Food preservation

Why do salting, drying, and pickling keep food edible for months?

Salt cod, raisins, jam, sauerkraut, vinegar pickles. They taste nothing alike and are made in wholly different ways, yet each keeps for a year where the fresh version would be inedible in a week. It is tempting to say salt and vinegar are simply poisonous to microbes — but a raisin has nothing added to it at all, and jam is mostly sugar, which microbes love. So the common thread cannot be a poison. What could a dried grape and a salted fish possibly share?

Begin with what has to be stopped. Food spoils because bacteria, yeasts and moulds grow in it. Growth means dividing, dividing means building new cells, and cellular chemistry happens in water. So rather than asking how to kill a microbe, ask what it must have in order to multiply — and water is at the top of that list.

Now the trap. Salted fish is not dry to the touch and jam is visibly wet. Plenty of water is present. So amount of water cannot be the quantity that matters. What might distinguish water that is there from water a microbe can use?

Consider what salt does once dissolved. Each ion drags a shell of water molecules around itself and holds them, so those molecules are no longer free for anything else. Sugar does the same through hydrogen bonding rather than ionic attraction, with an identical outcome: water present, water spoken for. And a microbe sitting in that solution has more dissolved material outside its cell than in, so water leaves it by osmosis. It does not merely fail to grow; it is drawn dry.

Food scientists name the useful quantity water activity, written aw — availability rather than amount, on a scale where pure water is 1.0. Fresh meat sits near 0.99. Drying removes water; salting and sugaring lock it up; all three land in the same place on the same scale. That is how a raisin and a salt cod are related: the same move, performed with different tools.

Because it is one scale, it has thresholds. Most spoilage and pathogenic bacteria stop below roughly 0.91. Moulds and sugar-tolerant yeasts persist further down but give up around 0.70 to 0.60, and below about 0.60 essentially nothing grows. This also explains the failures: a lightly salted ham that never reaches the threshold spoils, and a dried food that pulls moisture back out of humid air climbs across the line and moulds — which is why the packaging matters as much as the drying did.

Pickling is genuinely a different lever. Vinegar does not lock up water; a pickle is thoroughly wet. It changes acidity. Microbes need their internal chemistry near neutral, and in acid the undissociated acid molecule crosses the membrane, dissociates inside, and forces the cell to spend energy pumping protons back out until it cannot afford to grow. The critical figure is pH 4.6: below it Clostridium botulinum will not grow and produce toxin, which is why acidic foods can be canned in a boiling water bath while low-acid ones need a pressure canner. Most real preserved foods use both dials — a sauerkraut has salt for water activity and lactic acid bacteria for acid, the salt selecting for the bacteria you want while the acid they make finishes the job.

Two corrections. None of these methods sterilises; they hold organisms in check rather than killing them, and spores in particular survive salting and drying comfortably — rehydrate a dried food carelessly and you restore the conditions along with the water. And preserved is not unchanged: fats still oxidise into rancidity, enzymes still work slowly, colour and texture still drift. Water activity and pH stop microbes, not time.

• Why smoking and curing salts add antimicrobial chemistry on top of these two levers rather than more of the same.
• How canning and freezing fit the picture, and which methods actually sterilise.

Every term the collection defines is gathered in the glossary.