Cooking science

Water activity

Jam is about a third water and it keeps for a year. A fresh strawberry is ninety per cent water and it keeps for three days. The difference is not how much water is present but how much of it is available — and that is water activity.

Bound water and free water

Microorganisms need liquid water to live in. What they cannot use is water that is chemically tied up — held by dissolved sugar, salt or other molecules that grip water so tightly it is no longer available.

So a jar of jam contains plenty of water, and nearly all of it is bound to sugar. A cured ham contains a great deal of water bound to salt. Honey is around eighteen per cent water and almost none of it is free. In every case the food is wet and the water is not usable.

This is why preserving with sugar and salt works without drying anything out. Nothing was removed; the water was made unavailable, which amounts to the same thing from a bacterium's point of view.

The three ways to lower it

Remove water

Drying, dehydrating, air-curing, reducing a sauce. The obvious route.

Add sugar

Jam, marmalade, candied peel, condensed milk, honey. The sugar binds the water that is there.

Add salt

Cured meat, salt cod, preserved lemons, brined vegetables, salted butter.

Freeze it

Ice is not liquid water and cannot be used. This is a large part of why freezing preserves.

Combine them

Most traditional preserving uses two or three at once — salted and dried, or sugared and acidified.

Different organisms differ

Moulds tolerate lower water activity than bacteria, which is why jam grows mould long before it grows anything else.

What it explains

• Why low-sugar jam does not keep like a full-sugar one. Less sugar means more free water. It is a preservation change, not a taste preference.
• Why a cured ham hangs at room temperature while a fresh joint of the same meat does not.
• Why dried fruit goes mouldy if it is not dry enough, and why partly dried fruit needs refrigerating.
• Why honey never spoils despite being liquid.
• Why bread stales but does not usually rot, and why a wetter enriched loaf moulds faster than a lean one.
• Why fudge and caramel keep far longer than the cream they are made from.
• Why the surface of a food matters most — the surface dries or picks up moisture first, which is where mould appears.

What follows for a cook

1. Treat sugar and salt in a preserve as structural, not as seasoning. Reducing them changes how the food keeps.
2. Take the numbers from a food safety authority rather than adjusting to taste. Whether a preserved food is safe to store, and for how long, depends on numbers a food safety authority sets. FoodHQ publishes none of them — follow current Food Standards Agency guidance.
3. Dry things properly if you are drying them. Partly dried is the worst of both worlds — too wet to keep, too dry to be fresh.
4. Keep the surface of preserved foods covered — oil on labneh, syrup over fruit, fat over a pâté. Surfaces are where moisture and mould meet.
5. Store dried goods sealed. They pick moisture up from the air and their water activity rises.
6. Expect a low-sugar or reduced-salt version of any preserve to need refrigerating and to keep for less time.

Common questions