Shelf Life and Water Activity

Water activity (aw) measures how much water in a food is available for microbial growth and chemical reactions, on a scale from 0 (bone dry) to 1.0 (pure water). Fresh meat has aw 0.99, bread 0.95, jam 0.80, dried fruit 0.60, crackers 0.30. Most bacteria cannot grow below aw 0.85, most yeasts below 0.80, and most molds below 0.65. Staphylococcus aureus is exceptionally resistant, growing at aw as low as 0.83. Foods with aw below 0.85 are considered low-risk and do not require refrigeration—this is why honey (aw 0.55-0.65), beef jerky (aw 0.70-0.85), and hard cheeses (aw 0.80-0.87) are shelf-stable.