The Importance of Water Activity in Food Preservation

Food spoilage is one of the biggest challenges in the food industry. Every year, millions of tonnes of food go to waste because microorganisms – bacteria, yeasts, and molds – find the right conditions to grow. One of the most critical factors that determines whether these organisms can thrive is water activity (a_w). It’s not just about how much water a food contains; it’s about how much of that water is actually available for microbial use. Understanding and controlling water activity is at the heart of food preservation – from ancient salt-curing techniques to modern shelf-stable product design.

What is water activity? 🔗

Water activity, denoted as a_w, is a measure of the availability of free water in a food product for microbial growth and chemical reactions. It is defined as the ratio of the vapour pressure of water in a food to the vapour pressure of pure water at the same temperature. The U.S. Food and Drug Administration (FDA) formally defines a_w as this vapour pressure ratio under equilibrium conditions.

The scale ranges from 0 to 1.0, where pure water has an a_w of 1.0 and a completely dry substance approaches 0. Most fresh foods – fruits, vegetables, meat, and fish – have water activity levels above 0.95, while dried goods like biscuits and milk powder sit well below 0.60.

Water activity is not the same as moisture content 🔗

This is a common point of confusion. Moisture content measures the total water present in a food, while water activity measures only the free or unbound water available for biological and chemical processes. As the Province of Manitoba’s food safety resource explains, salami and cooked beef may have similar moisture content (around 60%), but their water activity values are very different – 0.82 for salami and about 0.98 for cooked beef. The difference exists because salt and other solutes in salami bind a large portion of the water, making it unavailable for microbial use.

This distinction is crucial. Two foods with identical moisture content can have entirely different shelf lives and spoilage risks because their water activity levels differ. That’s why food scientists rely on a_w, not moisture content, to predict microbial behaviour.

How water activity controls microbial growth 🔗

Every microorganism has a minimum a_w below which it cannot grow. This principle, first established by the researcher W.J. Scott in 1953, forms the scientific foundation of many preservation strategies. As reported by Food Safety Magazine, it is the water activity – not total water content – that sets the lower limit for microbial growth in food.

Different groups of microorganisms have different water activity requirements, which creates a hierarchy of concern for food producers:

Bacteria are the most water-demanding. Most spoilage bacteria need a_w above 0.90 to grow. Pathogenic bacteria like Salmonella, E. coli, and Clostridium botulinum generally cannot grow below a_w of 0.93. A notable exception is Staphylococcus aureus, which can grow at a_w as low as 0.86 under aerobic conditions – making it a particular concern in fermented and semi-dried products.

Yeasts are somewhat more tolerant of lower water activity, with most species growing at a_w levels as low as 0.88. Osmophilic yeasts, which thrive in high-sugar environments, can survive at a_w levels down to about 0.60.

Molds are the most resilient. Common molds can grow at a_w levels down to 0.70 or even 0.65. Xerophilic molds (those adapted to dry conditions) can colonise foods that bacteria and yeasts simply cannot survive in, such as dried grains, nuts, and spices. This is why mold growth is often the primary spoilage concern in low-moisture foods.

Water activity levels of common foods 🔗

Understanding where common foods fall on the a_w scale helps illustrate why some products spoil quickly and others last for months or years. Data from the USDA’s Pathogen Modeling Program and Kansas State University’s food safety fact sheet provide useful reference values:

High a_w foods (0.95-0.99): Fresh meat and fish (0.99), raw vegetables (0.99), raw fruits (0.98), fresh bread (0.95). These foods spoil rapidly without refrigeration because their high water activity supports virtually all types of microbial growth.

Intermediate a_w foods (0.80-0.93): Cured meats, cheddar cheese (0.85), fermented sausages, margarine, maple syrup. These foods have reduced spoilage risk but may still support the growth of certain bacteria and most yeasts and molds.

Low a_w foods (0.60-0.80): Jams and jellies (0.75-0.80), peanut butter (0.70), dried fruits (0.60-0.65), honey. These products are generally safe from bacterial growth but may still be susceptible to some molds and osmophilic yeasts.

Very low a_w foods (below 0.60): Biscuits (0.30), milk powder (0.20), instant coffee (0.20), dried vegetables. Below a_w of 0.60, virtually no microbial growth occurs, making these products highly shelf-stable.

Methods to control water activity in food 🔗

Food preservation has always revolved around reducing the availability of water to microorganisms. Whether our ancestors realised it or not, traditional methods like sun-drying, salting, and sugaring all work by lowering a_w. Modern food technology uses the same principles with greater precision.

Drying and dehydration 🔗

Removing water directly from food is one of the most straightforward ways to reduce a_w. Dried fruits, beef jerky, powdered milk, and freeze-dried coffee are all preserved this way. According to the UC Master Food Preserver Program, dehydrated foods typically achieve water activity levels below 0.75 – well under the threshold needed for most microbial growth. Modern dehydration techniques can push a_w even lower, often below 0.60, creating extremely shelf-stable products.

Adding solutes: salt and sugar 🔗

Salt and sugar work as humectants – substances that bind free water molecules, reducing their availability for microbial metabolism. Salt is far more effective than sugar on a weight basis. A roughly 13% salt solution achieves an a_w of about 0.91, sufficient to suppress most ordinary bacteria. Achieving the same a_w with sugar would require about a 55% solution. This is why salt curing has been one of humanity’s most effective preservation tools for thousands of years, while sugar-based preservation (as in jams and honey) requires much higher concentrations.

Freezing 🔗

Freezing works by immobilising water molecules, which slows both microbial metabolism and the rate of chemical reactions responsible for food degradation. While frozen water is technically still present, it is no longer available in a form that microorganisms can use. However, it’s important to note that freezing does not kill most microorganisms – they simply become dormant and can resume growth once the food thaws.

