a large group of substances essential to the working of the body and widespread in food. The scientific definition of an acid is a substance that dissolves in water to release hydrogen ions, dissolves metals releasing hydrogen gas, and reacts with a base to form a salt. All these properties are relevant to food.
First, the release of hydrogen ions—that is, hydrogen atoms with a positive electrical charge—means that acids tend to remove oxygen from other substances, and combine it with the hydrogen to form water. Oxygen tends to spoil foods (see, for example, fats and oils), so acids act as preservatives, as in pickles and some fermented foods such as sauerkraut and yoghurt.
Second, many compounds formed by acids and metals are important in foodstuffs. These include ordinary salt, sodium chloride, which can be made from sodium and hydrochloric acid. Other examples are bicarbonate of soda, cream of tartar, calcium oxalate (see oxalic acid), and saltpetre (see nitrates and nitrites).
Third, the reaction of an acid with a ‘base’ (roughly the same as an alkali) is the means by which baking powder evolves gas and thus raises cakes.
Acids may be classed as strong or weak, according to the quantity of hydrogen ions they can release. One of the strongest is hydrochloric acid, an inorganic acid, which is found in the stomachs of animals (including humans), where it helps to break down food. Strong acids are corrosive and this one is no exception; the stomach lining must be constantly renewed as it is eaten away. Many complex organic acids are so weak that their effect is negligible, for example the amino acids of which protein is composed. Some organic acids, however, are quite strong, such as citric acid in citrus fruit, malic acid in apples, and acetic acid in vinegar. The strength of acids (and of alkalis) is measured on the pH scale (see pH factor).
The old belief that certain foods are ‘acid forming’ and thus in some way bad for the body is no more than a myth.
Ralph Hancock is an encyclopedist with a special interest in food history and food science.
