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Characterizing Moisture Relations in Pet Food Formulations
By: Brady Carter
Introduction
Managing water is critical to ensuring the safety
and stability of pet food. Traditionally, formulators
have tried to control the moisture content (total
amount of water) in the recipe, but while moisture
content provides valuable information about pet
food quality, it is only one part of a complete
moisture analysis. Water activity is another
important moisture measurement that provides
essential information about the energy or
availability of water in a pet food. Understanding
the relationship between these two important
moisture measurements is the key to formulating
pet food with optimal safety and quality attributes.
Moisture Content
Moisture content is the total amount of water in a
product and is determined using many different
techniques such as Karl Fischer, loss on drying,
microwave, and NIR. It is a common measurement
in most labs and provides information about
nutritional labeling, concentration of solids, product
texture, and product weight.
Water Activity
Water activity measures the energy status of the
water in a product. It is equal to the relative
humidity of the air in equilibrium with a sample in a
sealed chamber. It ranges from 0 for a perfectly dry
sample to 1 for pure water. Water activity
measurements provide valuable information about
pet food safety and quality because they indicate
susceptibility to microbial spoilage, chemical
degradation, texture changes, and inhibited flow
properties.
Moisture Sorption Isotherms
The relationship between moisture content and
water activity is complex. An increase in moisture
content is usually accompanied by an increase in
water activity but the correspondence is not linear.
This relationship between water activity and
moisture content at a given temperature is called the
moisture sorption isotherm (Figure 1). The nature of
this relationship depends on the interaction between
water and other ingredients and provides valuable
insights into product characteristics. The amount of
water vapor that can be absorbed by a product
depends on its chemical composition, physical-
chemical state, and physical structure. The isotherm
shape is unique to each product type due to
differences in capillary, surface, and colligative
effects. For most foods, the isotherm is sigmoidal in
shape, although foods that contain large amounts of
sugar or small soluble molecules have a J-type
isotherm curve.
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Water Activity (P/Po)
Moisture Content (% d.b.)
Figure 1. Typical moisture sorption isotherm for dry dog food
at 25° C.
Measuring Sorption Isotherms
Constructing a moisture sorption isotherm involves
collecting a range of water activities and
corresponding moisture contents for a particular
sample. One of three isotherm methods is typically
used. For most sample types, the three methods
provide similar results (Figure 2). However, for
samples that experience a phase change during
sorption measurement or have slow diffusion
properties, the results may vary.
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Water Activity
Moisture Content (% d.b.)
Figure 2. Corn starch working isotherms when using
desiccators with saturated salts (■), Proximity Equilibration
Cell (●), DVS instrument 1 (), DVS instrument 2 (▲), and
DDI () (DDI data from Decagon Devices in-house testing,
data for all other methods taken from (Xin Yu, 2007).
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Traditional Desiccator Method
Traditional isotherm methods equilibrate the sample
to known water activities and then measure the
equilibrium moisture content of the sample. Most
methods control water activity levels using
saturated salt slurries, acid solutions, glycerol
solutions, or mechanical humidifiers. This is most
easily done by placing the sample in a sealed
chamber over a saturated salt slurry in excess.
Different water activity levels are achieved by using
different salts. Different concentrations of glycerol
or acid solutions can also be used. Another method
controls water activity by adjusting a mixture of wet
and dry air while monitoring the water activity with
a sensor. Equilibrium moisture contents are then
determined at each water activity level. Equilibrium
is reached when the weight of the sample stops
changing. This equilibration process can take
weeks. The temperature must be tightly controlled
during equilibrium and steps must be taken to
prevent microbial contamination at water activities
higher than 0.60.
Dynamic Vapor Sorption
Some instruments are programmed to automatically
change the water activity in a dynamic stepwise
progression. These instruments, often referred to as
controlled atmosphere balances, utilize the
Dynamic Vapor Sorption (DVS) method. The
instrument holds the sample at one water activity
level until the sample weight stops changing and
then dynamically moves to the next water activity.
Instrument temperature is held constant. Humidity
levels are usually controlled by mixing dry and wet
air.
Automatic isotherm generators are much faster and
less labor intensive than traditional desiccator
methods. They also make it possible to conduct
sorption kinetic studies. However, like traditional
dessicator methods, DVS instruments equilibrate
the sample to a known water activity level. Since
true equilibration between the sample and the vapor
source requires an infinitely long period of time,
they measure apparent equilibrium at the point
when the change in sample weight is negligibly
small. Increasing the tolerable weight change can
speed up the isotherm process but calls into
question the validity of the water activity values.
Dynamic Dew Point Isotherm Method
The Dynamic Dewpoint Isotherm (DDI) method
directly measures water activity while
gravimetrically tracking weight, so there is no
dependence on equilibration to known water
activity levels to determine water activity.
Adsorption occurs as saturated wet air is passed
over the sample. Desorption is accomplished as
desiccated air is passed over the sample. After
roughly a 0.015 change in water activity, airflow is
stopped and a snapshot of the sorption process is
taken by directly measuring the water activity and
the weight. Since the sample does not have to wait
for equilibration to a known water activity, this
method is faster without sacrificing accuracy. It is
also able to produce an unmatched number of data
points. Only water and desiccant are needed to run
the isotherm.
Uses for Moisture Sorption Isotherms
Moisture sorption isotherms can help pet food
formulators achieve specific qualities and attributes
(Bell and Labuza, 2000).
Drying And Wetting Curve
For any producer who dries or wets a pet food
product, the sorption isotherm serves as a drying
and wetting curve and provides information about
the moisture content of a product when dried or
wetted to a specific water activity. It can be used to
assist in process control by determining drying rates
and optimal endpoints.
