Water Activity Explained: The Science Behind Food Safety and Shelf Stability

1 day ago

10 min read

Updated: 9 hours ago

Water Activity vs. Moisture Content illustration.

A food can look dry, taste salty, or feel firm and still present a food safety risk.

Think about a dry-cured salami hanging in an aging room, fruit slowly drying after harvest, or a traditional preserve made with sugar. Each relies, in part, on controlling the availability of water. Yet appearance, texture, and even the amount of salt or sugar added cannot tell us whether a food will support microbial growth.

For generations, people have preserved foods through drying, salting, fermentation, and other methods. Long before modern laboratory instruments, producers understood through experience that controlling moisture and changing the conditions within a food could extend its useful life.

Today, we can measure one of the most important scientific principles behind these practices: water activity.

Understanding water activity helps producers evaluate preservation methods, develop consistent products, investigate food safety concerns, and make informed decisions about processing, packaging, and storage.

It also helps answer a question that becomes especially important when developing foods intended for room-temperature storage: What actually makes a product shelf stable?

Understanding Water Activity

Water activity, written as a_w, describes how available water is within a food to support microbial growth and other reactions.

The measurement ranges from 0 to 1.0, with pure water having a water activity of 1.0.

Technically, water activity is the ratio of the equilibrium water vapor pressure above a food to that above pure water at the same temperature. While that definition is important scientifically, the practical concept is much easier to understand.

Microorganisms need available water to grow. Reducing that availability can limit or prevent their growth.

Consider the role of salt in curing meat or sugar in making preserves. Both can reduce water availability, even when the food still contains considerable moisture. Drying accomplishes something similar by physically removing water.

This leads to an important distinction that is frequently misunderstood.

Moisture content measures how much total water a food contains, usually expressed as a percentage. Water activity describes how available that water is.

A food with a water activity of 0.80 does not contain 80% water. Likewise, two products with similar moisture contents may have substantially different water activities because of their ingredients, composition, and structure.

This is why moisture content and water activity are not interchangeable measurements.

Moisture content can help producers understand product yield, drying performance, and texture. Water activity provides different information, particularly when evaluating microbial growth potential and preservation.

Neither measurement tells the entire story on its own.

Why Microorganisms Respond Differently to Water Activity

Not all microorganisms require the same conditions to grow.

Many bacteria need relatively high water activity, while certain yeasts and molds can grow in foods with considerably less available water. The minimum conditions for growth also vary among bacterial species.

For example, many common foodborne bacterial pathogens require water activity above approximately 0.85 to grow. Staphylococcus aureus, however, can grow at lower levels under favorable conditions. Some specialized yeasts and molds tolerate even drier environments.

Below approximately 0.60, microbial growth is generally not supported.

These values help explain why lowering water activity can be such an effective preservation strategy. They also demonstrate why no single water activity number should be treated as a universal food safety standard.

Temperature, acidity, salt concentration, formulation, and competing microorganisms all influence the conditions under which growth can occur.

Another important consideration is toxin formation. Under certain conditions, Staphylococcus aureus can grow and produce enterotoxins that may remain hazardous even if subsequent processing reduces the number of viable bacteria. Preventing growth and toxin formation during processing is therefore an important part of controlling this hazard.

A product that prevents the growth of many bacterial pathogens may still support certain spoilage yeasts or molds. Conversely, a food that shows no visible signs of spoilage may still contain pathogens.

And that brings us to one of the most important distinctions in food safety.

Preventing a microorganism from growing is not the same as killing it.

Salmonella, for example, can survive for extended periods in certain low-moisture foods, even when the conditions do not allow it to multiply.

Similarly, a dried or fermented meat product may achieve its intended final water activity without having undergone a process that adequately reduces pathogens such as Salmonella or pathogenic E. coli.

USDA-FSIS has addressed this concern in guidance developed following foodborne illness outbreaks associated with fermented and dried meat products. Investigations identified products that achieved their final water activity targets without adequate validation demonstrating the necessary pathogen reduction.

This distinction matters because food safety frequently involves two separate objectives. The manufacturing process must adequately address relevant pathogens, and the finished product must maintain conditions that prevent unacceptable microbial growth and other hazards during its intended shelf life.

A satisfactory water activity measurement may support the second objective. It does not automatically establish the first.

