FSIS-GD-2014-0010
Water activity and shelf stability
Which published shelf-stability limit a given water activity and pH meets, and the moisture-protein ratio alongside it.
Water activity is the number that decides shelf stability, because Staphylococcus aureus grows at a lower water activity than the other pathogens and is therefore the last one still able to grow. The published limits are limits for that organism.
The FSIS jerky guideline is where the figures are stated, and the Ready-to-Eat Fermented, Salt-Cured, and Dried Products Guideline says the same water activity parameters can be applied to other shelf-stable meat and poultry products.
The published limits
| Condition | aw | pH | Source |
|---|---|---|---|
| Stored in air | 0.85 | - | FSIS-GD-2014-0010, Meat and Poultry Jerky, step 6 |
| Vacuum packed in oxygen-impervious film | 0.91 | - | FSIS-GD-2014-0010, Meat and Poultry Jerky, step 6 |
| pH and water activity together, vacuum packed | 0.96 | 5.1 | Tilkens et al. 2015, cited in FSIS-GD-2023-0002 appendix 5 |
| pH and water activity together, stored in air | 0.92 | 5.1 | Tilkens et al. 2015, cited in FSIS-GD-2023-0002 appendix 5 |
Stored in air. FSIS recommends an upper limit of 0.85 under aerobic conditions. ICMSF puts the growth limit for S. aureus with oxygen present at 0.83; FSIS sets its recommendation slightly above that because jerky-type products carry other barriers - nitrite, salt, competing microflora. Steps must also be taken against mould, which is not controlled by this figure.
Vacuum packed in oxygen-impervious film. The limit rises to 0.91 without oxygen. ICMSF puts the anaerobic growth limit at 0.90. FSIS adds that a product between 0.85 and 0.91 stops being shelf stable the moment the pack is opened, and should carry a "Refrigerate After Opening" statement.
pH and water activity together, vacuum packed. In ready-to-eat snack sausages containing salt, dextrose, sodium nitrite and sodium erythorbate, pH at or below 5.1 combined with water activity at or below 0.96 did not support S. aureus growth under vacuum. This is a hurdle result for one product with one formulation, not a general licence to stop drying at 0.96.
pH and water activity together, stored in air. The same study supports pH at or below 5.1 with water activity at or below 0.92 aerobically. Mould grew under those conditions in the study, so a product held there needs a separate answer for mould.
Calculate
Limits this product meets
- Vacuum packed in oxygen-impervious film - aw at or below 0.91. FSIS-GD-2014-0010, Meat and Poultry Jerky, step 6
Working
moisture-protein ratio = moisture % / protein %
= 40.00 / 28.00
= 1.429 : 1
FSIS states salami must reach 1.9:1 or less for standard of
identity purposes. Protein does not leave while water does, so
the loss needed to reach that ratio is:
water to lose = moisture - 1.9 x protein
already at or below 1.9:1
Moisture-protein ratio is a labelling number, not a safety number. FSIS says so directly: MPR values "do not take into account availability of the water". Water activity is the safety number. Neither of them is a lethality treatment: drying does not reliably kill Salmonella, which tolerates it.
Worked example
A salami at 42 percent moisture and 26 percent protein. MPR = 42 / 26 = 1.615 : 1 FSIS states salami must reach 1.9:1 or less. Protein stays put while water leaves, so: water to lose = 42 - 1.9 x 26 = 0.00 g per 100 g = 0.00 percent of the green weight
What this does not tell you
- None of these is a lethality limit. FSIS states that drying does not necessarily result in an adequate reduction of Salmonella, because the organism can be resistant to drying.
- Moisture-protein ratio is a labelling number. FSIS says MPR values do not take into account availability of the water.
Related: Staphylococcus aureus, Air drying.