified moisture
content and decides to re-wet the product back to
the moisture content specification, the results could
be disastrous. The product’s response to moisture is
no longer following the drying curve, but has
switched to the wetting curve. The moisture content
specification may correspond to a safe water activity
on the drying curve, but on the wetting curve, the
moisture content may be related to a very unsafe
water activity level.
Automatic Isotherm Generator
Decagon is pleased to announce the development of
an automatic moisture isotherm generator. It will be
available for a much lower price than currently
available isotherm instruments. The user will place
their sample into the isotherm chamber, set test
parameters, and then walk away. In one to three
days, a complete isotherm curve (both adsorption
and desorption) will be generated. It will be possible
to choose from several isotherm equations to
characterize the isotherm graph. The isotherm
coefficients will be displayed on the screen and
transferable to an additional application software
package. This instrument will facilitate sorption
isotherm development for any company that would
like to better understand the water in
their product.
Predicting Moisture Content from Water
Activity
An additional function of the isotherm is moisture
content prediction. Although water activity is a
much better predictor of safety and quality than
moisture content, there are times when it is necessary
to know both water activity and moisture content.
Moisture content measurements can be inaccurate,
time-consuming, and require a precision balance.
Thankfully, the sorption isotherm can be used to
predict moisture content based on water activity,
usually with better accuracy than actually running a
moisture content analysis and in much less time.
Thus, you will have the complete water picture for
your product in less than 5 minutes.
10
18
Moisture Content (%)
Drying and Wetting Curves
Moisture sorption isotherms provide
valuable information to a company.
For anyone who dries or wets their
product, the sorption isotherm serves
as a drying and wetting curve. It
shows the water activity of a product
when wetted and dried to a specific
moisture content. For anyone who
still relies on a moisture content
specification instead of a water
activity specification, the sorption
isotherm will clearly illustrate the
potential problems that can occur
when you wet to a moisture content
Moisture Sorption Isotherm
16
14
12
10
8
6
GAB
Isotherm
Generator
Data
4
2
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Water Activity ( a w)
0.8
0.9
1.0
Don’t be shortsighted when
protecting your products and
customers.
Isotherms Aid in Product Development
Isotherms are very valuable for formulation and
product development. By comparing the isotherms of
different formulations, it is possible to determine if a
product can be adjusted to allow higher moisture
content at a given water activity or a lower water
activity at a given moisture content. The result can
be a moister product that is still shelf stable. For
those producing multi-component products, it is
possible using the isotherms of the two components
to determine what the final water activity will be of
the mixture without actually making the product.
For dried products, the isotherm will predict the
moisture content of the product when it is dried to a
shelf stable water activity level.
Shelf-Life Prediction
Finally, sorption isotherms are valuable for shelf life
prediction. A product’s isotherm can be used to
determine package requirements depending on the
products sensitivity to moisture gain or loss.
Products with a crystalline type isotherm will be less
susceptible to caking than products with an
amorphous type isotherm. However, the water
activity of a crystalline material will increase very
quickly until the saturation point is reached—as
evidenced by the isotherm. Temperature can also
impact shelf life; sorption isotherms conducted at
different temperatures help determine the impact of
temperature abuse on moisture content and water
activity.
Affordable Isotherms
Isotherms are very valuable to anyone concerned
with product quality and safety. With the release of
Decagon’s affordable isotherm generator, it will be
possible for many more companies to begin
developing isotherms for their products. For more
information about isotherms and isotherm
generation, contact any of Decagon’s Application
Engineers. Water Activity
Seminar Schedule
for 2006
Dates and Venues
Princeton, NJ
October 10 (Food Seminar)
October 11 (Pharmaceutical Seminar)
Chicago, IL
October 12 (Food Seminar)
What are the water activity seminars all
about?
Decagon’s one-day short course on the fundamentals
of water activity will help you understand how the
water in your product relates to quality, shelf-life,
and safety. You will learn practical uses and
applications of water activity in product
formulation, processing, and storage. The concepts
will be illustrated with examples from the food and
pharmaceutical industry.
Register Now
$179 for one person.
$159 for two or more attendees.
www.decagon.com/seminar
Time to get
together and talk
about protecting your
products.
D
DECAGON
DEVICES
11
www.aqualab.com
From the President’s Desk
I recently read in Food Processing Magazine that the
#1 concern of food processors is food safety. Decagon can
help you with this concern. Our fast, accurate water
activity meters give you information that you can use to
assure food safety and improve product quality. We hope
this newsletter illustrates how measuring water activity
can benefit you!
You can contact me personally by e-mailing
[email protected] to offer your comments about this
newsletter or any of our products.
Sincerely,
Tamsin Campbell
President
Decagon Devices
Sponsors AACC
Symposium
Water Activity:
The More Important Water Analysis
T
his symposium will introduce the concept of
moisture management and the role of water
activity as a moisture management strategy. It will
cover a description of water activity, why it is important
for sweet and salty cereal foods, and how it differs from
moisture content. The impact of water activity on product
shelf life will also be discussed, specifically its use in
predictive models. In addition, the importance of
considering water activity during product development
will be discussed. Finally, water activity measurement
techniques will be discussed with a special emphasis on
the new concept of measuring water activity using near
infrared (NIR) spectroscopy.
Symposium Highlights
Strategies for Moisture Management
Shelly Schmidt, Ph.D.
What is Water Activity?
Anthony Fontana, Ph.D.
Shelf Life Prediction using Predictive Models
Cindy Stewart, Ph.D.
Water Activity in Product Development
Yang Kou, Ph.D.
Newly Redesigned
Pawkit
Water Activity Measurement
Brady Carter
Water Activity by NIR
Wes Shadow
D
uring the past year, Decagon completed a
redesign of the Pawkit portable water activity
meter. Easier-to-use than ever, the new Pawkit
features a robust case design and user-friendly sample
insertion while maintaining Decagon’s speed, accuracy,
and reliability in water activity measurement. This
lightweight, space-saving device is ideal for any on-the-go
health inspector, QC professional, or food scientist who
needs to quickly measure water activity and make
decisions in the field.
©2006 DECAGON
PRINTED IN USA
D
DECAGON
DEVICES
Decagon Devices, Inc.
950 NE Nelson Court
Pullman, WA 99163 USA
1-800-755-2751
509-332-2756
Fax: 509-332-5158
[email protected]
Contact us today to find out about trading in an existing
Pawkit for credit towards the new Pawkit. 12
www.decagon.com