Water activity, and the 0.75 line

Water activity is the variable that decides whether your preservative has an easy job or an impossible one, and most small formulators never measure it.

Not the same as water content

Water content is how much water is present. Water activity is how much of it is available to participate in reactions and to support growth. A product can hold a great deal of water and have much of it bound up by humectants, salts or sugars.

Organisms respond to availability, not to the total.

The threshold

Products with a water activity above roughly 0.75 support microbial growth. Almost every emulsion sits well above that, which is exactly why preservation is not optional in this craft.

Below it the environment becomes hostile without any preservative doing work. That is the reason an anhydrous balm is a different safety proposition rather than merely a drier one.

How to move it

Downward, by binding water: glycerin and other humectants at meaningful levels, salts, and in some formats sugars or polyols. Each has a cost in feel, so this is a formulation trade rather than a free lever.

You will rarely get an emulsion below the threshold. What you can do is move it in the right direction and reduce how hard the preservative has to work, which is what turns a marginal challenge result into a comfortable one.

Why it is the cheapest control you have

Because it is a design decision rather than a consumable. Raising preservative levels costs money on every unit forever, can break the emulsion by dehydrating the emulsifier, and invites sensitivity complaints. Lowering water activity costs nothing per unit once designed in.

If a challenge test comes back marginal, this is the first place to look, ahead of increasing the preservative. It is also the answer nobody reaches for first.

References