Ensuring Food Quality During Storage by Managing Water Activity

Every food product has an invisible number that quietly decides how long it will last on the shelf – and that number is its water activity (a_w). Water activity doesn’t just influence microbial safety; it directly determines what packaging a product needs, what storage conditions it demands, and how quickly it will lose quality. For food processors, getting this relationship right is the difference between a product that stays fresh for months and one that spoils within days.

What is water activity and why does it matter for storage? 🔗

Water activity is the ratio of the vapour pressure of water in a food to the vapour pressure of pure water under identical conditions. It is expressed on a scale from 0 (completely dry) to 1.0 (pure water). Unlike moisture content, which measures the total amount of water present, water activity measures how available that water is for microbial growth, chemical reactions, and enzymatic activity. According to the U.S. FDA, most fresh foods have a water activity above 0.95, providing enough moisture for bacteria, yeasts, and moulds to thrive.

A practical way to think about it: if you sealed a food item inside a jar and let it reach equilibrium, the relative humidity inside that jar – expressed as a decimal – would equal the food’s water activity. This equilibrium relationship is precisely what makes water activity so critical during packaging and storage. If the surrounding environment has higher humidity than the food’s a_w, the food will absorb moisture. If the environment is drier, the food will lose moisture.

How water activity affects microbial growth during storage 🔗

Microorganisms need available water to survive and reproduce, but each type has a specific minimum water activity threshold below which it cannot grow. As noted by the UC Master Food Preserver Program, most bacteria require a_w above 0.91, most yeasts need above 0.88, and moulds can persist at levels as low as about 0.65. The dangerous pathogen Clostridium botulinum has a minimum growth threshold of approximately 0.93 a_w.

This is exactly why the FDA uses the 0.85 a_w benchmark as a regulatory dividing line – foods with water activity at or below 0.85 are generally exempt from certain canned food regulations because most pathogenic bacteria simply cannot survive at those levels. During storage, if poor packaging allows the product’s water activity to rise above these critical thresholds, microbial spoilage becomes a serious risk, even if the food was perfectly safe at the time of packaging.

Beyond microbes: chemical and enzymatic degradation 🔗

Microbial growth is not the only concern. Water activity also influences the rate of chemical and enzymatic reactions that degrade food quality over time. As explained by AQUALAB, water can act as a solvent, a reactant, or a mobility enhancer in chemical reactions. Higher water activity speeds up Maillard browning (causing colour and flavour changes), lipid oxidation (causing rancidity), vitamin breakdown, and enzymatic reactions that alter texture and flavour.

Interestingly, lipid oxidation behaves differently – it actually accelerates at both very high and very low water activity levels. This means even very dry foods are not fully protected from rancidity. This U-shaped reaction curve is an important consideration when designing packaging strategies for low-moisture products like powdered milk, spice blends, or dried snacks.

Moisture migration: the silent quality killer 🔗

One of the most common storage problems linked to water activity is moisture migration. This occurs in two main ways: between the food and the external environment, and between different components within a multi-component product.

Environmental moisture exchange 🔗

When a packaged food product is stored in an environment where the relative humidity differs from its water activity, moisture will transfer through the packaging material toward equilibrium. For example, a product with a water activity of 0.40 stored at 60% relative humidity will gradually absorb moisture until it reaches an a_w of 0.60 – provided the packaging cannot prevent this transfer. For a product like a crispy snack, this shift means the product becomes soft and stale. For a product like dried fruit, it could mean mould growth once a_w crosses the 0.65 threshold.

Internal moisture migration in multi-component products 🔗

Multi-component foods – think cereal with dried fruit, biscuits with cream filling, or trail mix – face an additional challenge. Each component typically has its own water activity. Water always migrates from the component with higher a_w to the one with lower a_w, regardless of actual moisture content. This is a common misconception: water moves based on energy (water activity), not concentration (moisture content).

A familiar example is bhel puri, where the puffed rice becomes soggy after mixing with moist ingredients like onion and chutney. In packaged multi-component products, this same process happens more slowly but just as surely. The solution is to formulate all components at the same water activity before combining them, or to use internal barriers that separate components during storage.

