What is water activity and why does it matter?
Water activity is a thermodynamic property and is generally defined as the energy of water in a system or how easily water can escape into the vapor phase. This principle is vital in animal feed production because the higher the energy in the water, the more likely the water is to cause microbial growth, moisture migration, and chemical or physical reactions.
With this in mind, it's essential to understand the distinction between water activity and moisture content and that water activity is a far more precise predictor of spoilage, degradation, and other undesirable changes.
For example, microorganisms use the water in their surrounding environment to survive. If a microbe is surrounded by a lower water activity than its interior, it will experience osmotic stress. Osmotic stress causes water from the inside to move to the outside, and it will lose trigger pressure. In this scenario, when a microorganism can't match its surrounding environment, it will go dormant, meaning it can't grow or reproduce in that environment. Each microorganism has a specific water activity level under which it can't grow, making water activity a highly effective way to control microbial growth.
How to control water activity
Controlling water activity is incredibly important in product safety and quality. Let's examine how to do it.
Water activity is affected by chemical and physical bonding, meaning that when water is bound, its energy decreases. Salt and sugar are each an example of additives that bind water. Total moisture content also affects water activity because less water in a product equals less water available to react. And last but certainly not least, temperature is a critical component in water activity because as temperature increases, water activity also increases. Therefore, it is vital to understand the temperature conditions your product will encounter and how your product will react to temperature abuse.
Here are a few effective ways to control water activity:
• Dehydration – Increasing dehydration in kibble or feed will reduce the amount of water in the product
• Edible films and coatings – Using the correct films and coatings can prevent moisture from being able to travel
• Humectants – Using additives that bind with water such as salt, sugar, amino acids, and polymers can help control water activity
When water activity is used in conjunction with another control method, such as pH levels or additives, it is known as hurdle technology. Each method serves as an additional hurdle help to keep the product safe.