Water activity is often called “free” or “non-chemically bound” water in foods and other products. Although these terms are
easier to understand, they do not define all aspects of the concept of water activity.
The correct definition
“Water activity – a measure for the energy status of the water in a system” specifies
the relationship between the water vapor pressure of a product and the saturation
pressure of pur e water at the same temper ature. It is state d in “aw” in the range 0…1 aw
and is an important indicator for product quality in the industrial production of, for
example, plastics. The determination of water activity is also very important in the
food, tobacco, pharmaceutical and cosmetics industries. Water activity should not
be confused with the water content – the “chemically bound” water – of a product.
Tex tureTasteColorNutritional valueShelf life
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Water activity influences the following properties of a product:
• microbiological stability
• chemical stability
• enzymatic stability
• color, taste and nutritional value
• protein and vitamin content
• stability of the composition
• shelf life
• storage and packaging
• solubility and texture
Contents
Introduction to water activity 2
Applications 3
Product overview 4
Products 5 - 8
Software 9
Calibration 10
Theory 11
Rotronic worldwide 12
2
Rotronic – Water activity measurement
APPLICATIONS
All forms of life depend on water. Water activity indicates the amount of water which is biologically available to microorganisms. Each
species of microorganism (bacteria, yeast, mold…) has a minimum water activity value below which growth is no longer possible.
Water activity plays an important role for product quality in
various fields, including:
• Industrial manufacturing
• Food industr y
• Pharmaceutical and Cosmetics industry
• Tobacco industry
• Seed storage
The US Food and Drug Administration (FDA) has adopted the concept of water activity for establishing limits beyond which certain
types of foods are considered susceptible to mold and bacteria and have established the following table indicating which process
control procedure can be used. It specifies pathogens of concern and control methods for various product categories (including
examples of foods that may need to be evaluated for time/temperature control needs for safety).
Product category (examples of
possible foods for evaluation)
Meats and poultry
(fermented sausage)
Fish and seafood (smoked fish)Vibrio vulnificus, Vibrio parahaemolyticus, Vi-
Cereal grains and related products
(fresh pasta, foccacia bread)
Pathogens of concernTypes of process control 1
Clostridium botulinum5 and Clostridium
perfringens, Salmonella spp., enterohemmorr-
hagic Escherichia coli, Camplylobacter...
brio cholerae, C. botulinum5, L. monocytoge-
nes, Salmonella spp., Shigella spp., S. aureus
Salmonella spp., S. aureus, B. cereus, C.
botulinum5
Water activityContaminant
aw = 0.91...0.95Many bacteria
aw = 0.88Many yeasts
aw = 0.80Many mildews
aw = 0.75Halophile bacteria
aw = 0.70Osmiophile yeasts
aw = 0.65Xerophile mildew
(alone and in combination)
Time/temperature, pH, aw, preserva-
tives, moisture protein ratio, fermen-
tation, heat processing
Time/temperature, harvest site control, fermentation, pH, aw, water-pha-
se salt, preservatives, drying, salting
Cooking, aw, pH, preservatives, time/
temperature
The water activity (aw) in foods can be controlled by using various additives (humectants), using satisfactor y packaging materials
and by maintaining favorable maturation and storage conditions. Should too much water be available, there is a risk of microbial
growth and water migration. Food manufacturers today must prove to the FDA that the water activity of a product has been re-
duced sufficiently so that bacteria can’t grow.
The water activity (aw) of a product will always tr y to reach equilibrium with the surrounding atmosphere. Water will migrate inside
the product from regions with a high aw value to the regions of low aw. Water will migrate until equilibrium is reached! Therefore
it is crucial to measure the water activity level of all components of a product as well as the ambient atmosphere to ensure the
quality of a product. Water migration may cause the following issues: clumping, change of texture, reduced shelf life.
Water activity data may be used when deciding whether a product requires testing or not. The United States Pharmacopeia (USP)
<1112> states that pharmaceutical drug products with water activities well below 0.75 would be excellent candidates for reduced
microbial limit testing for product release and stability evaluation. Proposals exist stating that products with a water activity of
0.6 or less would not require routine testing for objectionable organisms.