Condensation during temperature fluctuations 🔗

Temperature changes during transport and storage create another moisture-related problem – condensation. When the temperature of a package drops below the dew point of the headspace air inside, water condenses on the inner surface of the packaging or the food surface. This localised increase in available water can trigger microbial growth in patches, even when the overall product a_w is within safe limits. This is especially problematic for refrigerated products moved through warmer distribution chains, or for foods stored in warehouses without climate control.

Selecting packaging materials based on water activity 🔗

Once the ideal water activity for a product is determined, the primary job of packaging is to maintain that a_w throughout the product’s shelf life. This means the packaging must resist moisture transfer from the storage environment. The key property that governs this is the Water Vapour Transmission Rate (WVTR) of the packaging material.

Understanding WVTR 🔗

WVTR measures how much water vapour passes through a material per unit area per unit time, typically expressed as g/m²/day. A lower WVTR means better moisture barrier performance. According to Measurlabs, WVTR testing must be conducted under conditions that simulate the intended real-world storage environment, including specific temperature and humidity levels, because barrier performance varies with these conditions.

The choice of packaging material depends heavily on the product’s sensitivity to moisture change. A dry snack stored in a humid tropical climate needs a much stronger moisture barrier than the same product stored in a temperate, dry environment.

Common moisture barrier materials 🔗

Different materials offer varying levels of moisture protection. Here is how the most widely used packaging materials compare:

Metalized PET (polyethylene terephthalate) is among the most effective options for maintaining crispness in dry snack foods and preventing moisture ingress. The thin metal layer dramatically reduces WVTR.

Polyethylene (PE) films are flexible and widely used as a base layer. They provide reasonable moisture barriers and excellent heat-sealing properties, making them suitable for general food packaging.

Polypropylene (PP), especially in its biaxially oriented form (BOPP), offers high clarity and good moisture resistance. It is commonly used for snack packaging and confectionery.

Ethylene vinyl alcohol (EVOH) is known for exceptional oxygen barrier properties and is frequently used in multilayer packaging structures for products that are sensitive to both moisture and oxidation.

Aluminium foil and aluminium oxide coatings provide the strongest barrier against both moisture and gas, making them ideal for long-shelf-life products like coffee, pharmaceuticals, and military rations.

Multi-layer laminates combine the strengths of different materials. A typical structure might include an outer layer of PET for strength and printability, a middle barrier layer of EVOH or aluminium, and an inner sealant layer of PE. This layered approach allows packaging engineers to optimise barrier performance while maintaining flexibility and cost-effectiveness.

Modified atmosphere packaging and water activity control 🔗

Modified Atmosphere Packaging (MAP) takes moisture and gas management a step further. In MAP, the air inside the package is replaced with a controlled mixture of gases – typically nitrogen, carbon dioxide, and sometimes a small amount of oxygen – to slow spoilage and preserve quality.

While MAP primarily targets oxygen reduction and carbon dioxide increase to inhibit microbial growth and oxidation, its effectiveness is closely linked to water activity management. As described in a review published by PMC (PubMed Central), HACCP plans for MAP-packaged products should incorporate monitoring of water vapour permeability alongside gas composition to ensure food safety.

MAP is particularly valuable for fresh produce, meats, dairy, and baked goods. For instance, baked foods packaged under modified atmospheres benefit from supplemental humidity that maintains their moisture content and prevents staling. In contrast, dry snack foods packaged with nitrogen flushing need the opposite – minimal moisture in the headspace to prevent sogginess.

Active packaging components 🔗

Modern packaging also employs active elements to manage moisture inside the package. Desiccant sachets containing silica gel or activated carbon actively absorb moisture from the headspace, keeping a_w low for moisture-sensitive products like dried foods, spices, and electronics. Oxygen absorbers, typically iron-based sachets, remove residual oxygen to prevent both oxidation and the growth of aerobic spoilage organisms.

