AskHACCP | Water Activity, pH, and Salt: Understanding Critical Technical Parameters in HACCP

When developing a HACCP plan for fermented, dried, cured, ready-to-eat, or shelf-stable foods, three technical parameters frequently enter the conversation: water activity, pH, and salt concentration.

Although these factors can work together to limit microbial growth, they measure different physical and chemical properties of food. They are not interchangeable, and documenting one does not necessarily demonstrate control of another. Each must be evaluated within the context of the specific product, process, and identified hazard. 

This distinction matters. A product may contain a substantial amount of salt yet still have enough available water to support microbial growth. A fermented product may reach its target pH but fail to achieve the required reduction quickly enough during processing. A dried product may feel firm and appear shelf-stable while retaining a water activity that requires refrigeration.

A compliant hazard analysis must explain not only which parameters are measured, but also why they control the hazards associated with the specific product and process.

Water Activity: How Much Water Is Available?

Water activity, written as aw, measures the amount of water in a food that is available for microorganisms to use. It is not the same as moisture content.

Two products can have similar moisture percentages and very different water activity values because ingredients such as salt and sugar bind water. Conversely, a product that appears dry may still contain enough available water to support certain pathogens or spoilage organisms.

Water activity is expressed on a scale from 0 to 1.00. Pure water has an aw of 1.00. As water activity decreases, fewer microorganisms can grow.

An aw of 0.85 is an important regulatory and food safety threshold for many foods. Foods at or below this level generally do not support the growth or toxin production of pathogenic bacteria, although certain yeasts and molds may still grow. FDA uses this threshold when distinguishing certain acidified and low-acid canned foods and provides additional guidance on the role of water activity in food safety and product classification.

However, 0.85 is not a universal answer for every product or process. A higher water activity limit may be appropriate when refrigeration, reduced-oxygen packaging, preservatives, pH, or another validated hurdle is also being used. The acceptable target must be connected to:

• The microorganism of concern 

• The food matrix 

• The packaging system 

• Storage and distribution conditions 

• The product’s intended shelf life 

• Other controls included in the process 

Water activity often becomes especially important in dried meats, jerky, snack sticks, dry sausages, shelf-stable foods, and products that may absorb moisture after processing.

Packaging also matters. Achieving an acceptable aw at the end of drying does not protect a product if the packaging allows it to absorb moisture during storage.

pH: How Acidic Is the Product?

pH measures the acidity or alkalinity of a food. The pH scale is logarithmic, meaning a change of one pH unit represents a tenfold change in hydrogen ion concentration. A product at pH 4.0 is therefore substantially more acidic than a product at pH 5.0.

In food safety, pH is commonly used to control the growth and toxin production of microorganisms that cannot grow under sufficiently acidic conditions. A finished equilibrium pH of 4.6 is a particularly significant threshold because it is associated with controlling Clostridium botulinum in applicable foods. This value is based on the growth characteristics of Clostridium botulinum and serves as one of several regulatory thresholds used for specific food categories. 

FDA defines a low-acid canned food, with certain exceptions, as a food with a finished equilibrium pH above 4.6 and an aw above 0.85. An acidified food is generally a low-acid food to which acid or acid foods have been added, resulting in a finished equilibrium pH of 4.6 or below and an aw above 0.85. FDA provides the current definitions and guidance for acidified and low-acid canned foods.

As with water activity, a pH of 4.6 should not automatically be applied as the critical limit for every fermented or acidified product. Some processes require a lower pH, a specific rate of acidification, or additional controls to address organisms that tolerate acidic environments.

For fermented meat and poultry products, the HACCP system may need to consider:

• Starting pH 

• Final or equilibrium pH 

• The time required to reach the target 

• Fermentation temperature 

• Starter culture activity 

• Product diameter or composition 

• Potential growth of Staphylococcus aureus during fermentation 

• Subsequent heating, drying, cooling, and storage conditions 

Simply documenting a final acceptable pH may not demonstrate that the product remained safe throughout fermentation.

