Acid.
When a stew tastes flat and more salt does not help, it usually needs acid. A teaspoon of vinegar can do what a tablespoon of salt cannot.
Sourness is the taste of acid: hydrogen ions on the tongue. A little of it makes rich food taste lighter and cuts through fat; it makes flavours seem sharper and more defined, which cooks call brightness. Many dishes taste "flat" for want of acid rather than salt. Lemon and lime juice (citric acid, pH about 2.3), vinegars (acetic acid, 4–6 %, pH about 2.5), wine, tomatoes, yoghurt, tamarind and pickles all supply it, each with its own aroma.
How sour a dish tastes depends on more than its pH. Weak acids like acetic acid release their hydrogen ions gradually, so the total acid matters as well as the pH; sugar suppresses sourness strongly, which is the basis of sweet-and-sour dishes from 糖醋 to agrodolce; salt and acid together each make the other taste stronger. The workshop dilutes a vinegar and balances it with sugar.
Below: the pH of a 5 % vinegar against dilution (exact weak-acid equilibrium), and the perceived sourness and sweetness of a sweet-and-sour sauce as you change its sugar (flavour bench, teaching model).
- Explain what acid does to flavour.
- Compute the pH of a diluted vinegar.
- Balance sour with sweet.
- Choose an acid for its aroma.
Brightness in a spoonful
Undiluted 5 % vinegar has a pH of about 2.42; diluted with twenty parts water it is still about 3.09. pH climbs slowly because acetic acid is weak: diluting it releases more of its held-back hydrogen ions. So pH alone does not tell how sour a dish will taste; the amount of acid does. In a sweet-and-sour sauce, 15 % sugar cuts the perceived sourness by about 40 % in this model, which is why 糖醋 sauces carry so much vinegar without tasting harsh.
Use acid like salt: in small steps at the end, tasting. Lemon brightens fish, vegetables and rich stews; rice vinegar is mild and sweet, black vinegar (镇江香醋) malty and round, sherry and balsamic vinegars nutty or sweet; lime suits Thai, Vietnamese and Mexican dishes; tamarind gives a deep, fruity sourness to Indian and Southeast Asian curries. Acid also changes texture and colour: it firms vegetables (V.4), turns green vegetables olive (XX.3), and "cooks" raw fish in ceviche by unfolding its proteins (XIX.6).
Taste a spoon of a rich stew or soup. Add a few drops of vinegar or lemon to a second spoonful and taste again. Most people find the second brighter even before it tastes sour.
Terms
- Sourness
- The taste of acid: hydrogen ions on the tongue.
- Brightness
- The sharp, lively quality acid gives to flavour.
- Weak acid
- An acid that releases only part of its hydrogen ions, such as acetic or citric acid.
- 糖醋
- Sweet-and-sour: sugar and vinegar in balance.
Sources
- A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
- H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.