Water Activity in Animal Feed

INTRODUCTION

The global feed industry is a cornerstone of modern animal agriculture, supplying nutritionally balanced feeds
to support livestock, poultry, aquaculture, and companion animals across diverse production systems. Valued at
hundreds of billions of dollars annually, the industry spans a complex supply chain that converts grains, oilseeds,
forages, and by-products into safe, high quality feedstuffs tailored to regional species, production intensity, and
regulatory requirements. Major multinational suppliers, such as Cargill, ADM, Bunge, Nutreco (SHV Group),
Charoen Pokphand Group, Land O’Lakes/Purina, Alltech, and ForFarmers, play a dominant role, alongside
countless regional mills and integrators. Together, these producers face growing pressure to deliver consistent
feed quality while managing biological risks, sustainability goals, and global variability in raw materials, making
feed safety and quality control central priorities across the industry.
Ensuring the safety and quality of feedstuffs is critical for animal health, performance, and regulatory compliance.
Fresh feed is preferred for all livestock, but it cannot be stored for later use and cannot be shipped to other
locations. Consequently, most feeds are dried and processed prior to storage or transport. However, these finished
feeds can still be vulnerable to spoilage mechanisms such as mold growth, yeast activity, bacterial proliferation,
and chemical degradation, all of which can lead to nutrient losses, mycotoxin formation, reduced shelf life, and
economic losses. While moisture content has traditionally been used as a quality indicator for processed feed, it
does not fully describe the biological and chemical risks associated with stored feeds.
Water activity (aw) provides a more direct and reliable measure of feed safety and stability. Unlike moisture
content, which quantifies the total water present, water activity describes the energy of water and directly
influences microbial growth and chemical reactions. In feed production, most problems do not start when they
become visible, but much earlier when water activity moves out of the safe range. Relying on moisture content
alone is not sufficient, as products with the same moisture can behave very differently in terms of stability and
risk.
Water activity gives you direct control over what really matters. It indicates whether mold can grow, whether
mycotoxins can form, and whether your product will remain stable during storage and transport. By measuring
water activity, feed producers can prevent mold growth and mycotoxin formation before they become a problem.
formation before they become a problem. At the same time, it allows precise control of the drying process,
avoiding both under-drying, which leads to spoilage, and over-drying, which wastes energy and reduces yield.
It also helps ensure consistent product quality despite variability in raw materials, and prevents physical issues
such as clumping, softening, or loss of texture.
In addition, water activity supports better decisions in packaging by matching barrier performance to actual
product needs, avoiding unnecessary costs. Without water activity, production remains reactive. With water
activity, it becomes controlled and predictable. It connects safety, quality, and efficiency in one measurable
parameter, making it one of the most effective tools to improve feed production performance. This paper
outlines the role that water activity plays in feed safety and quality, highlights typical water activity ranges for
common feedstuffs, and demonstrates how water activity measurement can complement traditional moisture
testing to improve feed management decisions.

THEORY OF WATER ACTIVITY

Water activity is defined as the energy status of water in a system and is rooted in the fundamental laws of
thermodynamics through Gibb’s free energy equation. It represents the relative chemical potential energy of
water as dictated by the surface, colligative, and capillary interactions in a matrix. Practically, it is measured as the
partial vapor pressure of water in a headspace that is at equilibrium with the sample, divided by the saturated
vapor pressure of water at the same temperature. The water activity covers a range of 0 for bone dry conditions
up to a water activity of 1.00 for pure water, resulting from the partial pressure and the saturated pressure being
equal. Water activity is often referred to as the ‘free water’ and while useful when referring to higher energy, it
is incorrect since ‘free’ is not scientifically defined and is interpreted differently depending on the context. As
a result, the concept of free water can cause confusion between the physical binding of water, a quantitative
measurement, and the chemical binding of water to lower energy, a qualitative measurement. Rather than a
water activity of 0.50 indicating 50% free water, it more correctly indicates that the water in the product has 50%
of the energy that pure water would have in the same situation. The lower the water activity then, the less the
water in the system behaves like pure water.
While water activity is an intensive property that provides the energy of the water in a system, moisture content
is an extensive property that determines the amount of moisture in a product. Water activity and moisture
content, while related, are not the same measurement. Moisture content is typically determined through losson-
drying as the difference in weight between a wet and dried sample. While useful as a measurement of purity
and a standard of identity, as this paper will describe, moisture content does not correlate as well as water
activity with microbial growth, chemical stability, or physical stability. Water activity and moisture content are
related through the moisture sorption isotherm.

