Water Activity in Food: What It Is, Why It Matters & How to Control It
When developing a food product, moisture content is only part of the story.
Two foods can contain similar amounts of water but have very different shelf lives. The reason is that not all of the water present in food is equally available for microorganisms and chemical reactions.
This is where water activity (aw) becomes important.
Water activity is a key concept in modern food formulation, preservation, drying, packaging and shelf-life management. Food manufacturers use it alongside factors such as pH, temperature, formulation, packaging and processing conditions to develop products with the desired stability and quality.
In this guide, we explain what water activity means, how it differs from moisture content, why it matters for food preservation and how manufacturers can control it.
What Is Water Activity?
Water activity, written as aw, describes how much water in a food is available to participate in microbial growth and other processes.
It is different from simply measuring the total amount of moisture in a product.
A portion of the water in food may be strongly associated with proteins, carbohydrates, salts and other components. This water is less available for microorganisms and chemical reactions.
Water activity is expressed on a scale from 0 to 1.0, with pure water having an aw of approximately 1.0.
The lower the water activity, the less available water there is for many microorganisms.
According to FDA technical guidance, controlling water activity is an important consideration in food stability and preservation.
Water Activity vs. Moisture Content
One of the most common misunderstandings in food science is assuming that moisture content and water activity mean the same thing.
They do not.
Moisture content
Moisture content tells you how much water is present in the food.
Water activity
Water activity tells you how available that water is.
For example, two products may contain comparable moisture levels but have different water activities because their ingredients bind or interact with water differently.
This distinction is especially important when developing:
Dried foods
Bakery products
Sauces
Confectionery
Meat products
Dairy products
Fruit preparations
Powdered ingredients
Ready-to-eat foods
Shelf-stable products
Therefore, measuring moisture alone may not provide enough information about product stability.
Why Does Water Activity Matter in Food Preservation?
Microorganisms require available water to grow.
Reducing water activity can therefore make the environment less favourable for many bacteria, yeasts and molds.
FDA guidance notes that most foods have relatively high water activity and that reducing available moisture can help inhibit microbial growth.
Water activity can also influence:
Microbial stability
Chemical reactions
Enzymatic activity
Texture
Crispness
Shelf life
Moisture migration
Product quality
For food manufacturers, controlling aw can therefore be an important part of developing a stable product.
The Relationship Between Water Activity and Microbial Growth
Different microorganisms have different requirements for growth.
In general:
Higher aw → greater availability of water
Lower aw → less available water
However, water activity should not be treated as a simple universal safety guarantee.
The effect of aw depends on the microorganism, food formulation, pH, temperature, packaging and other factors.
For example, FDA materials identify both pH and water activity as important intrinsic factors affecting microbial growth.
This is why professional food formulation considers multiple preservation hurdles rather than relying on one factor alone.
How Can Food Manufacturers Control Water Activity?
There are several ways to influence water activity during food formulation.
1. Drying and Dehydration
Drying removes water from food.
Traditional sun-drying has been used for centuries, while modern food manufacturing may use:
Hot-air drying
Vacuum drying
Spray drying
Freeze-drying
Dehydration systems
Reducing moisture availability can improve the stability of appropriately formulated dried products.
However, achieving the desired shelf life requires proper control of the finished product's water activity, packaging and storage conditions.
2. Salt
Salt can bind water and reduce its availability.
This principle is used in various traditional and commercial food-preservation applications.
Examples include:
Pickled vegetables
Salted fish
Cured foods
Brined products
Certain sauces
The amount of salt required depends on the specific product and its formulation.
Salt concentration should therefore be properly established rather than estimated from a general recipe.
3. Sugar
Sugar can also affect water activity by binding available water.
This is one reason high-sugar foods such as certain:
Jams
Jellies
Fruit preparations
Syrups
Confectionery products
can have lower water activity than fresh foods.
However, sugar concentration alone does not automatically guarantee microbiological safety. Product formulation, pH, processing, packaging and storage conditions still matter.
4. Formulation Ingredients
Different ingredients interact with water in different ways.
Hydrocolloids, salts, sugars, proteins and other food ingredients can influence the physical state and availability of water.
Food formulators therefore need to consider how every ingredient affects the finished product.
This is particularly important when changing:
A seemingly small formulation change can influence the final product's stability.
5. Packaging
Even when a product starts with the correct water activity, moisture can move between the product and its surroundings.
Poor packaging may allow moisture to enter or leave the product.
This can affect:
Crispness
Texture
Powder flow
Clumping
Microbial stability
Shelf life
For this reason, water activity and packaging should be considered together.
Water Activity and pH: Why Both Matter
Water activity is only one part of food preservation.
Another major factor is pH.
pH measures the acidity or alkalinity of a food, while water activity relates to the availability of water.
Together with other factors, they can form part of a food-safety and shelf-life strategy.
For example:
pH + aw + temperature + processing + packaging + formulation
can provide a much more complete picture of product stability than any single measurement.
FDA preventive-control guidance identifies both water activity and pH as potential formulation/process controls.
Why Food Manufacturers Measure Water Activity
Water activity testing can help manufacturers understand whether a product has reached the desired formulation target.
