Vinegar

Read the acidity on the label

Vinegar's strength is stated as acetic acid concentration, usually between 4 and 7%. Five and seven per cent are 40% apart, so the same tablespoon does noticeably different things.

When you do not know which vinegar a recipe assumed, add less and taste. Acid can be added later; it cannot be taken out.

The types are not an interchangeable list

• Rice vinegar: mild, faintly sweet (4-5%). The default in Korean and Japanese cooking, and the acidity behind sweet and sour pork.
• White (distilled) vinegar: sharp and neutral (5-7%). Having no aroma it is good for pickling; in a dish it reads as acidity and nothing else.
• Cider vinegar keeps some fruit character, which suits dressings and meat marinades.
• Wine vinegars: white is light, red is heavier. This is what dresses a Greek salad.
• Balsamic: low in acid, high in sugar and viscosity. It is there for sweetness and body rather than acidity, so used in place of another vinegar it changes the dish.
• Malt vinegar: grainy and distinctive. This is what goes on fish and chips.

Acid evaporates — so split it in two

Acetic acid boils off. Add it all at the start and there is no sourness left by the time the dish is done.

That is why vindaloo splits its vinegar: the first 150ml cures the meat, the final 2 tbsp restores aroma and edge. For any long-cooked dish that should taste sharp, the habit worth building is a little more in the last minute (balancing flavour).

What acid actually does

• Damps down salt and fat. Without one source of acid, a rich dish defeats you halfway through.
• Denatures protein. White fish and prawns firm up as though cooked within tens of minutes — that is cooking, not tenderising (what marinades do).
• Sets pectin firm. This is the one that matters in practice.

With beans and vegetables, add it late

Acid stiffens the pectin in vegetables and pulses, so vinegar or tomato added before the beans are tender leaves them hard forever. It is why a bean dish with tomato in it, like chana masala, observes the order it does (soaking).

The same property is occasionally useful: a splash in the water keeps boiled potatoes from breaking up.

Storage

• Vinegar does not spoil. It is too acidic for anything to grow in.
• Room temperature, out of the light. No refrigeration needed.
• The cloudy sediment or gelatinous lump that forms in an old bottle is a mother of vinegar. Strain it out and carry on using it.

Substitutes

• Lemon juice for vinegar works at the same volume, but lemon is more aromatic and slightly less acidic.
• White vinegar for rice vinegar: use 20% less and add a little sugar.
• Nothing substitutes for balsamic. If you must, a tablespoon of another vinegar plus a teaspoon of sugar approximates the direction.