An orange is a hesperidium — a berry with a leathery rind. Nearly everything that decides whether it is any good happens in three places: the skin, the juice vesicles, and the balance between sugar and acid inside them.
What is inside
- Flavedo — the coloured rind. Its orange appears as chlorophyll breaks down, and that needs cool nights.
- Albedo — the white pith. Cushions the fruit through a long sea voyage.
- Juice vesicles. Where the sugar and acid sit. Brix is measured from this juice.
- Segments. Ten or so, each wrapped in membrane. Thin membranes peel easily — hence “easy peeler”.
- Core. The central axis the segments radiate from.
- Navel. A second, tiny fruit growing inside the first. Navels set fruit without fertilisation, so there are no seeds.
An original schematic, drawn for this page. Proportions are illustrative rather than botanically exact.
The four numbers a buyer actually asks about
Why the ratio decides when fruit is picked
As citrus ripens, two things happen at once: sugar rises and acid falls. So the Brix:acid ratio climbs steadily as the season goes on — which makes it a convenient proxy for "is this ready?". Pick too early and the ratio is low: the fruit is sharp and thin. Leave it too long and it goes flabby.
| California navel maturity standard | A soluble-solids to acid ratio of 8:1, plus yellow-orange colour on at least 25% of the peel, for at least 90% of a lot |
|---|---|
| How old that standard is | Derived from USDA work and used by the California industry since 1915 |
| South African soft citrus, export | Maximum acid 1.5% (tolerance to 1.7% if Brix rises to match); minimum 50% juice for late varieties |
| A newer idea | BrimA — Brix minus acid — has been shown to predict how much people actually like the flavour better than the ratio alone |
Sources: CDFA citrus maturity regulations, South African export standards for citrus, Determinants of flavor acceptability during the maturation of navel oranges.
Seedless, and why
Navel oranges are parthenocarpic — they set fruit without being fertilised, so no viable seeds form. The navel itself is a small second fruit that grew inside the first, which is where the name comes from. A useful consequence: because a navel cannot be grown from seed, every navel orange tree in the world is a graft, propagated from cuttings of the original.
Not everything grown here is seedless. Navels are. Valencias carry a few seeds, and soft-citrus varieties vary — some are bred seedless, some are not. If seedlessness matters for your programme, ask the producer by cultivar rather than by region.
What "blemish-free" actually means
Clean, unmarked skin is mostly a grading outcome, not a natural property. Packhouses sort fruit by appearance against export standards; fruit that does not make the grade goes to the domestic fresh market or to juicing. Wind scarring or a bit of rind marking rarely says anything about how the fruit inside will taste — it is a cosmetic sort, and worth remembering when you are told that flawless skin equals better fruit.
What this valley contributes
Colour is the clearest link between place and fruit. Orange skin turns orange as chlorophyll breaks down, and that needs a spell of cool temperatures. The Western Cape's Mediterranean climate — cool winters, dry summers — suits varieties that need cool nights to colour properly, which is one reason the region is known for navels. The valley's other advantages are more prosaic: the Olifants River for water, deep well-drained soils, and a harvest that lands in the northern hemisphere's summer.
We do not publish Brix, acid or ratio figures for Citrusdal fruit, because we have not measured any and will not repeat numbers we cannot source. Everything above is general citrus science and published standards. For a spec sheet on actual fruit, ask a producer directly.
Last updated August 2026 · sourced in our fruit and rooibos notes.