It can be useful during:
Product development
When developing a new food, aw measurements can help compare different formulations.
Quality control
Manufacturers can monitor whether production batches remain within established specifications.
Shelf-life studies
Water activity can be evaluated alongside other factors during stability testing.
Troubleshooting
If a product develops mold, loses crispness or becomes unstable during storage, aw testing can help identify whether available moisture is contributing to the problem.
Examples of Foods Where Water Activity Matters
Water activity is relevant to a wide range of food products.
Bakery Products
Bread, cakes, biscuits and other baked products can behave differently depending on moisture distribution and water activity.
Dried Fruits
Removing moisture can reduce water activity and contribute to longer storage stability when combined with suitable packaging.
Sauces
Sauces may have relatively high moisture levels, making formulation, pH, preservatives, thermal processing and packaging important considerations.
Confectionery
Sugar concentration and formulation can strongly influence water activity and texture.
Meat Products
Cured and dried meat products may use combinations of salt, drying, processing and other preservation controls.
Powdered Foods
Low water activity can help reduce microbial growth and improve storage stability, while packaging helps prevent moisture uptake.
Water Activity and Shelf Life
Shelf life is not determined by water activity alone.
A food product can undergo several types of deterioration during storage, including:
Microbial growth
Oxidation
Enzymatic reactions
Moisture migration
Texture changes
Colour changes
Flavour deterioration
Therefore, manufacturers should evaluate the complete formulation and storage system.
A product with an appropriate aw may still require suitable packaging, temperature control, pH adjustment or other preservation measures.
The Importance of Measuring the Finished Product
Water activity should be evaluated in the actual finished product rather than assumed solely from the ingredient list.
This is because interactions between ingredients can influence the final result.
For commercial food products, manufacturers should establish appropriate specifications and validate their processes using suitable scientific methods.
FDA guidance notes that water activity may be controlled through established formulation approaches or through formulation development followed by finished-product testing.
Water Activity in Shelf-Stable Foods
Water activity becomes particularly important when developing foods intended for extended storage.
For example, commercial shelf-stable foods may rely on combinations of:
Thermal processing
pH control
Water activity control
Preservatives
Packaging
Hygienic manufacturing
Controlled storage
For acidified and low-acid canned foods, FDA specifically distinguishes products based on finished equilibrium pH and water activity and requires appropriate processing controls for products covered by its regulations.
This demonstrates an important principle:
Food preservation is a system, not a single ingredient.
Can Preservatives Replace Water Activity Control?
Not necessarily.
Preservatives can serve specific functions in suitable food formulations, but they should not be viewed as a universal replacement for controlling moisture, pH, processing or packaging.
Food manufacturers need to select preservation strategies based on the specific product and its intended storage conditions.
Ingredients such as Potassium Sorbate and Sodium Benzoate, for example, may be used in appropriate food applications where permitted and where their use fits the product's formulation and regulatory requirements.
Food Shield+ Ingredients for Modern Food Formulation
At Food Shield+, we provide a range of food-grade ingredients used by food manufacturers, formulators and food businesses.
Our range includes ingredients used for different formulation functions, including:
Food Acidulants
Citric Acid
Malic Acid
Lactic Acid
Tartaric Acid
Fumaric Acid
Cream of Tartar
Food Preservatives
Potassium Sorbate
Sodium Benzoate
Functional Ingredients
Xanthan Gum
Guar Gum
Calcium Chloride
Agar Agar
Gum Acacia
pH & Mineral Salts
Sodium Citrate
Potassium Citrate
Potassium Chloride
Calcium Citrate
Baking Soda
The right ingredient depends on the intended application, formulation, processing method, sensory requirements and applicable food regulations.
Water Activity and the Food Preservation Hurdle Approach
Modern food preservation often uses multiple controls rather than relying on a single preservation method.
This is sometimes described as a hurdle approach.
Potential hurdles include:
pH
Water activity
Temperature
Heat processing
Salt or sugar concentration
Preservatives
Packaging
Hygiene
When appropriately designed, multiple hurdles can work together to improve product stability.
However, each product requires its own formulation and process validation.
Key Takeaways
Water activity is an important concept in food science and preservation.
Remember:
Water activity is different from total moisture content.
Lower available water can make conditions less favourable for many microorganisms.
Drying, salt, sugar and formulation can influence water activity.
Packaging can help maintain the desired moisture balance.
pH and water activity should often be considered together.
Water activity can affect microbial stability, texture and shelf life.
Finished-product testing can help verify formulation targets.
Commercial food products require appropriate process and safety validation.
No single ingredient should be treated as a universal substitute for proper food-safety controls.
Final Thoughts
Understanding water activity gives food manufacturers another important tool for designing stable, consistent and high-quality products.
From dried foods and bakery products to sauces, confectionery, preserved foods and powdered ingredients, the availability of water can influence how a product behaves throughout its shelf life.
At Food Shield+, we believe better food formulation begins with understanding the science behind each ingredient.
Food Shield+ — Where Food Meets Science.
Explore our range of food-grade ingredients for preservation, acidity control, texture, stability and modern food formulation.