WATER ACTIVITY MEASUREMENT OF ANIMAL FEED

Water activity is measured by equilibrating the liquid phase water in the sample with the vapor phase water in
the headspace of a closed chamber and measuring the Equilibrium Relative Humidity (ERH) in the headspace
using a sensor. The relative humidity can be determined using a resistive electrolytic sensor, a chilled mirror
sensor, or a capacitive hygroscopic polymer sensor. Instruments from Novasina, like the Labmaster NEO, utilize
an electrolytic sensor to determine the ERH inside a sealed chamber containing the sample. Changes in ERH
are tracked by changes in the electrical resistance of the electrolyte sensor. The advantage of this approach is
that it is very stable and resistant to inaccurate readings due to contamination, a particular weakness of the
chilled mirror sensor. Additionally, some feed formulations may contain volatile materials that can interfere with
a chilled mirror sensor. These volatiles are overcome in Novasina instruments using protective filters that protect
the sensor, making the resistive electrolytic sensor the most accurate and precise instrument with the lowest
maintenance requirements capable of testing all types of feed products.
The water activity of feeds can cover a wide range depending on the type and post-harvest processing. Table
1 shows a range of water activity values for different feed products and indicates that some feed products can
have similar water activities, but different moisture contents. Test times ranged from 5 minutes to 22 minutes
depending on the product type. Feed samples do not typically contain volatiles that can interfere with the water
activity values, so no filters were used. The hay cube and feed pellets were crushed before testing while the other
samples were added to the testing cup with no special handling.

When testing the water activity of feed products, sample handling and presentation can play an important role
in achieving consistent and accurate readings. The key elements of achieving good results include:

Use the same sampling procedure for a product each time it is tested
Hard pelleted products are best tested crushed or ground to create a more homogeneous sample
If samples contain multiple components such as mixed ration, include all the components in the testing
sample
Minimize the time the sample is exposed to the room humidity during preparation steps and storage prior
to testing

WATER ACTIVITY AND MICROBIAL GROWTH IN FEED

The greatest potential risk presented by feed products is the potential for microbial contamination. Each
microorganism maintains an optimal internal water activity, which is essential for normal metabolic function,
growth, and reproduction. When exposed to an environment with a lower water activity than its cytoplasmic water
activity, the cell experiences osmotic stress and begins to lose water to its surroundings, as water naturally moves
from regions of higher water activity (energy) to lower water activity. This dehydration reduces turgor pressure
and inhibits key metabolic processes. To continue growing, the microorganism must lower its internal water
activity below that of the environment, primarily by accumulating compatible solutes to increase intracellular
osmolarity. The capacity to adjust internal water activity varies among species, resulting in organism‑specific
limiting water activity thresholds below which growth cannot occur (1, 2). Importantly, microbial growth depends
not on the total amount of water present (moisture content), but on the energy state of the water, its water
activity, and the microorganism’s ability to access and utilize that water.
A summary of the lower water‑activity limits for growth of common spoilage organisms is provided in Table 2.
These limits show that pathogenic bacteria are unable to grow at water activities below 0.87 aw, whereas the
growth of most spoilage yeasts and molds ceases at approximately 0.70 aw, often referred to as the practical
limit. Only xerophilic and osmophilic organisms can grow at water activities below 0.70 aw, and no microbial
growth has been observed at water activities less than 0.60. Microbial growth rates can also be modeled using
water activity in combination with other intrinsic and extrinsic factors such as temperature and pH.

Mold growth is of particular concern for feed products because of the potential of ingestion of harmful
spores or mycotoxins (3, 4). To prevent mold growth, the water activity must be reduced below 0.70 aw or
alternative interventions—such as preservative systems or vacuum packaging—must be employed. Thankfully,
a comparison of Table 2 and Table 3 indicates that even higher water activity levels are needed for toxin
production than are needed for growth. A mold that is actively growing will only produce toxin when growth is
established and the water activity levels are high enough. Toxin production can be viewed as a luxury that the
mold utilizes to make it more competitive, but only when conditions are ideal and there are no other hurdles
impeding its growth.