What Water Activity Looks Like in Real Food Production

The importance of water activity becomes clearer when we look at how different foods are produced.

In dry-cured and fermented meats, the process may involve salt, controlled fermentation, drying, temperature management, and other preservation measures. Each contributes differently to the finished product. Water activity is an important part of assessing microbial stability, but producers must also understand whether the processing conditions provide adequate pathogen reduction.

Drying fruits and vegetables presents different challenges. Moisture removal may vary according to product thickness, sugar content, equipment loading, airflow, and drying conditions. A piece of dried fruit may feel dry externally while retaining more moisture internally. Appropriate sampling and testing help producers understand those differences rather than relying entirely on appearance.

For jams and preserves, sugar traditionally plays an important preservation role by reducing water availability. When a producer reduces sugar to develop a new formulation, the change may affect microbial stability and require reconsideration of acidity, processing, packaging, and storage conditions.

Cheesemaking offers another example of how water activity interacts with other factors. Salt, pH, starter cultures, moisture, ripening conditions, and temperature all influence microbial behavior. A hard-aged cheese and a fresh cheese may have very different characteristics, yet neither can be evaluated for safety through water activity alone.

These examples illustrate an important principle: water activity must be understood within the food itself, not interpreted separately from its formulation and processing history.

This is especially relevant when working with traditional foods. The methods may have deep cultural and historical roots, but commercial production introduces questions involving batch consistency, distribution, labeling, shelf life, and regulatory expectations.

Scientific measurement gives producers a way to evaluate these conditions while preserving the characteristics that make their foods distinctive.

Does a Water Activity of 0.85 Mean a Product Is Shelf Stable?

This is one of the most common questions surrounding water activity, and the answer is not necessarily.

The value 0.85 appears frequently in food safety discussions because it has specific significance in federal regulations. For example, FDA definitions of acidified foods and low-acid canned foods include water activity greater than 0.85 among the criteria used to determine whether those requirements apply.

But 0.85 is not a universal approval for room-temperature storage.

A product measuring 0.85 or below may fall outside certain regulatory classifications while still requiring evaluation of other hazards, processing conditions, or applicable food safety obligations.

Shelf stability must be established for the specific product, considering its complete formulation, manufacturing process, packaging, and intended storage conditions.

For some foods, safety depends on several preservation controls working together.

Fermented and dried meats, for example, may rely on a combination of acidity, salt concentration, controlled fermentation, drying, temperature, and other processing conditions. This is often described as a multiple-hurdle approach, in which different controls work together to achieve the intended food safety outcome.

Our article, Water Activity, pH, and Salt: Understanding Critical Technical Parameters in HACCP, explores these relationships in greater detail.

The broader lesson is that shelf stability must be established through appropriate scientific support, rather than assumed from a single measurement.

A laboratory result is valuable scientific information. It is not, by itself, a shelf-stability determination.

From Product Development to Practical Measurement

Water activity deserves attention well before a product reaches its final packaging.

A producer may want to reduce sodium in a cured meat, shorten a drying cycle, modify the sugar content of a preserve, or change an ingredient to improve flavor or nutritional characteristics.

These decisions can have consequences beyond taste and texture.

Reducing salt or sugar may increase water activity. Modifying drying conditions may change moisture distribution. Increasing product thickness may make it more difficult to achieve uniform drying throughout a batch.

Even changes that appear relatively minor can affect the preservation system.

This is why product development and food safety evaluation should occur together rather than treating testing as something performed only after a formulation has been finalized.

Water activity is measured with specialized instruments that evaluate the equilibrium conditions between a food sample and the surrounding air inside a sealed chamber.

Testing may take place in a qualified laboratory or within a production facility using suitable equipment and established procedures.

But the quality of the measurement depends on more than the instrument.

Consider a producer drying several hundred pounds of meat in a commercial drying room. Products positioned near the airflow may dry differently from those farther away. Larger pieces may retain more moisture than smaller ones. Conditions near the surface may also differ from those inside a product.

Where samples are collected, when testing occurs, and whether the samples represent the actual production conditions all influence how useful the results will be.

Temperature, sample preparation, equipment calibration or verification, and product equilibration may also affect measurement reliability.

A single acceptable result does not necessarily demonstrate that every part of a production batch has achieved the intended conditions.

