Citrus × sinensis · Rutaceae
Botanically the orange is a hesperidium — a modified berry with a leathery, oil-bearing rind. It is not a wild species at all, but an ancient hybrid of the pomelo and the mandarin, propagated by grafting ever since.
Cut one across the equator and the whole structure declares itself at once.
Four to know
Nearly every orange you meet descends from a handful of chance mutations someone noticed on a single branch and then propagated. Each one below is, genetically speaking, a clone.
A mutation in which a second, tiny fruit grows at the blossom end, leaving the navel-like scar it is named for. Seedless and sterile, so the entire world's stock traces back to two trees shipped to Riverside, California in 1873. Peels cleanly; the juice turns bitter within hours of pressing.
The summer orange, and the one behind most commercial juice. Thin-skinned, deeply juicy, and slow to ripen — a tree can carry two crops at once. In warm springs the rind sometimes re-greens as chlorophyll returns, which affects appearance and nothing else.
A navel sport with pink-to-crimson flesh, coloured by lycopene rather than the anthocyanins of a blood orange. Lower acid than a standard navel, with a berryish note that reads almost like cranberry. Colour deepens later in the season.
The darkest of the blood oranges. The pigment is anthocyanin, and it only develops where nights turn cold — which is why the volcanic slopes below Etna produce them and most other regions cannot. Flavour runs sharp and faintly of raspberry.
Structure
Each of the four labelled parts in the diagram has a job, and knowing them explains most of what happens when you cook with an orange.
The coloured outer rind, packed with pockets of essential oil. This is zest. The oil is where nearly all the aroma lives, which is why zest perfumes a dish far more forcefully than juice.
The white pith beneath. Spongy, rich in pectin, and the source of the bitterness people complain about — but also the reason marmalade sets without added pectin.
The segments, each a sealed compartment lined with thousands of juice vesicles: individual fluid-filled hairs. Supreming a fruit means cutting those compartments away from their membranes.
The pithy core the segments radiate from, continuous with the albedo. In a navel orange it also holds the small conjoined second fruit.
In the market
Colour is the least reliable signal there is. An orange turns orange only when nights get cold enough to break down its chlorophyll; in the tropics fruit ripens fully while staying green, and some exported fruit is degreened with ethylene purely for looks.
Pick up two of the same size and keep the heavier one. Density means juice. A light fruit has dried out in storage, whatever the skin looks like.
The area around the stem should be firm. Soft or puffy skin that lifts away from the flesh means the fruit is past its best.
A week on the counter, up to a month in the refrigerator drawer. Bring back to room temperature before juicing — cold fruit gives up noticeably less.
Zesting a whole fruit is easy; zesting half a fruit is not. Take the peel first even if the recipe lists it second, and freeze what you don't use.
Every navel orange on earth is a cutting of a cutting — one branch, in one Brazilian orchard, two hundred years ago.