Table 3. A list of common mycotoxins found in animal feed, the organism that produces the toxin, where it
is commonly found, its effect on animal health, and the minimum water activity level needed for production
(3,4,5).

As shown in Table 1, the water activity levels of all feed products tested are well below the threshold required
for mold growth, let alone mycotoxin production. Consequently, feed that is properly processed, dried, and
stored is not typically a source of contamination. Problems arise when stored feed products are unknowingly
exposed to high humidity or when water infiltrates the storage container. Such exposure can raise the water
activity of the feed above the thresholds for both mold growth and toxin production, resulting in contaminated
feed that may cause severe illness in animals that consume it.

CASE STUDY: IMPROPERLY STORED FEED CONTAMINATES BEEF CATTLE

A well‑documented case of mycotoxin‑related livestock illness occurred on a family‑owned farm in the southern
United States, where a group of feeder steers developed progressive illness and multiple deaths after being
fed mold‑contaminated stored corn (6, 7). The corn had been improperly dried and stored under warm, humid
conditions following harvest, with significant insect damage to kernels, creating an ideal environment for
growth of Aspergillus flavus. Although the feed did not appear severely moldy at first glance, laboratory analysis
revealed aflatoxin concentrations of approximately 1,500 ng/g (1.5 ppm) in the corn, far exceeding safe feeding
limits. Clinically, affected cattle showed poor growth, depression, reduced feed intake, coughing, and respiratory
distress, and the herd responded poorly to antibiotic treatment. Necropsy and histopathological examination
of dead animals revealed liver lesions characteristic of aflatoxicosis, including centrilobular hepatocellular
vacuolation, and aflatoxin residues were detected in tissues. Once the contaminated stored feed was identified
and removed, no additional deaths occurred.
Importantly, routine monitoring of the water activity (aw) of the stored corn would likely have provided an
early warning that the feed was unsafe, as the readings would have indicated that there was the possibility of
fungal growth and mycotoxin production even when visible spoilage is limited. This case clearly demonstrated
that mycotoxins produced during feed storage, rather than in the field, can cause severe systemic disease and
mortality in livestock, highlighting the critical importance of proper grain drying, storage, and the monitoring of
objective indicators such as water activity to prevent mycotoxin exposure.

WATER ACTIVITY AND CHEMICAL STABILITY OF FEED

The water activity of intermediate‑moisture and dry feeds is typically below 0.70 aw, a level at which microbial
growth is unlikely to occur. However, products within this range do not possess unlimited shelf life. Instead, other
failure modes become dominant. For feeds with water activities between 0.40 and 0.70 aw, chemical degradation
is a primary concern because many reaction rates reach their maximum within this interval. Chemical processes
such as Maillard browning, lipid oxidation, and enzymatic reactions can negatively affect the flavor, appearance, and
nutritional value of the product. Water activity influences these reactions by reducing activation energy, increasing
molecular mobility, and thereby increasing the reaction rate constant. Consequently, reaction rates correlate
more strongly with water activity than with moisture content. In general, increasing water activity accelerates
most chemical reactions, although the specific relationship depends on both the product matrix and the reaction
mechanism (Figure 2). Most reactions reach peak rates at approximately 0.70–0.80 aw, where further increases in
water act as a diluent. Lipid oxidation is a notable exception, as its rate increases under low‑water‑activity conditions.
Vitamin degradation and rancidity are the reactions most likely to impact the quality and shelf life of feeds.
Vitamin and mineral loss most often occurs due to hydrolysis reactions where the vitamins are transformed into
compounds that cannot be utilized by the animals. Vitamin loss can be mitigated by drying to and maintaining
lower water activity levels where the reaction rate is slow enough to prevent substantial loss in nutrient quality prior
to consumption
Rancidity occurs when lipids are oxidized through a complex process that involves multiple pathways and requires
the presence of lipids, oxygen, and free radicals. As a result, lipid oxidation is typically mitigated by limiting oxygen
exposure, most often through nitrogen flushing or the incorporation of oxygen absorbers. Rancidity develops when
lipid oxidation produces volatile compounds that impart musty or off‑odors and flavors. Dry feeds are particularly
vulnerable because they are frequently coated with a surface fat layer to enhance palatability and nutritional value.
Once rancidity occurs, pets may refuse the food, or owners may discard it due to the objectionable odor. As noted
previously, lipid oxidation is unique among degradation reactions in that its rate increases not only with rising
water activity but also under low‑water‑activity conditions. This behavior makes the general assumption that
“lower water activity is always better” inapplicable for lipid stability.

