How Does Pickling Preserve Food? Salt, Acid and the Microbes on Your Side
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A cucumber left on the counter is soft and mouldy in a week; the same cucumber in a jar of brine is a pickle that keeps for a year, sour, crisp and, if it was made the old way, full of living bacteria that did the preserving. Pickling is the second oldest way of keeping food after drying, it is the reason the sauerkraut on a German sausage, the kimchi on every Korean table and the gherkin in a burger exist, and it works by a principle that a chemist can state in a word: pH. Below a certain acidity, the organisms that rot food cannot grow, and the trick is getting there.
Why food spoils
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.
Two ways to get the acid
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
What happens in the jar
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.
Crispness, colour and safety
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.
A short history
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, kept his crews free of scurvy on a three-year voyage, since fermentation preserves vitamin C. 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.
The takeaway
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.