The orange is a hybrid of ancient cultivated origin, possibly between pomelo (Citrus maxima) and mandarin (Citrus reticulata). It is a small flowering tree growing to about 10 m tall with evergreen leaves, which are arranged alternately, of ovate shape with crenulate margins and 4–10 cm long. The orange fruit is a hesperidium, a type of berry.
Oranges originated in Southeast Asia. The fruit of Citrus sinensis is called sweet orange to distinguish it from Citrus aurantium, the bitter orange. The name is thought to ultimately derive from the Sanskrit for the orange tree, with its final form developing after passing through numerous intermediate languages.
In a number of languages, it is known as a “Chinese apple” (e.g. Dutch Sinaasappel, “China’s apple”).
All citrus trees are of the single genus, Citrus, and remain largely interbreedable; that is, there is only one “superspecies” which includes grapefruits, lemons, limes, and oranges. Nevertheless, names have been given to the various members of the genus, oranges often being referred to as Citrus sinensis and Citrus aurantium. Fruits of all members of the genus Citrus are considered berries because they have many seeds, are fleshy, soft, and derive from a single ovary. An orange seed is called a pip. The white thread-like material attached to the inside of the peel is called pith.
Like all citrus fruits, the orange is acidic, with a pH level of around 2.5-3; depending on the age, size and variety of the fruit. Although this is not, on average, as strong as the lemon, it is still quite acidic on the pH scale – as acidic as household vinegar.
Oranges are widely grown in warm climates worldwide, and the flavours of oranges vary from sweet to sour. The fruit is commonly peeled and eaten fresh, or squeezed for its juice. It has a thick bitter rind that is usually discarded, but can be processed into animal feed by removing water, using pressure and heat. It is also used in certain recipes as flavouring or a garnish. The outer-most layer of the rind can be grated or thinly veneered with a tool called a zester, to produce orange zest. Zest is popular in cooking because it contains the oil glands and has a strong flavour similar to the fleshy inner part of the orange. The white part of the rind, called the pericarp or albedo and including the pith, is a source of pectin and has nearly the same amount of vitamin C as the flesh.