Controlling Microbial Growth in Foods Through Water Activity Management

Every food product contains water, but not all of that water is equally available to microorganisms. The key to understanding why honey can last for years while fresh bread grows mold in days lies in a single concept: water activity (a_w). By controlling water activity, food scientists and manufacturers can effectively prevent the growth of harmful bacteria, yeasts, and molds – making it one of the most powerful tools in food preservation and safety.

What is water activity and why does it matter? 🔗

Water activity is a measure of the available or “free” water in a food product that microorganisms can use for their metabolic processes. It is expressed as a ratio of the vapor pressure of water in a food to the vapor pressure of pure water under identical conditions. The scale ranges from 0 (completely dry) to 1.0 (pure water). Importantly, water activity is not the same as moisture content. Two foods may have the same moisture content but very different water activity levels, depending on how tightly the water is bound within the food matrix.

Most fresh foods – meat, vegetables, fruits – have a water activity above 0.95, which provides plenty of available moisture for microbial growth. Reducing this available water below certain thresholds can inhibit or completely stop the growth of specific microorganisms. This principle forms the foundation of many traditional and modern food preservation methods, including drying, salting, and the addition of sugar.

How microorganisms depend on water activity 🔗

Microorganisms need free water to carry out essential life processes – transporting nutrients into cells, eliminating waste, and facilitating reproduction. They take up water by moving it across their cell membranes from a higher water activity environment (outside the cell) to a lower one (inside the cell). When the water activity in the surrounding food drops below a critical point, it creates osmotic stress. The cell can no longer absorb sufficient water and becomes dormant. The organisms are not necessarily killed; they simply cannot grow enough to cause infection or spoilage.

Different types of microorganisms have different tolerances to low water activity, which is why understanding these thresholds is essential for designing effective preservation strategies.

Bacteria 🔗

Bacteria are generally the most sensitive to reduced water activity. Most spoilage and pathogenic bacteria require a_w values above 0.90-0.91 to grow. Some notable exceptions exist. Staphylococcus aureus, for instance, can grow at a_w as low as 0.86 under aerobic conditions, making it one of the most water-stress-tolerant foodborne pathogens. Clostridium botulinum, the bacterium responsible for botulism, has a minimum growth threshold of approximately 0.93-0.94 a_w. Other well-known pathogens such as Salmonella, E. coli, and Listeria monocytogenes all require a_w above 0.92-0.95 for active growth.

Yeasts 🔗

Yeasts can tolerate lower water activity levels compared to most bacteria. Most common yeasts need a_w above 0.88 for growth, but some specialised species are far more tolerant. Osmophilic yeasts, such as Saccharomyces rouxii (now known as Zygosaccharomyces rouxii), can grow at water activity levels as low as 0.62. This is why high-sugar products like syrups, concentrated fruit juices, and certain confectionery items can still be spoiled by yeasts even when bacteria cannot survive.

Molds 🔗

Molds are the most tolerant group when it comes to low water activity. While most common molds require a_w above 0.70-0.80 for growth, some xerophilic (dry-loving) molds such as certain species of Aspergillus and Eurotium can grow at levels as low as 0.61-0.62. This explains why products like dried fruits, nuts, and grains – which have water activity levels below the bacterial growth limit – can still develop mold over time if not properly stored. At the water activity range of 0.61-0.85, fungal spoilage occurs more frequently than bacterial spoilage simply because bacteria cannot compete in those conditions.

Critical water activity thresholds for food safety 🔗

Understanding specific a_w thresholds helps food manufacturers, regulators, and home preservers make informed decisions about food safety. Here are the key benchmarks:

a_w above 0.95: Nearly all bacteria, yeasts, and molds can grow freely. Fresh meat, fish, fruits, and vegetables fall in this range and require refrigeration or other preservation methods.

a_w 0.91-0.95: Most common foodborne pathogens can still grow. Foods such as cured ham, some cheeses, and evaporated milk fall in this range.

a_w 0.86-0.90: Bacterial growth becomes significantly limited. Only a few hardy organisms like S. aureus (under aerobic conditions) can survive. Fermented sausages and aged cheeses are examples of foods in this range.

a_w 0.70-0.85: No bacterial growth occurs. Spoilage is primarily due to molds and some yeasts. Products like jams, marmalades, dried meats, and sweetened condensed milk fall here.

a_w below 0.60: No microbial growth of any kind is possible. Products like crackers, dry milk powder, instant coffee, and dried spices are in this category.

The U.S. FDA uses a_w 0.85 as a regulatory threshold for low-acid canned foods. Products with water activity at or below this level are generally exempt from certain canning regulations because the risk of dangerous pathogen growth is significantly reduced.

Survival versus growth: an important distinction 🔗

One critical point that often gets overlooked is the difference between microbial growth and microbial survival. Lowering water activity inhibits growth, but it does not necessarily kill the organisms already present. Research on powdered milk and other low-moisture foods has shown that viable bacterial cells can persist at water activity levels far below their growth minimum, even though they cannot multiply. Over extended storage periods, bacterial numbers may gradually decline, but some cells can remain viable for surprisingly long times.

This has important implications for food safety. If a low-a_w product is later rehydrated or exposed to moisture – for instance, during improper storage – dormant organisms can “wake up” and begin multiplying rapidly. This is why controlling water activity should be viewed as a control step, not a kill step. Adequate initial processing (such as heat treatment) remains essential to reduce microbial loads before relying on low water activity for long-term preservation.

