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How does pickling preserve food?

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Quick answer

Pickling preserves food by making it too acidic for spoilage microbes and most harmful bacteria to grow. Either vinegar is added directly, or the food sits in salty brine while lactic acid bacteria ferment its sugars into acid. Salt, oil and spices add further protection by drawing out water and blocking air.

Pickling preserves food by making it acidic enough, usually below about pH 4.6, that harmful bacteria and spoilage microbes cannot grow. The acid comes either from added vinegar or from lactic acid bacteria fermenting the food in salty brine, often helped by salt, sugar, oil and spices.

Step by step

  1. Vegetables or fruit are washed, cut and usually salted, which draws water out of their cells.
  2. The pieces are covered with vinegar, salty brine or a salt, spice and oil mixture in a clean jar.
  3. In fermented pickles, lactic acid bacteria multiply in the brine and convert the food's sugars into lactic acid.
  4. Acidity rises until the pH falls below about 4.6, preventing harmful bacteria such as Clostridium botulinum from growing.
  5. The jar is kept sealed or covered by brine or oil, limiting air so moulds cannot settle on the surface.

Key things to understand

  1. Most bacteria that spoil food or cause illness grow poorly in acidic conditions, and very few can grow below about pH 4.6.
  2. Quick pickles are made by covering vegetables or fruit with vinegar, which contains acetic acid and acidifies the food immediately.
  3. Fermented pickles are soaked in salty brine, where salt-tolerant lactic acid bacteria multiply and convert the food's sugars into lactic acid.
  4. As acid builds up over days, it suppresses competing microbes, and salt further limits growth by drawing water out of the vegetables.
  5. Many Indian pickles add salt, mustard, chilli and a layer of oil, which keeps air away from the surface and slows mould growth.

Labelled diagram

  1. Salted produce

    Salt draws water from vegetables and holds back spoilage microbes.

  2. Vinegar route

    Added acetic acid acidifies the food immediately in quick pickles.

  3. Fermentation route

    Lactic acid bacteria in brine slowly convert sugars into lactic acid.

  4. Low pH

    Acidity below about pH 4.6 stops most harmful bacteria growing.

  5. Sealed jar

    Brine or an oil layer keeps air out, discouraging surface mould.

Two routes to an acidic, preserved pickle

Real-world examples

  • Indian mango pickle combines salted raw mango, mustard, chilli and a layer of mustard oil, then matures over days in the jar.
  • Kanji, a North Indian winter drink, is made by fermenting black carrots with mustard seed in water for several days.
  • German sauerkraut is shredded cabbage fermented in its own salty juice by lactic acid bacteria.
  • Dill cucumber pickles sold in supermarkets are usually quick pickles preserved with vinegar and then heat-treated.

Common mistake

✗ Myth: The oil layer on Indian pickles is what preserves them.

✓ Fact: Oil mainly seals the surface against air and mould. The real preserving work is done by salt, acidity from the fruit or fermentation, and spices such as mustard.

People also ask

How does vinegar preserve food?
Vinegar contains acetic acid, typically around 4 to 8 percent in kitchen vinegar. Soaking food in it lowers the pH far below the level most bacteria tolerate, and acetic acid also passes into microbial cells and disrupts them. Pickling recipes use enough vinegar to keep the food acidic even after the vegetables' own juices dilute it.
How do pickles last so long without refrigeration?
Traditional pickles combine several barriers. High salt removes available water, acidity from vinegar, fruit or fermentation stops most bacteria, spices such as mustard have mild antimicrobial effects, and brine or oil blocks air. Together these hurdles make the jar hostile to almost every microbe, so a well-made pickle keeps for months.
What is the difference between pickling and fermenting?
Pickling is any method that preserves food in an acidic liquid. Fermenting is one way to get that acid: bacteria produce lactic acid in salty brine over days or weeks. Quick pickles skip fermentation and use vinegar instead, so all fermented vegetables are a type of pickle, but not all pickles are fermented.
Why are some pickles kept in the sun?
Sunning Indian pickles in glass or ceramic jars warms the contents, which speeds fermentation and the softening of fruit, and helps evaporate excess moisture. Lower moisture and higher salt concentration make the pickle less hospitable to moulds.
Why do pickles need so much salt?
Salt draws water out of vegetables, creating the brine, and holds back spoilage bacteria and moulds while salt-tolerant lactic acid bacteria grow. In fermented pickles, too little salt can let soft-rot microbes take over before enough acid forms.
Are fermented pickles probiotic?
Unpasteurised fermented pickles such as traditional sauerkraut, kimchi and some kanji contain live lactic acid bacteria. Vinegar pickles and shelf-stable pickles that have been heat-treated generally contain few or no live bacteria.
Why does a pickle sometimes grow white film?
A flat white film on fermenting brine is often kahm yeast, which grows where the surface meets air. Fuzzy, coloured growth is mould, and mouldy pickles are generally best discarded.

Part of the guide: Food & Kitchen Science →