What is water activity, really?

Shelf life

Water activity is the most useful idea in food preservation that almost no one outside a lab talks about. Understand it, and a lot of food science suddenly makes sense — why honey never spoils, why dried fish keeps for months, and why “it doesn’t feel wet” tells you almost nothing about whether a product is safe.

Not how much water — how available the water is

Microbes — bacteria, yeasts, and moulds — need water to grow. But they cannot use every drop of water in a food. Some water is bound to sugar, salt, or proteins and is not available to them. The water they can use is called free water, and water activity measures exactly that.

Key term — Water activity (a_w): a number from 0 to 1 that describes how much free water is available in a food. Pure water is 1.0. The lower the number, the less water microbes can use, and the longer the food keeps.

Water activity is not the same as moisture content

This is the point most people miss. A food can be quite wet and still be shelf-stable, because its water is bound up rather than free. Honey is about 17% water, yet it never spoils — its sugar holds that water so tightly that its water activity is around 0.6, too low for microbes. Jam works the same way. So does the salt in prahok.

That is why you cannot judge safety by feel. “It doesn’t seem wet” is not the same as “it has low water activity”.

The numbers that matter

Above ~0.90 — most bacteria, including dangerous ones, can grow. Fresh foods live here.
Below ~0.90 — most bacteria stop, though some (like Staphylococcus aureus) can still grow down to about 0.86.
Below ~0.70 — most yeasts and moulds stop.
Below ~0.60 — essentially nothing grows. This is the safe zone for shelf-stable dried and high-sugar foods.

How you lower it

Three everyday methods, all of which show up in traditional Khmer food:

Drying — remove the water itself (dried fish, dried mango, dried chilli).
Adding salt — salt binds free water (prahok, salted fish, fish sauce).
Adding sugar — sugar binds free water too (jam, palm sugar, sweetened condensed products).

Practical tip: lowering water activity is often cheaper and more reliable than adding preservatives — and it is what your customers already trust in traditional foods.

You cannot eyeball it — you measure it

Water activity is measured with a small instrument called a water-activity meter, which reads the value in a few minutes. For any product you plan to sell as shelf-stable, this number is not a nice-to-have — it is how you prove your product sits in the safe zone.

Why this is a safety issue. A product that looks and feels dry but actually sits at a water activity of 0.92 can still grow dangerous bacteria. If you are selling shelf-stable food, measure water activity — do not guess it.

The short version

Water activity measures the free water microbes can use — not total moisture.
A wet-looking food can still be safe if its water is bound by sugar or salt.
Below 0.60, essentially nothing grows.
Lower it by drying, salting, or sweetening.
For shelf-stable products, measure it — never assume.

A note on this article. This is general educational information, not a substitute for professional food-safety testing. For any shelf-stable product you intend to sell, confirm its water activity and safety with proper measurement and, where required, an accredited laboratory.