Water Activity vs Moisture Content

Moisture content and water activity are related, but they are not the same. Moisture content is how much water is in the food by weight. Water activity is how available that water is for microbial growth. Some water can be tightly bound to the food structure and not available to bacteria or mold. Water activity is measured from 0 to 1. Most bacteria struggle below about 0.90. Many molds need higher than about 0.70. Properly freeze-dried foods are often far lower than those values. This is why thorough drying and good packaging matter. If food re-absorbs moisture from air, water activity can rise and stability drops. Understanding water activity helps explain why “looks dry” is not enough. You want the product truly dry and protected from moisture re-entry.