Methods for controlling water activity in foods 🔗

There are several practical approaches food processors use to reduce water activity and thereby limit microbial growth:

Drying and dehydration 🔗

Physically removing water is the most straightforward method. Dried fruits, jerky, powdered milk, and dried vegetables typically have water activity values below 0.75, which is well below the threshold for most microbial growth. Sun drying, hot-air drying, freeze-drying, and spray drying are all common techniques. The method chosen depends on the food type, desired final texture, and nutritional goals.

Adding solutes: salt and sugar 🔗

Salt and sugar act as humectants – they bind free water molecules and reduce their availability for microbial use. Salt is far more effective on a weight basis than sugar at lowering a_w. A roughly 13% salt solution brings water activity down to about 0.91, whereas a 55% sugar solution is needed to achieve the same level. This is why salt curing is highly effective for meats and fish, while sugar-based preservation works well for jams, jellies, and candied fruits. Other solutes like glycerol and propylene glycol are also used in certain processed foods and intermediate-moisture products.

Freezing 🔗

Freezing converts free water into ice crystals, making it unavailable for microbial activity. While freezing does not technically change a_w in the thermodynamic sense at the ice-water interface, it effectively immobilises water and dramatically slows both microbial metabolism and chemical degradation reactions.

Formulation and ingredient control 🔗

Many processed foods are carefully formulated to achieve a target water activity level. By adjusting the proportions of ingredients – proteins, starches, salts, sugars, and fats – manufacturers can precisely engineer the a_w of the finished product. This is especially important for intermediate-moisture foods (a_w 0.60-0.85), such as soft pet food, fruit bars, and certain bakery products, which need to remain soft and palatable while resisting microbial spoilage.

The hurdle technology approach 🔗

In modern food science, water activity control is rarely used in isolation. Instead, it is often combined with other preservation factors in what is known as hurdle technology. This approach uses multiple barriers – or “hurdles” – that a microorganism must overcome to survive and grow. Common hurdles include low or high temperature, low pH (acidity), reduced redox potential, preservatives, modified atmosphere packaging, and competitive microorganisms.

The beauty of this approach is that each individual hurdle does not need to be extreme. For example, a mildly reduced water activity combined with a slightly acidic pH and refrigeration can collectively prevent microbial growth just as effectively as a single, more extreme treatment – but with better retention of sensory and nutritional quality.

Traditional fermented sausages like salami are a classic example. During production, salt is added to reduce water activity, fermentation produces lactic acid to lower pH, and gradual drying further decreases a_w. Each of these steps adds a hurdle, and the finished product is shelf-stable at room temperature without the need for refrigeration.

Impact of storage conditions on water activity 🔗

Water activity is not a fixed property – it can change during storage depending on environmental conditions. Temperature, relative humidity, and packaging all play a role. A product that is shelf-stable at room temperature may become vulnerable to microbial growth if stored in a warm, humid environment where moisture migrates into the food through inadequate packaging.

This is why packaging design is critical. Appropriate moisture barrier materials prevent the food from absorbing atmospheric moisture. Manufacturers set both upper and lower critical water activity limits for their products – the upper limit protects against microbial growth, while the lower limit preserves texture and sensory quality (overly dry food can become unacceptably hard or crumbly).

Temperature also matters in another way: higher storage temperatures can increase the tolerance of some microorganisms to low water activity. A food that resists spoilage at 25°C may not do so at 35°C, even at the same a_w value. This is particularly relevant for food distribution in tropical and subtropical climates.

Water activity in HACCP and food safety regulations 🔗

Water activity is widely used as a critical control point (CCP) in Hazard Analysis and Critical Control Points (HACCP) programmes. By regularly monitoring a_w during production, food manufacturers can verify that their products meet safety specifications. The Food Safety Modernization Act (FSMA) in the United States also references water activity in its definition of potentially hazardous foods.

For food businesses, periodic a_w testing is essential – especially when ingredient suppliers change, which can alter the final water activity of the finished product. Even small variations can push a product above a critical safety threshold.

Practical takeaways for food preservation 🔗

Understanding water activity is not just academic – it has direct, everyday applications. Whether you are a food manufacturer designing a new product, a food safety inspector evaluating compliance, or a home preserver making jerky or jam, the same principles apply. Reducing a_w below the growth minimum for target organisms is one of the most reliable strategies for extending shelf life and preventing foodborne illness. Combined with proper sanitation, adequate heat treatment, correct packaging, and controlled storage temperatures, water activity management forms the backbone of a robust food safety system.

What do you think? How might changes in climate and rising global temperatures affect our current water activity-based food preservation strategies? And as consumer demand for minimally processed, “clean label” products grows, can water activity control offer a viable alternative to chemical preservatives?

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/microbial-growth
4. https://pmp.errc.ars.usda.gov/WaterActivity.aspx
5. https://pubmed.ncbi.nlm.nih.gov/30913613/
6. https://en.wikipedia.org/wiki/Water_activity
7. https://en.wikipedia.org/wiki/Hurdle_technology
8. https://www.sciencedirect.com/topics/food-science/hurdle-technology
9. https://aqualab.com/expertise-library/water-activity-for-food-safety-and-quality
10. https://www.food-safety.com/articles/4420-water-activitye28099s-role-in-food-safety-and-quality