Water activity ( aw) is the term for the amount of free (not chemically or physically bound) water, which is available for the growth of microorganisms. This information is particularly important, as higher amounts of free water favour the growth of microorganisms, while lower amounts (drier products) result in less microbial growth. Bacteria usually require at least aw 0.91 and fungi at least aw 0.71.
Aw can be measured by means of water activity meter.
Typical aw in Meat Products
| Product | aw range |
| Fresh meat | 0.99 (0.99 to 0.98) |
| Cooked ham | 0.97 (0.98 to 0.96) |
| Raw-cooked sausages | 0.97 (0.98 to 0.93) |
| Liver sausages | 0.96 (0.97 to 0.95) |
| Blood sausages | 0.96 (0.97 to 0.86) |
| Raw-fermented ham | 0.92 (0.96 to 0.80) |
| Raw-fermented sausages | 0.91 (0.96 to 0.70) |
| Dried meat | 0.70 (0.90 to 0.60) |
Limiting aw for the Growth of Microorganisms
| Microorganisms | aw |
| Pseudomonas | 0.93 |
| E. coli | 0.93 |
| Salmonella species | 0.91-0.95 |
| Listeria | 0.93 |
| Cl. botulinum types | 0.91-0.95 |
| Cl. perfringens | 0.93-0.95 |
| Bacillus species | 0.90-0.95 |
| Lactobacillus | 0.90 |
| Staph. aureus | 0.86-0.90 |
| Most yeasts | 0.87-0.90 |
| Most moulds | 0.80-0.85 |
Most bacteria between aw 0.91 – 0.96
Gunter Heinz & Peter Hautzinger – Meat Processing Technology
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