When water activity serves as a food safety control, producers need a scientifically supported target, a representative testing procedure, defined acceptance criteria, and a clear response when results fall outside those criteria.

The scientific support must also be relevant to the actual product and process. A published validation study involving one formulation, product thickness, or drying schedule cannot automatically be applied to a substantially different product or manufacturing environment.

Differences in ingredients, equipment, processing conditions, and product characteristics may affect whether that scientific evidence is applicable.

The purpose of testing is not simply to generate a number. It is to produce information that supports a meaningful decision.

That decision may involve continuing processing, evaluating affected product, changing a formulation, reviewing packaging, or determining whether additional scientific support is necessary.

Water Activity Can Change After Processing

Another important consideration is that water activity does not necessarily remain constant after a product has been manufactured.

Foods can absorb moisture from their surroundings, particularly when packaging does not provide an adequate moisture barrier.

Moisture may also redistribute within a product as it approaches equilibrium. This can be particularly relevant to foods with fillings, coatings, different layers, or uneven moisture distribution.

Consider a dried product that meets its intended water activity immediately after processing. If the packaging allows moisture to enter during storage, the product's water activity may increase. That change could affect texture, quality, and microbial stability.

Packaging selection, seal integrity, storage humidity, and distribution conditions are therefore part of the preservation strategy.

It is also important to distinguish between shelf life and food safety.

A product may remain microbiologically stable while losing desirable flavor, texture, or appearance. Alternatively, a product may look and taste acceptable while presenting a food safety concern.

These differences are explored in our AgriForaging educational illustration collection, including Shelf Life vs. Food Safety and Shelf Stable vs. Refrigerated.

Understanding how a product behaves throughout its intended shelf life is just as important as understanding its condition at the end of manufacturing.

Connecting the Science to Regulatory Compliance

For commercial food producers, water activity may influence product classification, licensing, HACCP plans, preventive controls, process validation, finished-product specifications, and storage requirements.

How it is applied depends on the product and the regulatory framework.

FDA-regulated foods and USDA-FSIS-regulated meat and poultry products may have different requirements, particularly when demonstrating that preservation methods adequately control food safety hazards.

State and local requirements may also apply, including those governing retail operations, specialty food manufacturing, and particular processing methods.

In some processes, water activity may be established as a HACCP critical limit. In others, it may be part of a preventive control, product specification, or broader scientifically supported manufacturing procedure.

The distinction between validation, monitoring, and verification is important here.

Validation provides scientific support that a process or control is capable of achieving its intended food safety outcome. This evidence must be appropriate to the product, formulation, equipment, and processing conditions being used.

Monitoring determines whether established criteria are being met during production.

Verification evaluates whether the food safety system and its controls are being implemented and functioning as intended.

Water activity testing can contribute to these activities, but the role of the measurement must be clearly established.

For producers developing new products or modifying established processes, understanding when additional scientific evaluation is needed can be just as important as understanding the testing itself. Our article Process Authority Review vs. HACCP Plan: What's the Difference and When Do You Need Each? explores that distinction.

The essential question is not whether a producer has a water activity result in their records. It is whether the result is scientifically meaningful and supports the product's food safety and regulatory decisions.

drying, salting traditional fermentations.

Where Traditional Preservation Meets Modern Food Science

Drying, salting, fermentation, and sugar preservation have shaped food cultures around the world for generations.

They reflect accumulated knowledge of ingredients, environmental conditions, and the changes that occur during food processing.

Modern food science does not replace that knowledge. It provides additional tools to understand the mechanisms behind it, recognize variability, and apply appropriate controls within commercial production.

As we discuss in Fermentation: Where Tradition Meets Food Safety Science, traditional methods and scientific understanding can complement one another.

At AgriForaging Compliance Services, much of our work involves connecting these principles to actual production. We work with farmers, processors, butchers, cheesemakers, specialty food businesses, and manufacturers to evaluate formulations, processing methods, validation needs, and regulatory expectations.

Water activity is one measurement within that larger system, but an exceptionally useful one when understood and applied correctly.

Ultimately, the most important question is not simply:

"What is my product's water activity?"

It is:

"What does this measurement tell us about the safety and stability of the product, and what scientific support do we have for the decisions we are making?"

That is where a measurement becomes a meaningful part of food safety.