Preserving and processing food

On a pebble-covered bank, long rows of fish dry on poles, with a few people in the distance. — Salmon set out to dry on poles near a fishing stage of the Nlaka’pamux (Thompson) people, at Spences Bridge, in British Columbia: photograph by George M. Dawson, 1889. George M. Dawson, CC BY-SA 4.0, source

In the Andes, the frost at night and the sun by day make it possible to keep potatoes for years.

Preserving food was, for almost the whole of human history, the number one problem: it arrives all at once, at harvest time or after a hunt, and it does not last.

Removing water

The micro-organisms that make food rot need water. Take the water away from them, and the food keeps.

This is the simplest technique, and it is found everywhere: drying in the sun, in the wind, in smoke. Fish, meat, fruit and chillies are dried.

In the Andes, an even cleverer process was invented. At high altitude, temperatures drop below freezing at night and the sun blazes by day. Potatoes are spread out of doors: they freeze, which bursts their cells, then they thaw in the sun. They are then trampled by the family, on areas set aside for the purpose, which drives out the remaining water and removes the skin. The operation is repeated.

After several days, what remains is a hard, light, almost black product that keeps for years. This is chuño.

This is no makeshift expedient: it is exactly the principle of freeze-drying, the technique used today for astronauts’ food — invented in the Andes long before.

Chuño forms a reserve against famine on the scale of an empire. Archaeologists have found two chuños about five hundred years old in a jar set into the floor of an Inca site, on the desert coast of Peru. Yet chuño can only be made high in the mountains. These two pieces of potato are enough on their own to prove that this food was carried from one end of the empire to the other.¹

Salt, and what it cost

Salt draws water out of food and stops microbes from settling in. Salted, a fish can last through a winter or cross an ocean.

That is why salt was, for centuries, one of the most precious commodities in the world. It was carried by camel across the Sahara, extracted from mines and salt marshes, and fought over.

That is also why states took an interest in it. Since no one can do without it, a salt tax is a sure source of revenue. Rome introduced one as early as 204 BCE. Comparable taxes existed in China, Japan and India.

A story best not repeated. You will read almost everywhere that Roman soldiers were paid in salt, and that our word “salary” comes from this. Half of it is true: salarium does derive from sal, salt. The other half has no basis. No ancient text attests that soldiers were paid in salt, or even that there was an allowance intended for buying it. The sentence from Pliny the Elder quoted in support — “the soldier’s pay was originally salt” — does not appear in this form in his text. What he actually writes is: salt is also related to magistracies and duty abroad, and that’s where we get the word “salaries”. He notes a link between two words; he does not say how soldiers were paid.
The researcher who took this story apart traces it back to the 1860s. The method is the same as for the legend of the diabolical zero: the birth of an invention can be dated by following who first wrote it down.
The story even has a happy ending. One of the greatest dictionaries of the English language has revised its entry and now cites this researcher in support of its caution. So a correction can travel up the chain, and not only down it.
Why a word for salt ended up meaning a payment remains unknown.²

In France, from the 1340s, the king taxed salt, then reserved its sale for himself. It could now be bought only in storehouses run by his agents, at the price he set, tax included. This tax was called the gabelle.³

The heaviest burden was not the tax itself. In some regions, every person was obliged to buy a minimum quantity of salt each year, whether they needed it or not. The price changed from one province to another: very expensive here, cheaper there, free in a few places. That is what made it the most hated tax in the kingdom.

It was abolished in 1790, reinstated in 1806, abolished again, reinstated again. It did not finally disappear until 1945.

Preserving one’s food was therefore expensive — not because of the technique, but because those in power had decided it should be so.

Letting the microbes work

The second strategy is the exact opposite of the first: instead of stopping micro-organisms from working, you let them get on with it — but you choose which ones.

This is fermentation. The good micro-organisms take up the space, transform the food, and stop the bad ones from settling in.

Almost every society has practised it, with whatever it had to hand. Bread and beer from cereals. Cheese and yoghurt from milk. Kimchi from vegetables, in Korea. Miso and soy sauce in East Asia. Sauces made from fermented fish, in South-East Asia as in the Roman world. Injera, in Ethiopia, from a tiny cereal called teff.

No one had ever seen a micro-organism, which stopped no one from using them.

Processing to make food edible

Some techniques go further than preservation: they make edible what is not.

Chuño already provides an example. There are two kinds. Black chuño is simply dried in the sun. White chuño, for its part, is made from bitter varieties of potato, toxic in their raw state, and it requires soaking in water for several weeks to remove the poison. Preserving and detoxifying, in a single operation.

Cassava is the clearest example. There are two kinds: sweet and bitter. Bitter cassava contains substances that release cyanide — the plant’s defence against those who would eat it.

To make it edible, it has to be peeled, grated, soaked at length, left to ferment, pressed, dried, then cooked for a long time. Each step removes part of the poison. Fermentation, together with pressing and washing the pulp, is the most effective step; drying in the sun or frying then removes part of the poison that remains.⁴

These steps are not mere habits but instructions, worked out in Amazonia, passed on for centuries, and followed today by hundreds of millions of people.

We must also mention what happens when they cannot be followed. Soaking cassava requires water, time, and something to eat while waiting. In times of drought, famine or war, families are forced to cut the soaking short. The poison remains, and a disease appears, konzo, which suddenly paralyses the legs and from which there is no recovery.⁵

A trap makes the situation far crueller.