Figure 2. Water activity stability map showing the typical response of modes of failure to increasing water
activity (by permission Ted Labuza).

WATER ACTIVITY AND PHYSICAL STABILITY OF FEED

8 APPLICATION NOTE
Physical stability is a key determinant of shelf life for low-water-activity (0.20–0.50 aw) feeds such as dry pellets or
cubes. These products are densified to make them easier to handle, store and transport. In this range, chemical
reactions and microbial growth are generally not the primary failure modes. Nevertheless, these products still
experience quality loss over time, most commonly due to changes in texture or becoming sticky. Water activity
affects both the structural and mechanical properties of feed matrices, and each product type has a specific
range in which its texture remains stable.
To achieve maximum shelf life, feed products must be manufactured within their optimal water activity range
and must maintain that level throughout distribution and storage. Low water activity is associated with a hard,
dry texture. When water activity rises above the desired range, the pellets and cubes absorb moisture, soften,
and become sticky, causing products to clump and are hard to handle and move through piping. Maintaining
the appropriate water activity range is therefore essential for preserving product quality and optimal handling
of feeds.
Investigations have shown documented changes in crispiness when equilibrating dry products to various water
activity values. (8, 9). Using texture profile instrumentation, a relationship between crispness and water activity
can be characterized, allowing identification of a water activity range where texture changed from acceptable
to unacceptable. In general, dry products will maintain their texture until they move beyond the critical water
activity where a sigmoidal loss in texture will occur (Figure 2).

Figure 3. Loss of crispness in low water activity products like dry feed due to changes in water activity at 3
different temperatures (9).

One mechanism by which the water activity of dry feed can change over time is moisture migration. Within
a package, water will transfer between individual particles whenever differences in water activity exist—
regardless of differences in total moisture content. This occurs because water moves from regions of higher
water activity (higher energy state) to regions of lower water activity, rather than simply from areas of higher to
lower moisture concentration.
When individual seed or grain components with differing water activity levels are combined, moisture
migration continues until equilibrium is achieved. This redistribution can lead to undesirable textural changes
in each component, such as softening or loss of hardness. To prevent these effects, all components should be
formulated and processed to reach the same water activity prior to blending. If components with different
initial water activities must be combined, predictive moisture migration models can be used to estimate the
final equilibrium water activity and anticipate resulting texture changes.
Another mechanism by which water activity—and consequently product texture—can change is exposure
to elevated ambient humidity. As described in the theory section, water activity corresponds to a product’s
equilibrium relative humidity (ERH) and is therefore directly influenced by environmental humidity. For
example, a product with a water activity of 0.40 aw placed in an environment with 60% relative humidity will
absorb moisture until its internal water activity equilibrates at approximately 0.60 aw. Although this process
occurs gradually, insufficient moisture barriers can allow the water activity to rise beyond its optimal range,
resulting in softening, loss of desired texture, and, at relative humidities above approximately 70%, increased
risk of mold growth.

PACKAGING SELECTION

Moisture‑barrier packaging slows moisture transfer by limiting the rate of water vapor ingress. When choosing
packaging materials, the goal is to provide adequate protection without overspending on unnecessarily
high‑barrier materials, which can lead to avoidable financial waste. Modeling tools are available to help
determine the ideal water vapor transmission rate (WVTR) required to maintain product quality based on
the target water activity range and expected storage conditions. In many cases, users discover they are using
packaging with a WVTR of 0.8 g/m²·day when a less expensive material with a WVTR of 1.0 g/m²·day would
perform just as well. Contact Novasina to learn more about using water activity data to optimize packaging
selection and reduce costs.

