Water activity (aW) represents the unbound water available within a product for chemical reactions and microbial growth. It is a fundamental property that quantifies how much of the water present is free to participate in these processes, rather than being chemically bound. The water activity scale ranges from 0 (completely dry) to 1 (pure water), and this measurement is a ratio of the vapor pressure of water in a food item to that of pure water at the same temperature. Water activity can also be understood as the equilibrium relative humidity (ERH) in the air surrounding a sealed food sample, expressed as a decimal fraction. For instance, a food with a water activity of 0.75 would create an equilibrium relative humidity of 75% in a closed container.
Why Water Activity Matters
Water activity directly influences microbial proliferation, making it crucial for product stability and safety. Microorganisms, including bacteria, yeasts, and molds, require available water to grow and reproduce. When water activity is sufficiently low, these microbes cannot absorb enough water, leading to osmotic stress that inhibits their growth. Controlling water activity below specific limits is essential to prevent spoilage and the formation of toxins.
Different types of microorganisms have varying water activity thresholds for growth. Most bacteria, for example, require a water activity of 0.90 or higher to thrive, with some pathogenic bacteria like Staphylococcus aureus capable of growing at levels as low as 0.86. Yeasts can grow at lower water activity levels, often down to 0.80, while molds are even more resilient, capable of growth at water activities as low as 0.60 to 0.70.
Beyond microbial control, water activity also impacts the rates of various chemical and enzymatic reactions affecting product quality. For instance, lipid oxidation, which can lead to off-flavors, is lowest at intermediate water activity levels (around 0.3-0.4) but increases at both higher and lower values. Maillard browning reactions, responsible for color and flavor development, occur most rapidly at water activity between 0.6 and 0.7. Enzymatic reactions generally require water activity above 0.8 to function efficiently, and the degradation of water-soluble vitamins can accelerate with increasing water activity.
Water Activity Versus Moisture Content
Water activity and moisture content are distinct measurements that offer different insights into a product’s water status. Moisture content quantifies the total amount of water present in a material, typically expressed as a percentage of the total weight.
In contrast, water activity measures the availability of that water for biological and chemical processes. A product can have a high moisture content but a low water activity if the water is largely bound by solutes like salt or sugar, making it unavailable. For example, strawberry jam has a lower water activity (around 0.76) than bread (around 0.96), even though jam appears to contain more water, which explains why bread molds more easily.
Water activity is a more reliable indicator of microbial stability and chemical degradation than moisture content alone. While moisture content can influence physical properties, texture, and yield, water activity directly predicts whether bacteria, yeast, and mold can grow. Both types of measurements are useful for optimizing product safety and quality, requiring separate testing methods.
Practical Applications and Management
These principles of water activity are widely applied across various industries to manage product safety, quality, and shelf life, especially in food preservation. For instance, dried fruits, jerky, and certain baked goods are stable at room temperature because their water activity has been reduced to levels that inhibit microbial growth.
Common methods for controlling or reducing water activity include drying, which physically removes water from the product, as seen in powdered milk or dried spices. Another method involves adding solutes such as salt or sugar. These substances bind with water molecules, making the water unavailable, a technique historically used in preserving items like salted fish or fruit jams.
Freezing also reduces water activity by transforming liquid water into ice, immobilizing it and making it inaccessible. This method is routinely employed for preserving vegetables, meats, and ready-to-eat meals. In pharmaceutical and cosmetic products, maintaining specific water activity levels ensures stability and prevents contamination.