Water activity is a fundamental factor influencing the growth of microorganisms, particularly bacteria. It measures the unbound water available for biological processes, distinguishing it from the total water content within a substance. Understanding water activity is important for ensuring the safety and extending the shelf life of food products, explaining why some foods remain stable without refrigeration while others spoil quickly.
Defining Water Activity
Water activity (aw) quantifies the amount of “free” or available water in a substance, which is not bound to other components like proteins or carbohydrates. It is formally defined as the ratio of the vapor pressure of water in a food to the vapor pressure of pure water at the same temperature. This measurement ranges from 0, representing a completely dry state, to 1.0, which signifies pure water.
The water activity value directly indicates how much water is accessible for microbial growth. Unlike moisture content, which measures total water, water activity specifically identifies water available to support biological activity. For example, honey and fresh meat can have similar moisture contents, yet honey is stable for years due to low water activity, while meat spoils quickly due to high water activity.
Minimum Water Activity for Bacterial Growth
The level of water activity significantly impacts the ability of bacteria to grow and multiply. Most bacteria require a relatively high water activity, typically above 0.91, to proliferate effectively. Pathogenic bacteria, which can cause illness, often need even higher levels, generally above 0.94. However, some bacteria can tolerate lower water activity levels.
Clostridium botulinum, a bacterium known for producing a potent neurotoxin, generally requires a water activity of approximately 0.94 or higher for growth and toxin formation for proteolytic strains. Non-proteolytic strains of Clostridium botulinum may require an even higher water activity, around 0.97, for growth. In contrast, Staphylococcus aureus is an exception among pathogenic bacteria, capable of growing at water activity levels as low as 0.86, and some studies indicate growth as low as 0.83. The ability of Staphylococcus aureus to thrive in lower water activity environments makes it a concern in some intermediate-moisture foods.
How Water Activity Prevents Bacterial Growth
Low water activity inhibits bacterial growth by creating an osmotic imbalance. This occurs when water moves out of the bacterial cell into the surrounding environment, where water activity is lower. Bacteria need available water to carry out metabolic processes, transport nutrients, and remove waste products.
This outward movement of water leads to cellular dehydration and osmotic stress. As a result, bacteria cannot perform essential functions or multiply effectively. While low water activity generally prevents bacterial multiplication, it does not necessarily kill microorganisms. Instead, bacteria may become dormant, capable of resuming growth if water activity levels increase.
Applying Water Activity in Food Preservation
Controlling water activity is a widely used strategy in food preservation to prevent bacterial growth and extend shelf life. Traditional methods often reduce available water in food products.
One common approach is drying, which physically removes water from the food matrix. Examples include dried fruits, beef jerky, and powdered milk, which have significantly reduced water activity levels, typically below 0.75, making them stable against most microbial growth.
Another effective method involves adding solutes, such as salt or sugar, to bind water molecules and make them unavailable for microorganisms. This principle is applied in curing meats with salt or preserving fruits as jams and jellies with high sugar concentrations. These techniques reduce water activity, inhibiting bacterial proliferation and enhancing food safety.