Water Activity, Explained With a Bag of Chips

I want to explain one number that governs almost everything in your kitchen, and I want to do it with a bag of chips, because the chips are the clearest example and everybody has one in the cupboard.

Open a bag of chips and leave it out on a humid day. Come back in three hours and they are soft. Nothing was added to them. Nothing was taken away. The air did all of it.

What Actually Happened

The chips were dry. The air was wet. Dry things and wet air want to reach an agreement, and the agreement is that water moves from the air into the chips until the two match. That is it. That is the whole mechanism.

This is called reaching equilibrium, and it is not a chemical reaction. It is just water finding the level. The chips did not spoil. They picked up moisture, and moisture is what makes a crisp thing stop being crisp.

Now here is the part that matters. The number that describes how much water is AVAILABLE to do that is called water activity, and it runs from zero to one. Zero is bone dry. One is pure water. Everything you eat sits somewhere in between.

Why It Is Not the Same as Water Percentage

This is where most people's intuition breaks, so it is worth going slowly.

You can have two foods that are both five percent water by weight and behave completely differently. One keeps for a year on a shelf. The other grows mold in a week. The percentage does not tell you which is which.

What tells you is how tightly that water is held. In honey, the water is bound up so tightly by the sugar that nothing can use it. Honey sits at a low water activity despite being liquid, which is why it does not spoil. In a fresh strawberry, the water is loose and available, which is why it goes soft in the drawer.

So the useful question is never how much water is in this. It is how much of that water is free.

Where the Danger Line Is

Nothing that spoils food can grow below a water activity of about 0.6. That is the line. Below it, bacteria and mold and yeast are not dead, they are dormant, and dormancy means they cannot multiply.

Freeze-dried food sits down around 0.08 to 0.33. That is not close to the line. It is in a different country from the line, which is why a properly sealed pouch keeps for years and why the date on it is really a statement about the seal.

A fresh strawberry sits up near 0.95. A slice of bread is around 0.95. Cheese is somewhere in the 0.9s. Every one of those needs refrigeration or a fast clock, and now you know why: they are all comfortably above the line.

The Practical Bit

You do not need to measure anything to use this. You need one idea: dry things stay dry only if you keep them away from wet air, and the container is the only thing standing between them and the room.

Which is why the desiccant packet in a jar is doing real work. It is not decoration. It is pulling water out of the headspace so your food is not competing with the air inside the container for dryness.

And it is why an open bag of anything crisp goes soft in a humid week, and why the same bag survives a dry January. The food did not change. The room did.

Stay curious. Stay crunchy. Stay cosmic.

— Sara Jane Shelvock, CEO, Torus Coffee Company LLC