Salt Concentration: An Ingredient Level Is Not Automatically a Safety Control

Salt can inhibit microbial growth, support curing, influence fermentation, bind water, and contribute to flavor and product texture. But the percentage of salt in a formulation is not the same measurement as pH or water activity. Its antimicrobial effect depends not only on salt concentration but also on available water, product composition, and its interaction with other preservation hurdles. 

A recipe stating “3% salt” does not, by itself, establish how much water remains available to microorganisms. The effect of salt depends on the amount of moisture in the product, how uniformly the salt is distributed, the food matrix, and the presence of other ingredients.

In some applications, water-phase salt is more meaningful than salt as a percentage of the total formulation. Water-phase salt describes the concentration of salt in the water portion of the food rather than in its total weight.

This distinction becomes especially important for brined, cured, and certain seafood products. Even then, a calculated salt value should not automatically be substituted for a measured aw result unless scientific support establishes a reliable relationship for that particular product and process.

Salt may be:

• A formulation control 

• Part of a validated multi-hurdle process 

• A factor supporting pathogen inhibition 

• A quality or identity requirement 

• A curing component used with nitrite or nitrate 

• An operational specification that supports, but does not independently establish, food safety 

The hazard analysis should clearly describe which role salt performs.

Why These Parameters Are Not Interchangeable

It is tempting to reason that a salty product must have a low water activity, or that an acidic product must automatically be shelf-stable. Neither assumption is reliable.

Parameter	What it measures	What it can help control
Water activity	Water available for microbial growth	Pathogen growth, toxin formation, spoilage and shelf stability
pH	Acidity or alkalinity	Growth and toxin production of organisms sensitive to acidic conditions
Salt concentration	Amount of salt in the product or water phase	Microbial inhibition, curing, fermentation performance and reduction of available water

Measurements should be selected based on their ability to verify the safety outcome identified in the hazard analysis, not simply because they are easy to collect. 

For example, monitoring the amount of salt added to a batch may confirm that the formulation was followed. It does not necessarily confirm the finished product’s water activity. Likewise, documenting drying time does not establish that every product reached the required aw unless the process has been validated and consistently controlled.

Hurdle Technology: When Several Controls Work Together

Many fermented, cured, dried, and shelf-stable products do not rely on one factor alone. Instead, they use hurdle technology: several barriers that work together to prevent pathogen survival, growth, or toxin formation.

Potential hurdles include:

• Reduced pH 

• Reduced water activity 

• Salt 

• Nitrite or nitrate 

• Heat treatment 

• Competitive starter cultures 

• Preservatives 

• Refrigeration 

• Reduced-oxygen or moisture-resistant packaging 

• Controlled fermentation and drying conditions 

Each hurdle may be insufficient by itself, but the combination can create a safe process.

Ready-to-eat, shelf-stable fermented, salt-cured, and dried meat and poultry products frequently rely on multiple hurdles, including salt, nitrite, reduced pH, reduced aw, drying, and, in some cases, heat. USDA FSIS emphasizes that the scientific support for these controls should reflect the establishment's actual formulation, process, and finished product. Additional information is available in the USDA FSIS Compliance Guideline for Ready-to-Eat Fermented, Salt-Cured, and Dried Meat and Poultry Products.

“Hurdle technology” should not become a vague justification, however. A HACCP plan must identify the relevant hazards and explain how the selected combination of controls prevents, eliminates, or reduces those hazards to an acceptable level.

What Belongs in the Hazard Analysis?

The hazard analysis should identify the reasonably likely food safety hazards associated with the product, ingredients, process, packaging, and intended use. It should then explain whether pH, aw, salt, or a combination of parameters is necessary to control those hazards.

Depending on the process, the analysis may need to address:

• Pathogen growth during fermentation 

• Inadequate lethality 

• Survival during drying 

• Toxin production before an inhibitory pH or aw is reached 

• Outgrowth during cooling or stabilization 

• Growth during refrigerated or ambient storage 

• Post-lethality contamination 

• Moisture migration or rehydration 

• Risks associated with reduced-oxygen packaging 

If a parameter must remain within a defined limit to control a hazard at a particular step, that parameter may become a critical limit at a Critical Control Point. Other parameters may serve as prerequisite program specifications, formulation controls, operating limits, or supporting conditions within a validated process.

