Vinegar (Acetic Acid)

Vinegar is a dilute solution of acetic acid (CH3COOH) in water. Typical vinegar contains about 5 to 8 per cent acid; the rest is water and some flavour molecules. Vinegar has been made and used by humans for at least 7,000 years for cooking, pickling, medicine and cleaning. The sharp, mouth-puckering tang of pickled onions or salt-and-vinegar crisps is exactly the same chemistry as the acid eating away at the limescale in your kettle. Vinegar is one of the simplest, oldest, most useful chemicals in everyday life.

  • Main ingredientAcetic acid (CH3COOH)Around 5-8 per cent
  • pHAround 2.5A weak acid
  • Made fromFermented alcoholWine, cider, malt, rice
  • Word originFrench vin aigreMeans "sour wine"
  • Bacteria responsibleAcetobacterTurn alcohol into acid
  • Used forCooking, cleaning, picklingAnd many old remedies

What is acetic acid?

Acetic acid is a small organic molecule, just 8 atoms in all: 2 carbons, 4 hydrogens and 2 oxygens, arranged as CH3-COOH. The COOH part (called a carboxyl group) is what makes it an acid, releasing an H+ ion in water.

Pure acetic acid (called glacial acetic acid) is a clear, colourless liquid that solidifies just below room temperature (16.6 degrees Celsius). It freezes into ice-like crystals, which is where the "glacial" name comes from. Glacial acetic acid is very corrosive and dangerous to handle. The everyday vinegar in your kitchen is mostly water, with the acid diluted to a safe concentration.

How vinegar is made

Vinegar is made in two steps:

  1. Fermentation: yeasts turn sugar in fruit or grain into alcohol (ethanol). This is the same process that makes wine and beer.
  2. Acid fermentation: bacteria called Acetobacter use oxygen from the air to turn the alcohol into acetic acid. (CH3CH2OH + O2 -> CH3COOH + H2O)

Different vinegars come from different starting alcohols:

  • Wine vinegar: from grape wine
  • Cider vinegar: from apple cider
  • Malt vinegar: from beer-like grain mash (the dark vinegar on fish and chips)
  • Rice vinegar: from fermented rice wine, common in Asian cooking
  • Balsamic vinegar: from grape juice aged in wooden barrels for years, sometimes decades
  • White vinegar: from distilled alcohol, used mainly for cleaning and pickling
Fact Vinegar is so old that the ancient Babylonians were making it as early as 3000 BC. Egyptian and Roman texts mention it. The famous Egyptian queen Cleopatra is said to have once won a bet with the Roman general Mark Antony by dissolving a precious pearl in a glass of vinegar and drinking it, proving she could consume the most expensive meal in history. Pearls (calcium carbonate) do react with vinegar, so the story is at least chemically possible, but historians think the tale is mostly legend.

Vinegar in cooking

Vinegar is one of the most useful kitchen ingredients.

  • Flavour: it adds brightness and sharpness to almost any dish.
  • Salad dressing: the basis of vinaigrette, mixed with oil, salt and herbs.
  • Pickling: high acidity stops bacteria growing, so pickled food keeps for months. Onions, cabbage, cucumbers, beetroot, eggs, herring and many other foods are preserved this way.
  • Tenderising meat: marinating meat in vinegar breaks down tough fibres.
  • Activating baking soda: in recipes that contain baking soda, a splash of vinegar (or other acid like buttermilk) triggers the reaction that releases CO2 and makes cakes rise.
  • Stopping browning: a few drops of vinegar in water keeps cut apples or potatoes from going brown.

Vinegar for cleaning

Vinegar is a mild but effective cleaner because its acidity tackles many kinds of dirt.

  • Limescale: deposits in kettles and shower heads are mostly calcium carbonate. Vinegar reacts with them and dissolves them away.
  • Glass and mirrors: leaves a streak-free shine.
  • Soap scum: cuts through the build-up on bathroom tiles.
  • Kitchen surfaces: kills some bacteria, though not as effectively as proper disinfectants.
  • Window cleaner: a 1:1 mix of vinegar and water with a tiny drop of washing-up liquid makes a good basic cleaner.
  • Coffee machines: regular descaling with diluted vinegar keeps them flowing freely.
Did you know? Vinegar can soothe a wasp sting because wasp venom is mildly basic, so the acid neutralises it. Bee venom, however, is mildly acidic, so for a bee sting you should use a base like baking soda or toothpaste instead. The simple rule: "acid for wasps, alkali for bees". Both will help take the sting out a little, though severe reactions still need medical attention.

Vinegar in industry

Beyond the kitchen, acetic acid is a major industrial chemical:

  • Plastics: starting material for some plastics including PET (used in clear drinks bottles) and polyvinyl acetate (PVA glue).
  • Textiles: used in making rayon and cellulose acetate fabrics.
  • Aspirin manufacture: acetic acid is one of the ingredients in making aspirin (acetylsalicylic acid).
  • Inks and dyes: many ink and dye recipes contain small amounts of acetic acid as a solvent or stabiliser.
  • Food preservatives: used in tiny amounts in many packaged foods.
Try this Show that vinegar can react with limestone. Drop a small piece of chalk or eggshell (both are calcium carbonate) into a cup of vinegar. Watch bubbles of CO2 fizz off the surface as the acid eats the chalk: a slow, fascinating chemical reaction. Now try the same with an iron nail: tiny hydrogen bubbles form much more slowly on the nail surface (vinegar is a weak acid, so it reacts slowly with iron). One cup, two different acid-base reactions.
Deeper dive: vinegars long history as medicine

For most of human history, before modern medicine, vinegar was one of the most widely used remedies. Ancient texts from Babylonia, Egypt, Greece, Rome and China all describe vinegar as a treatment for an extraordinary range of illnesses.

The Greek physician Hippocrates, often called the father of medicine, prescribed vinegar mixed with honey (called oxymel) for coughs, fevers and stomach troubles around 400 BC. Roman soldiers drank a diluted vinegar called posca as a daily refreshing drink, which probably helped kill germs in dirty water and prevented some diseases on campaign.

During the Black Death of the 1300s, doctors and grave-diggers wore protective masks soaked in herb-infused vinegar (probably more for the smell than any real protection), and many people believed that vinegar could help ward off the plague. During the 1918 flu pandemic, some hospitals washed surfaces with vinegar to slow the spread.

Modern science has confirmed that vinegar does kill some bacteria and fungi (the acid environment is hostile to many microbes). It is now used as a mild antiseptic and food preservative. Some studies suggest small amounts of vinegar with meals may help with blood sugar control, though results are mixed.

Vinegar is not the cure-all that ancient doctors thought, but its role in human history is remarkable. From the kitchen jar of pickles in your fridge to the foundations of food preservation and many ancient medical traditions, this simple chemical (CH3COOH) has been part of human life longer than almost any other.

For more, see what is an acid and common acids in everyday life.