Acids are substances which release hydrogen ions (positively charged hydrogen atoms, i.e. H+) when they dissolve in water, or release hydrogen gas when dissolving metals, or react with bases to form salts. From a theoretical standpoint, pH is the logarithm to base 10 of the reciprocal concentration of hydrogen ions, or single positively charged atoms. In practice, it is measured on a scale from 0 to 10, based on standard solutions. Common acidic foods include lemon juice (citric acid) with pH of 2.1, weak wine vinegar (acetic acid) a pH of 2.5 and yoghurt (lactic acid) a pH of 4.0. Most foods are acidic or neutral; they are usually used in their natural form, but sometimes are used as pure chemicals, e.g. citric acid or tartaric acid.
Acids are vital to human bodily functions. For food biology, acidic foods are important for:
- Taste: their taste - sourness;
- Preservation: their action as preservatives because they are toxic to some bacteria - vinegars are used to preserve vegetables as in pickles or fermented foods - sauerkraut or yoghurt. The "toxicity" of acids' preservative action arises because its hydrogen ions (positively charged) react with oxygen (negatively charged) to create water (neutral); this removes any "free" oxygen that would spoil foods through oxidation or microbial action.
- Reactivity: acting as a base in chemical reactions - acids react with bicarbonate of soda to release carbon dioxide as in baking powder; forming useful salts when reacting with alkalis, e.g. sodium chloride, bicarbonate of soda, cream of tartar or saltpetre.
In flavouring, adjusting the level of sourness is a crucial last minute task of cooking, so adding a twist of lemon or a dash of vinegar enlivens a meal. On the other hand, sugar does not neutralize acidity, so you get a pleasing sweet and sour flavour in your mouths, so essential to Chinese cuisine.