Preserving food means taking away one of the four things microbes need: water, warmth, oxygen or a low-acid environment.
What spoils food are bacteria, yeasts, moulds and the food's own enzymes. All of them need four conditions: water, warmth, oxygen and a low-acid environment. Every preservation method removes at least one. Understanding which one lets you improvise instead of following recipes.
Removing water
Drying is the oldest method and the most economical. Below about 15 % residual moisture, almost nothing grows any more.
What matters isn't heat but air movement and thin slices. Drying too slowly gives mould time to settle in the middle; drying too hot hardens the surface and traps the moisture inside. Three to six millimetres thick, in the shade, in a draught, is the universal compromise.
Salting removes water another way: salt draws water out of cells by osmosis, including out of bacterial cells, which dry out. Sugar at high concentration does exactly the same, which is how jam works.
Removing warmth
Cold kills nothing, it slows everything down. Every ten-degree drop divides the speed of spoilage by roughly two or three.
Without electricity, there are three free sources: the ground, steady at around ten to twelve degrees (50 to 54 °F) one metre down in temperate climates; running water, noticeably colder than a shelter in summer; and evaporation, where a wet cloth over a porous container in a draught lowers the temperature by several degrees. None of them keeps food for long, but all of them buy days.
Removing oxygen
Set fat over a jar, oil covering a vegetable, submersion in brineSalted water in which food is submerged to preserve it.: all of them cut off contact with the air and stop mould.
Careful: this is where the only real danger in this sheet lies, covered in Les conserves en bocal. An environment with no oxygen, low acidity, little salt and at room temperature is exactly what the bacterium behind botulism is looking for. It changes neither the taste, nor the smell, nor the look, and its toxin is one of the most powerful known. That's why jarred vegetables, meat and fish can't be improvised: they need pressure sterilisation, which plain boiling doesn't reach.
Acidifying
Below a certain level of acidity, almost all dangerous bacteria stop growing, including the one behind botulism. That's what makes vinegar and lactic fermentationNatural bacteria turning sugars into lactic acid, which makes the food acidic and protects it. so safe.
Fermentation is the most elegant of the methods: you don't get rid of microbes, you favour the ones that suit you. Lactic bacteria, naturally present on vegetables, tolerate salt and the lack of air better than their competitors. You let them win, they produce the acid, and the acid protects the rest.
The methods in practice
Drying. Slice thin and even. Hang or spread on a rack, in the shade, in a draught, protected from insects. The food is dry when it snaps or cracks instead of bending. Store it in the dark in a closed container, and check after a week: leftover moisture shows up as condensation.
Lactic fermentation of vegetables, detailed in La fermentation lactique. Slice, put in a jar, cover with brine, keep everything submerged under a weight, let it release its gas at room temperature for a few days, then store somewhere cool. The rule of thumb is simple: anything sticking out of the brine will go mouldy.
Preserving in fat. Cook slowly in the meat's own fat, put it in a jar, and cover with a layer of set fat that seals the surface. Keep it cool and in the dark.
Burying. Roots and tubers keep for months in a clamp: a pit, a bed of straw or dry sand, unwashed vegetables, an insulating cover, and drainage so water doesn't pool. See Collecting and storing rainwater for the principle of the buried container.
By climate and means
What to preserve first follows from real needs, detailed in Ce que le corps consomme.
You have no salt. You lose salting and fermentation gets harder. Drying and cold remain. In that case, salt becomes one of the most sought-after trading goods there is — see Trading, negotiating, keeping your word.
You're in a humid climate. Air drying fails. Switch to smoking, which dries and at the same time deposits antimicrobial compounds, or to fermentation, which doesn't mind humidity. Smoking needs a controlled, long-lasting fire: see Making and keeping a fire.
You're in a hot climate. Everything goes faster, both ways: drying works in a day, and so does rotting. Process food the same day, never the next.
You have to judge a doubtful food. Trust smell and appearance first, but know that no visible sign proves nothing for jarred preserves. A bulging lid, contents that foam or spurt when opened: throw it away without tasting. With preserved food, doubt isn't something you test, it's something you settle by throwing it out.