How does salt preserve food?

Salt lowers a food's water activity, meaning the fraction of water available to microorganisms. Through osmosis, it also dehydrates the bacterial and fungal cells trying to grow in it. Below a certain water-activity threshold, most pathogenic bacteria can no longer multiply. Brining adds the gradual acidification of the medium, which further reinforces this barrier effect.

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Salting and brining: preserving food with salt

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Maomao investigates in the kitchens after a bureaucrat’s poisoning. Facing suspicion falling on some seaweed, she immediately rules out the idea of a year-old stock, pointing out that brine does not guarantee indefinite preservation: even soaked in salt, seaweed harvested the previous year would not have held up. This reasoning gives her a chronological clue for dating the goods and directing her investigation. Later, she mentions that brine-preservation is the normal way to import this seaweed, which explains how it could travel from a distant region without rotting along the way.

The subject in depth

Preservation by salt relies on two linked mechanisms. The first is the reduction of water activity, denoted a_w, a value that measures the fraction of water in a food that is actually available for chemical and biological reactions ^[Wikipedia]. A fresh food has a water activity close to 1: the water is free, and microorganisms can use it for their enzymes, metabolism, and reproduction. By saturating a food with salt, part of that water becomes chemically bound to sodium and chloride ions, which drives a_w down. Below 0.9, most pathogenic bacteria stop multiplying; below 0.7, nearly all common microorganisms are inhibited.

Diagram of osmosis during salting and the loss of water activity in salt-preserved food — Salt takes the water microbes need, so they cannot multiply

The second mechanism is osmosis ^[Wikipedia]. Salt creates a high ionic concentration outside microbial cells. Through osmosis, water leaves those cells toward the more concentrated surrounding medium, dehydrating them until they can no longer function. This osmotic pressure is also why meat or fish placed in a strong brine themselves lose water: the salt does not soak into the food, the food dries out from the inside.

Salting takes two main forms. Dry salting involves rubbing or coating the food with crystalline salt, which gradually draws out moisture and forms its own brine on the surface. Wet brining plunges the food directly into a saturated or semi-saturated saltwater solution; diffusion is faster and more even, which suits thick cuts or preparations meant for import and transport.

Acidity plays a complementary protective role. In brine, halophilic (salt-tolerant) lactic acid bacteria ferment the food’s sugars and produce lactic acid. This low pH creates a second barrier for microorganisms that cannot tolerate acid. Many traditional salt-based preparations, from sauerkraut to fermented fish to olives, rely on this combined effect: salt selects for lactic acid bacteria, and those bacteria then acidify the environment, making the niche hostile to pathogens.

But this protection is neither absolute nor permanent. Some halophilic or halotolerant microorganisms resist high salt concentrations. Others, like Clostridium botulinum, the cause of botulism, can grow under anaerobic, low-a_w conditions if the salt concentration is insufficient. And time itself matters: salt can redistribute over time, the food can absorb ambient moisture, and a_w can rise back up. This is exactly what Maomao expresses in the story when she says that even in salt, a year-old batch of seaweed eventually degrades.

In real life

Salting is one of the oldest known preservation techniques, attested on several continents well before our era. In China, salted fish, or xian yu, was a staple food as far back as Antiquity. Ancient Egypt salted meat and fish to feed armies and construction crews. In Mediterranean Europe, Roman garum, a fermented fish sauce made in brine, and Iberian salted tuna and mackerel were exported across the Empire. Further north, salt cod, dried and re-salted, shaped the Atlantic economy for centuries, allowing entire populations to have access to protein without refrigeration. Salt itself was a strategic resource: trade routes, taxes, and even some wages depended on it in many cultures.

Today, salting is still practiced on a large scale in the food industry, no longer as the sole line of defense but combined with refrigeration, vacuum packaging, and microbiological controls. Water activity has become a measurable regulatory parameter, allowing safety levels to be guaranteed that ancient cooks could only estimate empirically.

The fact-checker's eye

Maomao’s reasoning is scientifically sound. Brine does preserve food well, but not indefinitely: the duration depends on salt concentration, the type of food, storage temperature, and the microorganisms present. Using the preservation limit as a chronological clue is a valid argument in a real food-safety investigation. The series does not simplify the mechanism, it applies it with precision, which is rare.

One nuance, though: in the scene, Maomao dismisses year-old seaweed as necessarily spoiled, without specifying the brine’s concentration or the storage conditions. In practice, some very concentrated preparations, like salted anchovies (Engraulis encrasicolus), can keep properly for several years. The reasoning still holds as an investigative heuristic, but seaweed preserved in a very strong brine in a cool environment could theoretically hold up for more than a year.

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Published on May 30, 2026

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