Water activity (aw) measures how much of the water in a food is free to be used by bacteria, molds and yeasts, on a scale from 0 to 1. The formula is the vapor pressure of water in the food divided by the vapor pressure of pure water at the same temperature, so pure water is 1.00 and a dry cracker might be 0.30. It isn't the same as moisture content: honey is about 17% water and still keeps for years, because the sugar holds the water tight. Water activity is one of the two numbers, with pH, that decide whether a food can safely sit on a shelf without refrigeration, and it appears in FDA's rules and the Food Code. Below is what it means, the limits that matter, how to test it, and how to keep the readings.
The short version
- Water activity = vapor pressure of the food's water ÷ vapor pressure of pure water, at the same temperature.
- It runs from 0 to 1; lower means less water available for microbes.
- It's not moisture content: two foods with the same moisture can have quite different water activity.
- FDA's acidified and low-acid canned food rules apply to foods above 0.85.
- The Food Code uses water activity with pH to decide which foods need refrigeration.
- Test it with a water activity meter, or send samples to a lab.
- TaroStack keeps the reading on every batch record, tied to its lot. More below.
Water activity against moisture content
| Moisture content | Water activity | |
|---|---|---|
| Measures | How much water, by weight | How freely that water is available |
| Units | Percent | 0 to 1, no units |
| Honey | About 17% | About 0.6 |
| Fresh bread | About 35% | About 0.95 |
| What it predicts | Weight, texture, yield | Microbial growth, shelf stability |
Sugar, salt and other dissolved solids bind water, which is why jam, honey and cured foods keep at room temperature with plenty of water in them. For safety, water activity is the number that matters.
The limits in the rules
FDA defines water activity as the quotient of the water vapor pressure of the food and that of pure water at the same temperature, and its acidified foods and low-acid canned foods rules apply to foods with a water activity above 0.85 (21 CFR 114.3). Below 0.85, those particular rules don't reach the food.
The FDA Food Code, which states adopt for retail and food service, uses water activity together with pH to decide which foods need time and temperature control. For a food that isn't heat-treated, or is heat-treated but not packaged, its table reads (FDA Food Code 2022):
| Water activity | pH under 4.2 | pH 4.2 to 4.6 | pH over 4.6 to 5.0 | pH over 5.0 |
|---|---|---|---|---|
| Under 0.88 | No temperature control needed | No | No | No |
| 0.88 to 0.90 | No | No | No | Assessment needed |
| Over 0.90 to 0.92 | No | No | Assessment needed | Assessment needed |
| Over 0.92 | No | Assessment needed | Assessment needed | Assessment needed |
For food that's heat-treated and then packaged, a second table is more generous: at a water activity of 0.92 or below, no temperature control is needed at any pH. "Assessment needed" means the food has to be evaluated, for example by a process authority or a lab, before it can be treated as shelf-stable. Some state cottage food laws use a water activity limit too; Hawaii's, for example, is covered in Hawaii cottage food law. This is general information, not food safety or legal advice.
Testing it
| Option | How | Good for |
|---|---|---|
| Lab testing | Send samples; get a report | Setting a process, a product launch, a buyer's request |
| Benchtop water activity meter | Put a sample in the chamber; read in minutes | Checking every batch in-house |
| Both | Lab to set the limit, meter to check batches | A small maker with a shelf-stable product |
Readings depend on temperature, so test at the temperature your meter or lab specifies, and calibrate a meter with its standards. Test the finished product as sold: granola after baking and cooling, jerky after drying, a jam after it sets. Water activity also drives quality, not only safety: crisp snacks go soft as their water activity rises, which is part of shelf life testing.
Recording it by batch
A reading only protects you if it's tied to the batch it came from. Record the water activity, the temperature, the pH if you test it, the date, who tested, and the lot code of the batch, every time. If a buyer or an inspector asks how you know a product is shelf-stable, the batch records are the answer. How a batch record should look is in what is a batch record.
Where this stops working
On paper, readings go on a clipboard by the meter, the batch record is in a binder, and the lot code on the package comes from a stamp. The three are supposed to match. When a batch reads high and gets re-dried, the retest goes on a sticky note, and a year later nobody can show which reading belongs to which lot.
How TaroStack does it
TaroStack keeps the reading on the batch, and the batch on the lot, so the evidence travels with the product.
The reading is part of the batch record. Batch record forms are filled in when the batch is recorded, with your own fields: water activity with its limit, temperature, pH, initials and sign-off. A reading outside its limit is recorded and flagged, and the flag is marked reviewed with what was done about it, so a high reading can't pass quietly. Recording the batch takes ingredients from the lots used, notes them automatically, and gives the finished product its own lot code and date.
A batch that fails can wait. Stock on hold sits apart from what's available to sell, so a batch waiting for a retest, or one that read high, can't be picked or shipped by mistake. If it can't be saved, writing it off records the reason and the cost.
And the lot leads back. The lot label prints from the lot, and one report walks any lot back to its batch record and supplier lots, and forward to every customer who got it.
| The job | Clipboard and binder | In TaroStack |
|---|---|---|
| Reading for each batch | A clipboard by the meter | A field on the batch record, flagged outside its limit |
| A batch that read high | A sticky note | On hold until it's retested |
| Which reading goes with which lot | A guess | The lot's own batch record |
| A buyer's question | Search the binder | One report, lot to customers |
Lots, holds and recalls are on every plan, from $49 a month; batch records with checklist fields are on Standard at $99. The first 30 days are free, with no credit card. If you'd like a hand setting up, ask, and we'll do it with you.