Rotronic – Water activity measurement
3
PRODUCT OVERVIEW
Rotronic offers a complete range of products for measurement of water activity. The instruments are accurate and boast high
efficiency, compatibility and simple calibration. Combine the measurement heads, insertion probes, benchtop display units and
handheld instruments as you need.
Product overview
Measurement unit
Display unit
Stationar y
Portable
Interchangeable probe
connector
Probe connector fitted
USB port/connector
Ethernet inter face
AwTherm
HygroLab C1
HP23-AW-A
HC2-AW-USB
HC2-AW-USB-SW
HC2-AW
HC2-P05
HC2-HP28
42
HC2-HP50
AwQuick function
Includes PC software
HW4-P-Quick
Dew and frost point
calculation
HW4 compatibility
Temperature controlled
4
Rotronic – Water activity measurement
PRODUCTS
AwTher m
Temperature-stabilized measurement
The AwTherm is a professional high-end laboratory analyser for temperature-stabilized measurement of water activity in the food,
pharmaceutical, cosmetics and other industries.
The wide control range permits measurements to be integrated directly in the temperature controlled manufacturing or storage
process. Water activity measurement reacts ver y sensitively to variances in temperature. Stabilizing the temperature prevents im-
precise results due to external temperature influences. Tested at 25 °C, AwTherm therefore fulfills the requirements of ISO 21807
(Microbiolog y of food and animal feeding stuffs – Determination of water activity), which requires measurement at 25 °C.
Order code
AwT herm-PS14
AwT herm-PS40
AwT herm-SET
AwT- MHS
AwT-PS14
AwT- PS40
!
Interchangeable measurement head
A further advantage of AwTherm is the removable measurement head, which means
that the probe can be temperature calibrated or adjusted to achieve high precision.
User friendliness
In stand-alone use, the AwTher m excels with its easy handling and clarity and, together
with the Rotronic HW4 software (FDA 21 CFR part 11 compliant), should be available in
any laboratory.
Features
• Accuracy: ±0.005 aw, ±0.1 K
• Temperature control range: 0...60 °C
• High temperature stability: ±0.01 °C / min
• Measurement repeatability: 0.002 aw
• Variable sample container sizes: 14 / 40 mm
• Interchangeable reference probes for cleaning or calibration
• Aw Quick function for fast measurement results
(typically 4-5 minutes after reaching a stable temperature)
• Aw Equilibrium method available
AwT-MHS: Interchangeable measurement head
Fully automatic aw measurement can be planned if the AwTherm is connected to the HW4 software. The instrument then moves to preset temperature points automatically and carries out a
pre-selected Aw Quick or Aw Equilibrium measurement.
Rotronic – Water activity measurement
5
PRODUCTS
HygroLab C1
Laboratory application
The HygroLab C1 is a laboratory analyzer for water activity measurements with up to four interchangeable measurement heads.
The possibility to combine measurement heads and insertion probes makes the HygroLab C1 extremely flexible.
Order code
HygroLab C1
Hygr oL ab C1-SET-40
Validated software
The HygroLab C1 is easy and intuitive to use. It offers a high degree of user friendli-
ness and the possibility to connect the benchtop display unit to a PC running Rotronic
HW4 software (included in the SET) using a USB cable and via the Ethernet.
The HW4 sof tware can be used to configure instruments and as a remote control for
the complete system.
Features
• 4-channel benchtop display unit for the measurement of water activity,
relative humidity and temperature
• Aw Quick function for fast measurement results (typically 4-5 minutes)
• Aw Equilibrium method available
• Multi-channel display
• Suitable for applications throughout industr y
• Audible alarm to indicate completed aw measurement
HygroPalm23-AW-A
Mobile application
As a mobile laboratory unit, the HygroPalm HP23-AW-A is perfect for on-site water activity measurements to determine the sta-
bility and preservability of the sample being analyzed. Like the HygroLab C1, the HP23-AW-A can also be used with measurement
heads or insertion probes (two can be connected). The HygroPalm can be controlled externally using the HW4 software running
on a PC.