Other humectants 🔗

In commercial food production, substances like glycerol and propylene glycol are also used to reduce water activity. These compounds are especially important in the manufacture of intermediate moisture foods (IMFs) – products like soft cookies, some sausages, and certain pet foods that maintain a soft texture while resisting spoilage. These products typically have a_w values between 0.70 and 0.85.

Water activity and chemical reactions in food 🔗

Microbial growth isn’t the only thing influenced by water activity. Several chemical and enzymatic reactions that degrade food quality are also a_w-dependent.

Lipid oxidation, which causes rancidity in fats and oils, is significantly affected by water activity. Interestingly, oxidation rates are lowest at intermediate a_w levels (around 0.20-0.40) and increase at both very low and very high water activity values. This means that over-drying a fatty food can actually accelerate rancidity.

Enzymatic browning and Maillard browning (non-enzymatic) reactions are also influenced by a_w. Maillard reactions, which cause browning and flavour changes in foods during storage, tend to peak at intermediate water activity levels (around 0.60-0.70). This is relevant to the quality of stored dried foods, baked goods, and powdered products.

Vitamin degradation, particularly of vitamins B1, B2, and C, can also be accelerated at certain a_w levels. For food manufacturers, this means that controlling water activity is about maintaining not just safety but also nutritional quality and sensory attributes over time.

Water activity in food safety regulations 🔗

Regulatory agencies worldwide recognise a_w as a critical food safety parameter. The FDA uses an a_w threshold of 0.85 to define whether a low-acid canned food or acidified food falls under certain safety regulations (21 CFR Parts 108, 113, and 114). Foods with a_w at or below 0.85 in the finished product are exempt from these regulations because the risk of pathogenic bacterial growth is considered minimal at that level.

Water activity is also widely used as a Critical Control Point (CCP) in HACCP (Hazard Analysis and Critical Control Points) programs. Food manufacturers regularly test product samples during production to verify that a_w remains within specified safe ranges. This proactive approach to monitoring water activity helps prevent food safety incidents before products reach consumers.

Water activity and the hurdle technology approach 🔗

In modern food science, water activity is rarely used as the sole preservation strategy. Instead, it is combined with other preservation methods in what is known as hurdle technology – a concept developed and refined by food scientist Lothar Leistner. The idea is that multiple preservation factors (or “hurdles”) work together, each contributing a barrier that microorganisms must overcome to survive.

Common hurdles combined with water activity include temperature control (refrigeration or heat treatment), pH reduction (acidification), modified atmosphere packaging, and the addition of preservatives. According to a review published in Critical Reviews in Food Science and Nutrition, the intelligent combination of these hurdles has become increasingly prevalent as our understanding of how preservation factors interact has deepened.

A classic example is traditional fermented salami. During production, salt reduces a_w, lactic acid bacteria lower the pH through fermentation, and gradual drying further reduces moisture availability. The result is a product stable at room temperature for extended periods – not because any single hurdle is extreme, but because their combined effect creates an environment hostile to pathogens.

Practical guidelines for maintaining safe water activity levels 🔗

For food producers, processors, and even home food preservers, keeping a_w under control is essential. Here are key practical considerations:

Monitor regularly: Water activity meters (such as dew point hygrometers) provide fast and accurate readings. Regular monitoring during production and storage ensures a_w remains within target ranges.

Account for temperature effects: Water activity can change with temperature. Some foods see increased a_w at higher temperatures due to changes in solute solubility or the physical state of the food matrix. This means that storage temperature matters – a product safe at one temperature may not be safe at another.

Control packaging: Moisture-barrier packaging is critical for maintaining desired a_w levels during distribution and storage. Without proper packaging, dry products can absorb ambient moisture and reach water activity levels that support microbial growth.

Understand moisture migration: In multi-component foods (like a cereal bar with a fruit filling), water migrates from high-a_w areas to low-a_w areas over time. This can make the dry component soggy while drying out the moist component, affecting both quality and safety. Food formulators design products with matched water activity levels to minimise this migration.

Why water activity matters beyond the laboratory 🔗

Water activity is not just a concern for food scientists in laboratories or large-scale manufacturers. It is relevant to anyone who handles, stores, or processes food. Home canners selecting recipes for shelf-stable preserves, small-scale artisan producers developing dried or fermented products, farmers storing grain – all benefit from understanding a_w principles. Even the simple act of properly drying herbs or storing spices is governed by water activity.

In developing countries, where refrigeration infrastructure may be limited, controlling water activity through drying, salting, and sugaring remains one of the most practical and affordable strategies for reducing post-harvest food losses and preventing foodborne illness.

What do you think? How might a better understanding of water activity change the way you store or preserve food at home? And considering the growing global challenge of food waste, could wider awareness of a_w principles help reduce spoilage in regions where cold-chain infrastructure is still developing?

References

1. https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/inspection-technical-guides/water-activity-aw-foods
2. https://www.gov.mb.ca/agriculture/food-safety/education-resources/print,water-content-water-activity.html
3. https://www.food-safety.com/articles/4420-water-activitye28099s-role-in-food-safety-and-quality
4. https://pmp.errc.ars.usda.gov/WaterActivity.aspx
5. https://bookstore.ksre.ksu.edu/download/water-activity-of-foods_MF3674
6. https://ucanr.edu/program/uc-master-food-preserver-program/article/water-activity-and-its-role-food-preservation
7. https://en.wikipedia.org/wiki/Hurdle_technology
8. https://www.tandfonline.com/doi/full/10.1080/10408398.2012.761594