When there is drought, the plant itself produces much more poison than usual. Cassava, stressed by the lack of water, increases its defence.⁶

In other words: at the very moment when there is not enough water to soak the roots, when people are hungry and cannot wait, the roots are more toxic. The three factors reinforce one another, and they arrive together.

A fourth factor adds to them. To neutralise cyanide, the body needs the sulphur in certain proteins. A person who eats little protein is therefore less well defended — and it is exactly the same circumstances that deprive people of protein and force them to cut the soaking short.⁷

It is not knowledge that is lacking. The knowledge is sound and it works. What is lacking is water, time, food and choice.

The doctor who gave this disease its name said so bluntly: one should not say that cassava causes konzo. Poverty, malnutrition, conflict, the absence of roads and markets are also needed. And he added the reason that matters here: if the true cause is not found, one does not act correctly.

A more encouraging note to finish: a quicker way of removing the poison from cassava flour has been developed, then taught to the women of affected villages. In one of them, the proportion of children exposed to the risk fell from more than half to less than a fifth.⁸

The most astonishing case is that of maize. In Mesoamerica, it is not simply cooked: it is soaked and cooked in water with lime added — further north, wood ash was used instead. This process is called nixtamalisation.⁹

This removes the skin of the grain, which is convenient. But above all, it releases a vitamin, niacin, which in raw maize remains locked up: it is indeed there, but the body cannot use it.¹⁰

The rest of the story is instructive. When maize travelled around the world, the plant travelled, but not the technique. In Europe and then in North America, poor populations ate untreated maize as their staple food. They developed a deficiency disease, pellagra, which damages the skin, the gut, then the mind, and which kills.

People died because the plant had been adopted without the know-how that went with it.

Cold

The last strategy: slow things down. In the cold, micro-organisms work slowly.

Long before electricity, ice houses were built. The most impressive is Iranian, and it bears a name meaning “ice pit”: the yakhchal.

One clarification is needed first, because it changes what goes into it. It is not a refrigerator. Food was not stored in it: ice was made and kept in it, and then distributed in the town during the summer. The ice, for its part, served many purposes — including sorbets.

The yakhchal is a great dome of earth set over a deep pit, with walls at least two metres thick, made of a special mortar mixing sand, clay, lime, egg white, goat hair and ashes. In winter, water is left to freeze in pools nearby — the water arrives through an underground channel, a qanat, like those described on the page on mastering water. The ice is piled into the pit. It lasts until the height of summer, when the temperature outside exceeds forty degrees.

How long has it existed? No one really knows, and that is interesting. Digging a pit to keep ice or snow is a very old practice: a Greek author relates that Alexander the Great had them dug during his campaign in India, more than two thousand three hundred years ago. But between that hole in the ground and the great brick dome, the whole story is missing: no text, no excavation tells how one led to the other. Many of the domes still standing are much more recent than people believe.

Yet one date is found everywhere, always the same, always phrased in the same way. That is precisely what should make us wary: when a sentence is copied word for word, it often has only one source.

A delightful detail to finish: in Persian, the word yakhchal still means refrigerator today.

At the end of the eighteenth century (the years 1700 to 1799), the French government promised a reward to whoever found a good way of preserving food. A confectioner, Nicolas Appert, set to work on it in 1795. He sealed food in carefully stoppered glass jars, then heated them in a water bath. He tried, failed, started again — for fifteen years.

The method worked. In 1810, he was given twelve thousand francs, on one condition: that he publish his method instead of keeping it to himself. He agreed, and his book was printed in a run of six thousand copies.¹¹

But no one, not even he, knew why it worked. It would be another fifty years or so before the role of micro-organisms was understood. Throughout that time, food was preserved with a method that worked without anyone being able to explain it.¹²

Then came industrial refrigeration, and with it a new problem: between the farm and the plate, food passes through trains, warehouses, shops. If a single link is too warm or too dirty, everything is lost.

It was an American chemist, Mary Engle Pennington, who brought order to this field at the beginning of the twentieth century (the years 1900 to 1999). She measured the exact temperatures at which bacteria grow, set the rules for building refrigerated railway wagons, developed quick freezing, and wrote the first rules of what is today called the cold chain.¹³

One detail of her hiring deserves to be reported precisely. It was not she who had this idea. Her future director registered her under the name “M. E. Pennington”, so that no one would see that she was a woman.¹⁴

She came top in the examination.¹⁵

When the administration realised that she was a woman, some wanted to reverse her appointment; her director intervened, and she took up the post. In 1907 she became the first woman to head a laboratory of the United States government (some sources date her appointment to the end of 1906; the year 1907 is the one most generally accepted).¹⁶

Four ways to stop rotting

Principle	Techniques	Examples
Removing water	Drying, frost and sun, smoking	Dried fish, Andean chuño
Making the environment uninhabitable	Salting, sugar, vinegar	Salted fish, confits
Choosing the micro-organisms	Fermentation	Kimchi, cheese, injera, miso
Slowing down	Cold, freezing	Cold chain

Note 1

ALBERTA, CANADA—Two freeze-dried potatoes estimated to be 500 years old were discovered in a ceramic jar set into a floor at Tambo Viejo, a coastal Inca site in southern Peru’s arid Acarí Valley, according to a Phys.org report.

Saraceni, Jessica Esther, “Preserved Potatoes Discovered in Inca Site in Southern Peru”, Archaeology (Archaeological Institute of America), 16 June 2026