Order code
HP23-AW-A
HP 23 -AW-SE T-14
HP23-AW-SET-40
6
Rotronic – Water activity measurement
Portable and handy
The handheld instrument for measurement of water activity has the same functions
as the HygroLab C1, but is handier in its portable version.
Features
• 2-channel handheld instrument with multi-channel display for measurement
of water activity, relative humidity and temperature
• Aw Quick function for fast measurement results (typically 4-5 minutes)
• Aw Equilibrium method available
• Audible alarm to indicate completed measurement
• Saves up to 10,000 measured values
• Battery charging function via a USB mains adapter cable (AC1212)
Aw measurement head HC2-AW
PRODUCTS
The aw measurement head was developed for use in combination with the HygroLab C1
and the HP23-A-AW handheld analyser. This digital probe enables quick and easy
measurement of water activity. It can be calibrated via the HygroLabC1, HP23-A-AW
or via the HW4 PC software.
The sample holder size is reduced to a minimum, thereby making it possible for
humidity equilibrium to be reached quickly for all products being tested, while the
metal housing ensures high temperature stability. All critical surfaces are made of
chrome steel for minimal soiling.
You can find information on sample holders, disposable plastic sample containers
and the clamp sealing mechanism under Accessories on the Rotronic website and in
the main catalogue.
Aw measurement head HC2-AW-USB
This USB measurement head can be connected directly to a PC and comes in a set
including software or as a single instrument for addition to a system.
The set HC2-AW-USB-SW contains the HW4 software with AW Quick function,
thereby opening up all the possibilities of water activity measurement. It can be ex-
tended into a multi-station application with up to 64 measurement heads used with
a powered USB hub.
Order code
HC2-AW
Order code
HC2-AW-USB
HC2-AW-USB-SW
Features
• Measurement range: 0…1.00 aw (0…100 %RH), -40…85 °C
• Direct USB PC connection
Clamp sealing mechanism
Under certain circumstances additional mechanical sealing of the AW measurement
station and sample holder may be necessary to prevent external conditions influenc-
ing the sample. The AW-KHS ensures a strong mechanical seal and is compatible with
the WP-40 and WP-40TH sample holders.
Order code
AW-KHS
Rotronic – Water activity measurement
7
PRODUCTS
AW insertion probes HC2-P05, HC2-HP28, HC2-HP50
Direct measurement
This probe is suitable for direct measurement of water activity in bulk material sam-
ples such as powder, granulated materials, corn and grain. The HC2-HP is equipped
with a robust stainless steel probe with a diameter of 10 mm and interchangeable
sinter steel dust filter for measurement in dusty bulk materials. The HC2-P05 is the
ideal measuring instrument for dust-free applications such as tablets, gel capsules
and granulated plastics.
Features
• 5 mm insertion probe, for water activity measurement
in dust-free bulk materials (P05)
• 10 mm insertion probe for water activity measurement
in dusty bulk materials (HC2-HP28 and HC2-HP50)
Water Activit y sets
The different aw start sets contain ever ything needed
to measure water activit y and calibrate the measuring
instruments to confirm sensor performance.
HP23-AW-SET-40
Order code
HC2-HP05
HC2-HP28
HC2-HP50
HP28
Order code
HP 23 -AW-SE T-14
HP23-AW-SET-40
HygroLab C1-Set-40
AwT herm-Set
P05
HP50
Sample holders and disposable sample containers
Sample holders and disposable sample containers
The sample holders ensure optimal temperature stability for the product. The WP-40TH
sample holder can be combined with a water jacket for additional stabilization of the
temperature.
The disposable sample containers (PS-14 and PS-40) ensure optimal utilization of
the volume in the sample holder. They prevent the sample holders from coming into
direct contact with the product being tested, thereby preventing soiling or cross con-
tamination. The disposable sample containers also provide a convenient means of