Importance of Water Activity (aw) in Microbial Control of Dried Foods

Water is present in almost every food we eat – but not all of it is available for microorganisms to use. This distinction between total moisture and available moisture is at the heart of food preservation science. The concept that governs this availability is called water activity (a_w), and it is arguably the single most important factor in determining whether bacteria, yeasts, or molds can grow in a food product. If you work with dried foods or study food microbiology, understanding water activity is essential for predicting shelf life, preventing spoilage, and ensuring safety.

What is water activity (aw)? 🔗

Water activity is a measurement of the free water in a food – the water that is not bound to food molecules like proteins, starches, or sugars, and is therefore available for microbial growth and chemical reactions. According to the U.S. FDA, water activity is defined as the ratio of the vapour pressure of water in a food to the vapour pressure of pure water under the same conditions. It is expressed on a scale from 0 (completely dry) to 1.0 (pure water).

A crucial point to understand here is that water activity is not the same as moisture content. Moisture content tells you how much water is present by weight, while water activity tells you how much of that water is actually accessible to microorganisms. For instance, honey contains about 18% water by weight, but its water activity is only around 0.6 because the high sugar concentration binds most of that water tightly. This is precisely why honey resists spoilage naturally. On the other hand, fresh meat may have a similar or even lower moisture content than some preserved foods, yet its high a_w (around 0.99) makes it highly perishable.

As the UC Master Food Preserver Program explains, you can think of water activity as the relative humidity inside a sealed jar containing a food sample, once the air and the food have reached equilibrium. If you divide that equilibrium relative humidity by 100, you get the a_w value. Most fresh foods – fruits, vegetables, meat, fish – have water activity values above 0.95, making them ideal environments for microbial proliferation.

Why aw matters more than moisture content for microbial control 🔗

In 1953, scientist W. J. Scott demonstrated that microbial growth correlates with water activity, not water content. This was a landmark finding. Before Scott’s work, food scientists tried to link bacterial growth potential with total water content and found the values were inconsistent across different food products. Scott established that every microorganism has a minimum a_w below which it simply cannot grow – a principle that now underpins modern food safety regulations worldwide.

When the water activity outside a microbial cell drops too low, the cell cannot draw water across its membrane. This creates osmotic stress – the cell becomes dormant and unable to multiply. It is important to note that reducing water activity is a control step, not a kill step. The microorganisms are not destroyed; they are simply prevented from growing to levels that cause infection or spoilage. Over time, bacterial numbers in low-a_w environments do decline, but viable cells can persist at very low water activity levels for extended periods.

Minimum water activity levels for different microorganisms 🔗

Not all microorganisms require the same amount of available water. This difference forms the basis of many dried-food preservation strategies and helps food scientists predict which organisms pose the greatest risk in a given product.

Bacteria 🔗

Bacteria are the most demanding when it comes to water requirements. Most foodborne pathogenic bacteria, including Salmonella and E. coli, need a_w above 0.90-0.91 to grow. Staphylococcus aureus is an exception – it can grow at a_w as low as 0.86 under aerobic conditions, making it one of the more resilient bacterial pathogens. The minimum a_w for Clostridium botulinum growth is approximately 0.93, according to FDA technical guidelines. This is why properly dried, salted, or cured foods are generally safe from most bacterial pathogens.

Yeasts 🔗

Yeasts are somewhat more tolerant of low water activity. Most yeast species can grow at a_w levels down to about 0.85-0.88. Certain osmophilic (sugar-loving) yeasts can survive at even lower levels, around 0.60-0.62. This explains why you might occasionally see yeast contamination in foods with moderate moisture reduction – products like low-sugar jams or partially dried fruits.

Molds 🔗

Molds demonstrate the greatest tolerance for low-a_w environments. Some species can grow at water activity levels as low as 0.65-0.70. According to the USDA Pathogen Modeling Program, most molds stop growing below about 0.80, but it is recommended to keep products below 0.75 to provide a safety margin. Mycotoxin-producing molds have a lower growth limit around 0.78 a_w. This tolerance is why mold is the primary spoilage concern in dried grains, nuts, spices, and stored cereals – foods where bacteria and yeasts cannot thrive.

Below a_w of 0.60, virtually no microorganism – bacteria, yeast, or mold – can proliferate. This threshold is a key benchmark in designing shelf-stable dried food products.

How water activity is reduced in dried foods 🔗

There are several established methods for lowering a_w in food products, each working through different mechanisms.

Dehydration 🔗

Drying is the most direct approach. Whether through sun-drying, hot-air drying, or freeze-drying, removing water physically lowers the a_w. Dried fruits, jerky, and powdered milk typically achieve water activity levels below 0.75, well below the threshold for most microbial growth. Modern dehydration techniques can bring a_w down to 0.60 or even lower, producing shelf-stable products that resist spoilage without refrigeration.

Adding solutes – salt and sugar 🔗

Salt and sugar reduce water activity by binding free water molecules, making them unavailable to microorganisms. Salt is significantly more effective than sugar on a weight basis. As noted by the UC Master Food Preserver Program, a 13% salt solution achieves an a_w of approximately 0.91, sufficient to suppress most common bacteria. Reaching the same a_w with sugar requires roughly a 55% solution. This is why traditional preservation methods such as salting fish, brining vegetables, and making high-sugar jams and preserves have been effective for centuries.

Other humectants 🔗

Food manufacturers also use compounds like glycerol and propylene glycol to bind water and reduce a_w. These are commonly found in intermediate moisture foods – products like soft cookies, some sausages, and pet foods that typically maintain a_w levels between 0.70 and 0.85. These products remain soft and palatable while resisting microbial spoilage.

