"Measurement of
Water Activity (AW) in
Foods"
Introduction
"Understanding Water activity
is the key to food and
preservation"
It's a critical factor affecting food safety
and preservation. It is the measure of
unbound water in a product and affects
microbial growth, chemical reactions,
and physical stability.
Understanding aw is essential for
developing safe and stable food
products.
What is Water Activity?
Water activity (aw) is crucial
for food preservation and
safety.
It is defined as the ratio of the
vapor pressure of water in a
food to the vapor pressure of
pure water at the same
temperature (Karel 2021) .
Factors Affecting Water
Activity
Several factors affect aw,
including temperature, pH, and
the presence of solutes. Lowering
the aw of a food product can be
achieved by removing water or
adding solutes.
Preservation techniques such as
drying, salting, and adding
preservatives reduce aw and
extend the shelf life of food
products.
Microbial
Growth
Water activity is a pivotal factor in
regulating microbial activity in food and
other materials.
By reducing the water activity of a
product to a point where it falls below
the growth threshold for these
microorganisms, food manufacturers can
effectively inhibit spoilage and extend
the product's shelf life.
This makes water activity a cornerstone
of food preservation (Ryffel, 2019).
Food Quality
and
Texture
Water activity profoundly
impacts the texture, flavor,
and overall quality of
products.
Lower AW values are
associated with crispness
and dryness, contributing
to the desired
characteristics of
products like crackers and
dehydrated snacks
(Labuza & Riboh, 2020).
Chemical
Reactions
Water activity can
significantly influence
chemical reactions in
various products,
particularly in the food
industry.
One notable example is
the Maillard reaction, a
complex browning
reaction between amino
acids and reducing sugars
(Barbosa-Cánovas 2020) .
Shelf
Life
Water activity is a critical
determinant of a product's
shelf life. Products with
higher water activity are
more susceptible to
degradation, microbial
growth, and spoilage.
By controlling and
minimizing water activity,
manufacturers can extend
the stability and longevity of
their products (Labuza &
Riboh, 2020).
Mechanisms
and
Tools/Equipment
Used
1. Vapor Pressure
Water activity is
fundamentally based on the
principle of vapor pressure.
It involves measuring the
vapor pressure of water in a
sample and comparing it to
the vapor pressure of pure
water at the same
temperature and pressure
conditions (BarbosaCánovas 2020).
2. Hygrometers
Various types of
hygrometers are employed
to measure vapor pressure
and, by extension, water
activity. These
instruments are capable of
quantifying the moisture
content or humidity in the
surrounding environment.
Two common types of
hygrometers used in water
activity measurement include:
• Psychrometers: It measure
humidity by assessing the
temperature difference
between a dry bulb and a
wet bulb thermometer (Karel
et al., 2021). This
information is used to
calculate the water activity
of the sample.
• Hygrometers: These
advanced instruments
employ a cooled
surface to determine
the dew point
temperature, which is
then correlated with
water activity (Ryffel,
2019).
3. Moisture Sorption
Analyzers
sophisticated devices
designed to generate
moisture sorption
isotherms.
The data collected from
these sensors can be used
to assess water activity
accurately.
Examples of
Water Activity
(AW)
in Different
Foods
Bread
Freshly baked bread typically
exhibits a water activity of
around 0.95 (Labuza & Riboh,
2020).
This high AW contributes to
the soft and moist texture of
the bread. To extend its shelf
life, it's important to reduce
the water activity, which can
be achieved by storing bread
in a dry environment.
Dried Fruits
Dried fruits, such as raisins, are
known for their extended shelf
life, and this is largely due to
their low water activity values,
which typically range from 0.6 to
0.7.
The reduction in AW during the
drying process inhibits microbial
growth and spoilage, allowing
dried fruits to remain safe and
flavorful for extended periods.
Cheese
The water activity of cheese
can vary significantly
depending on the type.
Hard cheeses like Parmesan
typically have lower AW values,
often around 0.8 to 0.85
These low AW values
contribute to the hardness and
crumbly texture of hard
cheeses.
Crispy Snacks
Snacks like potato chips are
beloved for their crispiness,
which is preserved by their
extremely low water activity,
typically ranging from 0.1 to
0.2.
The minimal water content and
low AW prevent the chips from
becoming soggy and maintain
their desirable texture.
Conclusion
Water activity is a critical factor in food safety and
preservation. Understanding how to control aw is
essential for developing safe and stable food products.
Preservation techniques such as drying, salting, and
adding preservatives can reduce aw and extend the
shelf life of food products. Controlling aw is also
critical for preventing microbial growth and foodborne
illness.