How Does Pickling Preserve Food? Salt, Acid and the Microbes on Your Side

Fresh food is eaten by microbes before people get to it: bacteria, moulds and yeasts on its surface and in the air multiply in the water and sugar it contains, and the results are the slime, the off smells and the toxins of spoilage, of which the botulism bacterium is the dangerous one. Most spoilage organisms need a pH above about 4.6, neutral being 7, and water they can use; take either away and they stop. Drying and salting remove the water, freezing locks it, canning kills the microbes with heat and seals them out, and pickling makes the food too sour, which is also why it tastes as it does.

The acid can be added or grown:

• •Vinegar pickling: the food is packed in a solution of vinegar, water, salt and often sugar and spices, usually heated, and the acetic acid of the vinegar, at a pH of 3 or so, does the preserving at once; quick, reliable and the method of most commercial pickles, and the result is sharp and stable but not alive
• •Fermentation, or lacto-pickling: the food is packed in salt or brine without vinegar, and the lactic acid bacteria that live on every vegetable, tolerating salt that their competitors cannot, eat the sugars and excrete lactic acid, which drops the pH over days until nothing else can grow; sauerkraut, kimchi, real dill pickles and the sour pickles of Eastern Europe are made this way, and the result is complex, fizzy at first and full of live cultures

A fermenting crock runs a succession. In the first day or two, in the salty, oxygen-free brine, the bacteria that can tolerate salt, mainly Leuconostoc species, begin turning sugar to lactic acid and carbon dioxide, which is the bubbling; as the acid rises they are succeeded by Lactobacillus species that tolerate more acid and push the pH down to about 3.5 over one to four weeks depending on the temperature. The salt, at two to three percent, is what selects the right microbes at the start and keeps the vegetable crisp by drawing water from its cells; the vegetable must stay under the brine, since mould grows where there is air, and the scum that forms on top is yeast and is skimmed. The same succession, on cabbage, is sauerkraut; on cabbage with chilli, garlic and fish sauce, kimchi; on cucumbers with dill, the pickle of the New York deli.

The pickler's enemies are softness and cloudiness. Enzymes in the vegetable, and in the blossom end of a cucumber especially, break down the pectin that holds the cells together, which is why recipes trim the ends, add a tannin-rich grape or oak leaf, or use the calcium salts that commercial makers add for crunch; heat sets the pectin too, which is why vinegar pickles are often crisper than they have a right to be. Green vegetables turn olive as acid replaces the magnesium in chlorophyll. Safety is a matter of acid: a fermented or vinegar pickle below pH 4.6 cannot support botulism, and the deaths from home preserving are almost all from low-acid vegetables canned without enough heat, not from pickles, which is why the instruction to use the stated amount of vinegar and salt is not a matter of taste.

Cucumbers were being pickled in Mesopotamia by 2400 BC, Cleopatra credited them with her looks, the Roman legions carried them, and sauerkraut, which Captain Cook took to sea by the barrel in 1768, was credited with keeping his crews free of scurvy on a three-year voyage. Kimchi has been made in Korea for at least a thousand years and its making in autumn, kimjang, is on UNESCO's heritage list; the Dutch gave New York its pickles and the word; and the cornichon, the gherkin, the pickled egg, the pickled herring and the lime pickle of India are the same principle applied to whatever a region had too much of in season. The revival of fermented foods since the 2010s, on evidence that live cultures help the gut, has put crocks back on kitchen counters that had not seen one since refrigeration, and the cucumber in the jar is doing what it did for the pharaohs.

If you ferment vegetables at home, the jar will change in ways that can look alarming but are usually signs of success. The brine turns cloudy within a few days because it is full of lactic acid bacteria, and a fine white sediment settles at the bottom as dead cells collect. Bubbles rise when you press the vegetables down. A thin, flat, whitish film on the surface is usually a harmless yeast, often called kahm yeast, which can be skimmed off. Measuring salt by weight rather than by spoon helps too, since a two percent brine means twenty grams of salt for every kilogram of vegetables and water.

Other signs mean the batch has gone wrong. Fuzzy mould in blue, green, black or pink, a slimy texture, vegetables that turn mushy all the way through, or a rotten rather than sour smell suggest that the wrong microbes won, usually because there was too little salt, the vegetables floated above the brine, or the room was too warm. Mould that has spread through the jar cannot be safely scraped away, because its threads reach further than you can see. Salt with anti-caking agents can also cloud a vinegar pickle, which is why many recipes ask for plain pickling or sea salt.

Not every pickle contains living microbes. Vinegar pickles never did, and most fermented products sold in jars or tins on ordinary shelves have been heated so they keep, which kills the bacteria that made them. If you want live cultures, the clue is a refrigerated product labelled unpasteurised or raw. Even then, most of the bacteria pass through the gut rather than settling there for good. A small trial at Stanford University in 2021 found that adults who ate several servings of fermented foods a day for ten weeks developed a more varied mix of gut microbes and lower markers of inflammation, which is encouraging but far from settled.

Salt is the other side of the ledger. A pickled cucumber or a serving of kimchi can carry a noticeable share of the recommended daily salt intake, and in countries where very salty pickled and preserved foods were eaten in large amounts every day, studies have linked them with higher rates of stomach cancer. The International Agency for Research on Cancer lists traditional Asian pickled vegetables as possibly carcinogenic, the lowest of its three categories for agents that may cause cancer. Stomach cancer rates fell in many countries during the twentieth century, and the spread of refrigeration, which reduced reliance on heavy salting, is often given as one reason.

Scurvy, caused by a lack of vitamin C, killed vast numbers of sailors on long voyages. On his first great voyage of exploration, from 1768 to 1771, James Cook carried large quantities of sauerkraut, fermented cabbage that keeps for months, and insisted that his crew eat it, at first by having it served to the officers so that the sailors would want it too.

Cook's ships suffered remarkably little from scurvy, though historians think fresh food gathered at landfalls and his general emphasis on hygiene and diet probably mattered more, since sauerkraut contains only modest amounts of vitamin C. The episode shows how preserved foods made long journeys possible, and how hard it was to tell which of several practices was actually working before the cause of scurvy was understood.

Pickling preserves food by making it acidic enough, below about pH 4.6, that spoilage microbes and botulism cannot grow, either by packing it in vinegar, which is quick and stable, or by salting it and letting the lactic acid bacteria already on the vegetable ferment its sugars into acid over weeks, which gives sauerkraut, kimchi and sour pickles their complexity and their live cultures. Salt selects the right microbes and keeps the crunch, air must be kept out, and the method is at least four thousand years old.