Some advanced packaging systems combine these active components – using oxygen absorbers alongside desiccants – for products that are vulnerable to both moisture uptake and oxidative degradation simultaneously.

Storage conditions and their interaction with packaging 🔗

Even the best packaging cannot fully compensate for poor storage conditions. Temperature, relative humidity, and duration of storage all interact with packaging performance to determine final product quality.

Temperature control 🔗

Water activity increases with temperature. This means a product that is safe at refrigeration temperatures might approach dangerous a_w levels at room temperature. Additionally, higher temperatures increase the WVTR of most packaging materials, meaning more moisture can permeate through the barrier. Products stored in hot climates or uncontrolled warehouses therefore face a double challenge – the a_w of the food rises and the packaging becomes less effective at the same time.

Relative humidity of the storage environment 🔗

The driving force for moisture transfer through packaging is the difference between the a_w inside the package and the relative humidity outside it. In regions with high ambient humidity – such as tropical and coastal areas – the moisture gradient is steep, accelerating moisture ingress. Packaging designed for storage in arid environments may be completely inadequate in humid regions.

Shelf life prediction 🔗

Shelf life ultimately depends on how quickly the product’s water activity moves toward a critical limit. As outlined by AQUALAB, rates of moisture exchange through the packaging combined with the rate of a_w change in the food toward critical values will determine shelf life. Knowing the storage temperature, ambient relative humidity, and critical a_w values allows food engineers to select packaging with precisely the right barrier properties for the desired shelf life.

Practical strategies for managing water activity in storage 🔗

Effective water activity management during storage requires a combined approach:

Formulate to an ideal a_w. Before even thinking about packaging, adjust the product formulation to achieve the target water activity using humectants like salt, sugar, or glycerol, or through dehydration.

Match a_w across components. In multi-component products, design all components to the same water activity to prevent internal moisture migration. If matching is not feasible, use internal barriers like edible films or separate compartments.

Select packaging by WVTR requirements. Calculate the maximum allowable moisture gain over the desired shelf life and choose a packaging material whose WVTR – under expected storage conditions – will stay within that limit.

Use active packaging where needed. For highly sensitive products, include desiccants, oxygen absorbers, or both inside the package.

Control the storage environment. Maintain warehouses and retail environments at appropriate temperature and humidity levels. Use climate-controlled logistics for sensitive products.

Monitor regularly. Routine water activity testing during storage and distribution helps catch problems before they reach consumers.

Real-world examples 🔗

Dried snacks and chips: These products typically have a_w around 0.3-0.4. Packaged in metallized BOPP with nitrogen flushing, they stay crispy for months. Without adequate moisture barriers, they quickly absorb humidity and go stale.

Dried fruits: With a_w around 0.55-0.75, dried fruits sit just below most microbial thresholds. Packaging must prevent any moisture uptake that could push a_w above 0.65, where mould growth becomes possible, as noted by the USDA Pathogen Modeling Program.

Bakery products: These often need to retain moisture to stay soft. MAP with controlled humidity helps maintain their texture, while the packaging must also prevent external contamination.

Powdered foods: Products like milk powder, spice mixes, and instant coffee are extremely hygroscopic. Even small increases in a_w can cause caking, clumping, and loss of flowability. Aluminium foil laminates with desiccant inserts are standard for these products.

What do you think? How might the climate of your region influence the packaging requirements for locally produced food products? And if you were designing a multi-component snack, what strategies would you use to prevent moisture migration between ingredients?

References

1. https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/inspection-technical-guides/water-activity-aw-foods
2. https://ucanr.edu/program/uc-master-food-preserver-program/article/water-activity-and-its-role-food-preservation
3. https://aqualab.com/en/knowledge-base/expertise-library/water-activity-food-safety-and-quality
4. https://measurlabs.com/methods/water-vapor-transmission-rate-wvtr/
5. https://pmc.ncbi.nlm.nih.gov/articles/PMC11794936/
6. https://pmc.ncbi.nlm.nih.gov/articles/PMC7089433/
7. https://pmp.errc.ars.usda.gov/wateractivity.aspx