The interaction between water activity, pH, and temperature 🔗

Water activity does not work in isolation. Its effectiveness as a preservation tool is influenced by other environmental factors, and understanding these interactions is critical for designing safe food products.

Water activity and pH 🔗

Acidity (pH) and water activity have a synergistic relationship when it comes to microbial control. According to AQUALAB, when both pH and a_w are reduced together, the combined inhibitory effect is greater than either factor alone. This means a food does not need to achieve extremely low water activity if its pH is also low, and vice versa.

For example, strawberry preserves have relatively high water activity, but their low pH (due to citric acid) is sufficient to prevent microbial growth. Similarly, maple syrup has a nearly neutral pH but stays safe because its high sugar content keeps water activity low. The FAO/WHO Codex guidelines note that reducing a_w to 0.94 is generally adequate to suppress most pathogenic bacteria, especially when combined with mildly acidic conditions. Products that are not heat-treated typically need either a_w below 0.88 or pH below 4.2 to be considered safe – but higher values of one can be acceptable if the other provides additional protection.

Water activity and temperature 🔗

Temperature affects both water activity readings and how microorganisms respond to low-a_w conditions. Higher temperatures generally make microorganisms more sensitive to low water activity. However, the relationship is not straightforward – some products increase in a_w with temperature, others decrease, and many high-moisture foods show negligible change. Even slight temperature variations during a_w measurement can produce significant errors. The FDA notes that a temperature difference of just 1°C between the sample and the surrounding air can shift the a_w reading by 0.05 – a potentially critical margin when dealing with pathogen growth thresholds.

For practical food storage, this means a dried product stable at room temperature may become vulnerable to spoilage in warm environments, even if its water activity has not technically changed. Conversely, refrigeration can sometimes allow certain microorganisms to persist at lower a_w levels than they would tolerate at higher temperatures.

Hurdle technology: combining water activity with other preservation factors 🔗

Modern food preservation increasingly relies on a concept called hurdle technology, which combines multiple preservation methods – each acting as a “hurdle” that microorganisms must overcome to grow. Water activity reduction is one of the most important hurdles, but it is often combined with pH control, temperature management, modified atmosphere packaging, and preservatives for maximum effectiveness.

A classic example is fermented sausage (salami). During production, salt is added first (lowering a_w), followed by fermentation that produces lactic acid (lowering pH), and then a drying period that further reduces water activity. As described by AQUALAB’s hurdle technology guide, the initial hurdles (salt and nitrate) weaken over time during ripening, but water activity progressively becomes the dominant preservation factor as the sausage dries. The result is a product that is shelf-stable at room temperature for extended periods.

This combined-methods approach, formally developed by food scientist Lothar Leistner in the 1970s, is now used worldwide. It allows food producers to use lower intensities of each individual preservation method – less salt, less acid, lower heat – while still achieving effective microbial control. This results in products with better sensory qualities and nutritional value compared to relying on a single extreme preservation method.

Regulatory significance of water activity 🔗

Water activity has been incorporated into food safety regulations at the highest levels. The U.S. FDA uses a_w of 0.85 as the regulatory threshold for determining whether a low-acid canned food or acidified food falls under certain processing regulations (21 CFR Parts 108, 113, and 114). Foods with a_w at or below 0.85 are exempt from these regulations because they do not support the growth of most pathogens, including C. botulinum.

Water activity is also used as a critical control point (CCP) in HACCP (Hazard Analysis and Critical Control Points) plans across the food industry. Manufacturers routinely measure a_w during production to ensure products meet safety specifications. The FDA Food Code defines potentially hazardous foods as those requiring time and temperature control – but foods with a_w of 0.85 or below, or pH of 4.6 or below, are excluded from this definition.

Water activity and chemical changes in food 🔗

Beyond microbial control, water activity also influences the rate of chemical and biochemical deterioration reactions in food. At intermediate a_w levels (roughly 0.20-0.50), lipid oxidation – which causes rancidity in fats and oils – reaches its minimum rate. At very low water activity levels, the protective water layer around food components is removed, and oxidation can actually accelerate again.

Non-enzymatic browning (Maillard reaction) peaks at intermediate water activity levels (around 0.60-0.70) and slows significantly at both higher and lower values. Enzymatic reactions generally require higher water activity and are largely inhibited below 0.85. Understanding these relationships allows food manufacturers to target specific a_w ranges that optimize both microbial safety and chemical stability for a given product.

Practical considerations for dried food safety 🔗

For anyone preserving food at home or in small-scale production, a few practical points about water activity are worth keeping in mind. Achieving the right a_w is more important than simply removing a certain amount of moisture. When making jerky, dried fruit, or dehydrated vegetables, the goal is to bring water activity below the threshold where target organisms can grow – not just to reduce weight or volume.

Storage conditions matter significantly. Dried foods can reabsorb moisture from humid air, raising their water activity back to levels that support microbial growth. Proper packaging in airtight, moisture-proof containers and storage in cool, dry conditions are essential to maintain the low a_w achieved during processing.

It is also important to understand that freezing preserves food primarily by immobilising water molecules and slowing metabolic activity – not by reducing water activity in the same way drying does. Frozen foods can spoil quickly once thawed if they have not been preserved by other means.

What do you think? Given that water activity interacts with factors like pH and temperature, how might a food producer decide the optimal combination of preservation hurdles for a new dried product? And in home food preservation, how confident are you in achieving safe water activity levels without specialised measurement equipment?

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://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://aqualab.com/en/knowledge-base/expertise-library/how-water-activity-and-ph-work-together-control-microbial
6. https://www.fao.org/4/ac334e/ac334e02.htm
7. https://aqualab.com/expertise-library/using-hurdle-technology-for-safer-and-fresher-food