The isotherm will also show whether a product
exhibits hysteresis and what impact that will have
on water activity after drying to a given moisture
content. Hysteresis describes a condition when the
moisture content at a given water activity depends
on the wetting or drying history of the sample. In
almost all cases, if hysteresis occurs, the moisture
content will be higher at a given water activity for
desorption than for adsorption. The cause of
hysteresis remains unclear, but the practical
implications are important. In a product that
exhibits hysteresis, a moisture content that is safe
when drying a sample (because it corresponds to a
safe water activity of 0.6 aw or below) may not be
safe when wetting the sample (because it
corresponds to an unsafe water activity of above
0.70 aw) (Figure 3).
A working isotherm represents the drying and
wetting characteristics of a product from its native
state and is a scanning curve that connects the
adsorption and desorption curves. It most correctly
describes the changes experienced by a product
when it is wetted or dried from its native state.
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0
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Water Activity (P/Po)
Moisture Content (% d.b.)
Figure 3. A working isotherm (♦) superimposed over a full
isotherm curve (■) indicating that the working isotherm scans
between the adsorption and desorption curves of the full
isotherm. Also shown are the different water activities
associated with 7% moisture depending on if the product is
wetted or dried.
Moisture Content Prediction
An additional function of the isotherm is moisture
content prediction. Since both water activity and
moisture content are needed in certain situations, it
would be advantageous to measure both
simultaneously. In addition, moisture content
measurements can be inaccurate, time-consuming
and require expensive instrumentation. As an
alternative to moisture content measurement
methods, the sorption isotherm can be used to
determine moisture content based on water activity,
usually with better repeatability than actually
running a moisture content analysis and in much
less time. This requires determination of a product’s
isotherm, characterization of the isotherm by a
model or equation, and then loading that model into
a specialized water activity instrument.
Temperature Abuse Modeling
Isotherms can be used to determine the effect of
temperature on a product’s water activity and
moisture content. A product with a safe water
activity at room temperature may become unsafe
under temperature abuse conditions. Isotherms
conducted at several different temperatures will
show the temperature at which a product in a sealed
package (at constant moisture content) will be at
unstable water activity levels. For example, soft dog
bits at 25% moisture content will have a water
activity (0.60) at 15°C that will not support mold
growth; however, at 40°C, the water activity will be
high enough (0.70) for mold to grow (Figure 4).
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Water Activity (P/Po)
Moisture Content (% d.b.)
Figure 4. Moisture sorption isotherms of soft dog bits
conducted at 15°C (♦), 25°C (■), and 40°C (▲).
Formulating For Water Activity Control
Isotherms can be 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.
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 water will adjust between the materials
until an equilibrium water activity is obtained.
Thus, to prevent moisture migration in a multi-
component product, one should match the water
activity of the two components. A great example of
this type of product is dog food with hard kibbles
and soft bits. The kibbles and bits have very
different moisture contents, and hence very
different textures, but the same water activity
(Figure 5). This provides a product with variety that
is still shelf stable.
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Water Activity (P/Po)
Moisture Content (% d.b.)
Figure 5. Moisture sorption isotherms for hard kibbles (♦), one
type of soft bits (■), and another type of soft bits (▲). The line
at 0.70 aW represents the natural water activity of the product.
Dry Ingredient Mixing
For those producing multi-component products, it is
possible using the isotherms of the two components
to determine what the final water activity of the
mixture will be without actually making the
product. The equation for this process is:


sii
siii
Wb
Wba
eq
a
where aeq is the initial aW of each ingredient (i), bi is
the absolute value of the moisture isotherm slope of
each ingredient, and Ws,i is the weight of dry solids
of each ingredient (Bell and Labuza, 2000).
Shelf Life Determination
Sorption isotherms are valuable for shelf life
prediction. An isotherm can be used to determine a
pet food’s monolayer moisture content and the
corresponding water activity, which represents its
most stable state. The monolayer value is
determined by modeling isotherm data using the
GAB or BET equations. It is usually around 0.2-0.3
aW. Increasing water activity 0.1 above the
monolayer value will decrease shelf life by two to
three times.
A product’s isotherm can also be used to determine
package requirements depending on the product’s
sensitivity to moisture and the conditions it may be
exposed to.
Phase Changes And Critical Water Activities
The shape of the isotherm can provide information
about the level of amorphous to crystalline material
in a product. Sharp inflection points in the isotherm
indicate phase transitions (equivalent to a glass
transition) and indicate critical water activities for
maintaining texture properties and preventing
caking and clumping (Figure 6). If the water
activity of a product moves above the critical water
activity for phase transition, the stability of the
product will decrease as time dependent processes
such as caking and crystallization speed up
significantly.
Increasing temperature will lower that critical water
activity point and can also result in loss of stability
with no change in water activity. Determining phase
transitions using isotherms is like determining glass
transitions with differential scanning calorimetry,
except instead of holding water activity constant
and scanning temperature, the isotherm analysis
holds temperature constant and scans water activity.
Figure 6. Moisture sorption isotherm for spray dried milk
powder showing a phase change occurring at a critical
water activity of 0.43.
Conclusion
Moisture sorption isotherms are a blueprint for
moisture relations in a pet food product. They
provide valuable information concerning product
safety and quality and can be an effective tool for
the pet food formulator.
Reference:
Bell,L.N., and T.P.Labuza. 2000. Moisture
sorption: practical aspects of isotherm
measurement and use. American
Association of Cereal Chemists, St. Paul,
MN.
Xin Yu. 2007. Investigation of moisture sorption
properties of food materials using saturated
salt solution and humidity generating
techniques. Ph.D. thesis, University of
Illinois at Urbana-Champaign.
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Water Activity (P/Po)
Moisture Content (% d.b.)
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