INGREDIENT INSPECTION

Once an ideal water activity specification has been established to minimize microbial, chemical, and physical
degradation, the next challenge is to consistently manufacture product at that target level. In principle,
production parameters such as oven temperature, extruder settings, and belt speed could be fixed from run
to run, yielding product with uniform water activity. In practice, however, external factors require continual
adjustment of these settings. Variability in incoming ingredient properties—particularly moisture content
and water activity—as well as fluctuations in the production environment can alter drying behavior. When
process settings are calibrated based on an assumed water activity of raw materials, any deviation from that
assumption will result in finished product with inconsistent water activity. Typically, this discrepancy is not
detected until the first batch completes processing and undergoes quality‑assurance testing. At that point, if
the product fails to meet specification, it must be reworked or discarded, resulting in inefficiency and waste.
An effective strategy to reduce problems caused by variability in incoming ingredients is to monitor their water
activity and establish an acceptable range that ensures finished product meets specification with minimal
process adjustment. This can be readily achieved by collecting a subsample of each incoming ingredient lot
and measuring its water activity. If the measured value falls outside the acceptable range, the ingredient can
either be rejected or used with predefined adjustments to processing conditions, knowing that standard
settings will not produce the desired result. The use of water activity testing as an incoming‑ingredient
screening tool is another powerful application for water activity and incorporating this practice could provide
an additional level of process control and help reduce rework, variability, and waste.

WATER ACTIVITY: THE MOST IMPORTANT SPECIFICATION FOR FEED

For feeds, establishing an ideal water activity specification is a critical step in processing and storing products
for both safety and quality. This specification helps prevent microbial proliferation, reduce undesirable
chemical reactions, minimize physical and structural degradation, and limit moisture migration during storage.
The optimal water activity target depends on the most likely mode of failure for a given product type, such as
texture loss in dry pellets, chemical degradation in nutritional supplements, or microbial growth in wet feeds.
Once this target value is identified, a combination of storage strategies and processing controls can be used to
consistently achieve and maintain the desired water activity. In addition, the careful monitoring of the water
activity of product on the production line will eliminate unnecessary energy waste and weight loss due to
processing to lower than ideal water activities, which will maximize revenue. Finally, feed products should be
stored in packaging that possesses adequate moisture barrier properties to slow water activity change over
time. In summary, feed manufacturers should:

Establish an ideal water activity specification
Process to meet that specification
Monitor production with frequent water activity testing to ensure specifications are met
Package product in quality moisture barrier packaging
Monitor stored product over time for changes in water activity
This will ensure a safe, quality product with an optimal shelf life and maximum revenue. In short, water
activity is the most important feed specification.

REFERENCES

Beuchat, L. 1983. Influence of water activity on growth, metabolic activities and survival of yeasts and
molds. Journal of Food Protection 46(2):135-141.
Scott, W. 1957. Water relations of food spoilage microorganisms. Advances in Food Research 7:83-127.
Mostrom, M. 2021. Overview of mycotoxicosis in animals. MSD Manual. Overview of Mycotoxicoses in
Animals – Toxicology – MSD Veterinary Manual
Muñoz-Solano, B., Lizarraga Pérez, E., González-Peñas, E. Monitoring Mycotoxin Exposure in Food-
Producing Animals (Cattle, Pig, Poultry, and Sheep). Toxins 2024, 16, 218. https://doi.org/10.3390/
toxins16050218
Stein, R.A., Bulboacӑ, A.E. Chapter 21 – Mycotoxins, Editor(s): Christine E.R. Dodd, Tim Aldsworth, Richard
A. Stein, Dean O. Cliver, Hans P. Riemann, In Foodborne Diseases (Third Edition), Academic Press, 2017,
Pages 407-446, https://doi.org/10.1016/B978-0-12-385007-2.00021-8
Colvin, B.M., Harrison, L.R., Gosser, H.S., and Hall, R.F. 1984. Aflatoxicosis in feeder cattle. JAVMA 184: 956-
958.
Clark, Kim. 2026. Aflatoxin in Corn. Nebraska Dairy Extension. Aflatoxin in Corn | Nebraska Dairy Extension
| Nebraska.
Katz, E.E. and Labuza, T.P. 1981. Effect of water activity on the sensory crispness and mechanical
deformation of snack food products. Journal of Food Science 46, 403.
Carter, B.P., Galloway, M.T., Campbell, G.S., and Carter, A.H. 2015. The critical water activity from dynamic
dewpoint isotherms as an indicator of crispness in low moisture cookies. Journal of Food Measurement
and Characterization 9(3):463-470.