Food Chemistry

The aW scale ranges from 0 (completely dry) to 1.0 (pure water). Most fresh foods have a high aW (e.g., raw meat and fresh produce are above 0.95), making them highly perishable. As you lower the aW, you restrict the ability of bacteria, yeasts, and molds to grow.

Here's a diagram illustrating the general effect of water activity on different types of spoilage:

Measuring aW typically involves specialized equipment like a water activity meter. This device equilibrates the food sample's vapor pressure with its headspace, then measures the relative humidity of that headspace to determine the aW.