The HACCP plan and supporting records should make these roles clear.

What Belongs in the Supporting Scientific Documentation?

Scientific support should establish why the process works. It may include:

• Regulatory guidance 

• Peer-reviewed research 

• Published processing authority recommendations 

• Challenge studies 

• Inoculated-pack studies 

• Predictive microbial modeling 

• Product testing conducted under representative conditions 

• A validated scheduled process 

• Documentation showing that the cited research closely matches the actual product and process 

The support must be relevant to the establishment’s formulation, equipment, product dimensions, processing temperatures, humidity, fermentation time, drying profile, packaging, and storage conditions. Scientific support should also represent the establishment's worst-case conditions, including product size, formulation, drying profile, packaging system, and other variables that could influence food safety. 

A published study involving a thin product with a different salt level and drying schedule may not adequately support a thicker product produced under substantially different conditions.

Practical Product Examples

Fermented sausage

A fermented sausage may rely on controlled fermentation to reduce pH, followed by heating and drying to reduce pathogens and prevent subsequent growth. The plan may need to monitor fermentation time and temperature, the rate or endpoint of acidification, lethality conditions, and final aw.

Final pH alone may not address toxin formation that could occur while the product is slowly acidifying.

Jerky and dried meat

A jerky process usually requires more than simply drying the product until it looks or feels done. The establishment must support both pathogen reduction and safe stabilization. Thickness, humidity, airflow, time, temperature, and final aw can all affect the outcome.

Drying is not automatically equivalent to lethality.

Cured ready-to-eat meat

Salt, nitrite, heat treatment, refrigeration, and packaging may work together. The amount of salt added can be an important formulation specification, but it should not be assumed to establish shelf stability unless the complete process has been scientifically supported.

Shelf-stable product

A product may qualify as shelf-stable because of its pH, its aw, a validated combination of barriers, or an appropriate thermal process. Product classification should be based on finished-product characteristics and validated processing, not on appearance, traditional preparation, or expected customer use. 

Multi-component food

Sauces, fillings, toppings, and inclusions can create areas with different pH and aw values. Measuring a blended sample may mask a higher-risk component. The evaluation may need to consider equilibrium conditions, moisture migration, and the characteristics of each component.

Common HACCP and Regulatory Documentation Pitfalls

Several recurring problems can weaken a HACCP plan:

• Treating moisture content and water activity as equivalent 

• Assuming a salt percentage guarantees a particular aw 

• Monitoring final pH without addressing the acidification rate 

• Using a regulatory threshold without determining whether it applies to the product 

• Declaring a product shelf-stable based only on appearance or tradition 

• Citing research that does not match the actual formulation or process 

• Failing to account for product variability and worst-case conditions 

• Using a laboratory result without documenting the sampling method 

• Omitting meter calibration, testing temperature, or sample preparation procedures 

• Ignoring packaging integrity and moisture migration 

• Listing multiple hurdles without explaining their individual and combined roles 

• Confusing pathogen growth inhibition with pathogen destruction 

One of the most important distinctions is the difference between preventing growth and achieving lethality. Reaching an aw or pH that prevents further growth does not necessarily destroy pathogens already present in the product.

Start With the Hazard, Then Build the HACCP System 

Water activity, pH, and salt are useful technical tools, but none should be added to a HACCP plan merely because it is common for the product category.

Start with the hazard. Identify the organism or food safety concern, determine when it could survive, grow, or produce toxin, and then establish which control or combination of controls addresses it. From there, define measurable limits, monitoring procedures, corrective actions, verification activities, and scientifically valid supporting documentation.

Effective HACCP systems do more than document measurements. They demonstrate, through science, validation, ongoing verification, and accurate recordkeeping, that the selected controls consistently manage the hazards associated with the product and process. The strongest HACCP plans connect sound food science with practical operational controls that can be implemented, monitored, and confidently defended during regulatory review. 

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