Walk through an Indian fruit market in late October and you will see heaps of Nagpur oranges where green, yellow and orange fruit sit side by side. Guess which ones are ripe? The honest answer is that the peel will not tell you. In citrus, the colour of the skin and the ripeness of the fruit inside run on different clocks.

This page is a reading of the citrus research and of FSSAI's guidance on ripening. It is about how fruit colour works. It does not make any claim about what an orange does for your health.

The green is chlorophyll, and the orange is carotenoids

The outer, coloured layer of citrus peel is called the flavedo. While the fruit grows, it holds chlorophyll, the same green pigment found in leaves. As the fruit matures, two changes can happen: the chlorophyll breaks down, and yellow and orange pigments called carotenoids build up. A review of chlorophyll and carotenoid metabolism in citrus, published in Reviews in Agricultural Science, describes this "colour break" in detail.

A 2007 study in Postharvest Biology and Technology names some of the pigments involved in orange peel, including violaxanthin and β-citraurin, a reddish pigment. The Florida citrus review adds β-cryptoxanthin, which it calls orange-red. The mix of these pigments sets the final shade.

Cool nights flip the colour, not ripeness

The key trigger is temperature. The Reviews in Agricultural Science paper says most citrus starts to lose its green when field temperatures drop, and that night temperatures below about 13 °C with moderate days near 20 °C stimulate colour break. It says day temperatures above about 25 °C inhibit chlorophyll breakdown and carotenoid production, and states plainly that this is why citrus grown in the tropics has a greenish peel.

A study of lemons in the Journal of Experimental Botany found that moderately low storage temperatures, from 5 to 20 °C, triggered degreening compared with 25 °C, and that this happened even when ethylene action was blocked. The authors suggest that low temperature plays a leading role in natural peel colour change on the tree.

The Florida review makes the same point from the orchard. Early varieties there can reach full maturity in early autumn with sweet, good-flavoured juice and a green peel. It says that in tropical climates green or mostly green fruit is the accepted colour. Much of India's citrus crop ripens while nights are still warm, so a green or patchy peel is normal.

Did you know an orange can turn green again?

The Reviews in Agricultural Science paper says that Valencia oranges left on the tree into spring or summer can turn from orange back to green as the soil and air warm. During this regreening, genes that make chlorophyll are switched up and genes that break it down are switched down. The fruit has not gone back to being unripe. Only the peel pigment has changed.

What degreening is

Ethylene is a plant hormone. The FSSAI guidance note on artificial ripening of fruits describes it as a hormone produced naturally within fruits. Packers can expose harvested citrus to a low level of ethylene so the green fades faster. This is called degreening.

A study of Nagpur mandarin treated yellowish-green fruit with ethylene at about 5 ppm, at 27 to 29 °C and 90 to 95 percent humidity. In 48 hours the fruit turned yellow to yellowish-orange and the green disappeared. It found 5 ppm as effective as 25, 50 or 100 ppm. It also reported that degreening increased stem-end rot during storage, which is why handling after treatment matters.

The 2007 Postharvest Biology and Technology study found that ethylene reproduced and sped up the pigment changes that happen naturally as citrus matures. The Florida review notes one catch: the temperatures used for degreening there break down chlorophyll but hold back some orange pigments, so the fruit can come out yellow rather than deep orange.

What FSSAI allows

The FSSAI guidance note says ethylene gas can be used for artificial ripening at up to 100 ppm (100 µl/L), and that no source of ethylene may come into direct contact with the fruit. It says calcium carbide and acetylene gas are not permitted under the Food Safety and Standards Regulations, 2011. It also advises consumers to buy from known sellers who declare that their fruit has not been ripened with banned chemicals.

When Indian oranges come to market

A NABARD model profile for citrus cultivation describes two main crops of mandarins and sweet oranges. The Ambia bahar crop flowers in January, when mangoes flower, and its fruit is available from October to December. The Mrig bahar, or monsoon, crop flowers in June and July and is harvested from February to April. It says mandarins and sweet oranges take about 240 to 280 days to mature, and that mature fruit at the colour-break stage is picked in two or three rounds. So the oranges in markets right now are mostly the Ambia crop, picked in a warm October.

What this means when you buy oranges

  • Do not judge by colour alone. A green or patchy Nagpur orange can be fully mature inside.
  • An even, bright orange is not proof of tree-ripening either. It may reflect cool nights, the variety, or degreening.
  • Buy from sellers you know. That is FSSAI's own advice for avoiding fruit ripened with banned chemicals.

For food businesses, the same point applies to fruit raw materials: peel colour is a poor stand-in for maturity, which is why fruit specifications rely on measured values rather than appearance. See How to read a fruit preparation spec.

Sources

Hero photo: "NAGPUR ORANGE" by Ranjithkumar Murugesan, Wikimedia Commons, CC0. See Credits.

Questions

Is a green orange unripe?

Not necessarily. A Florida citrus review published in the Proceedings of the Florida State Horticultural Society says that, in its semitropical climate, the peel colour of some varieties has little relation to their maturity, and that early varieties can have sweet, good-flavoured juice under a green peel. It also says that in tropical climates green or mostly green is the accepted colour for fruit sold locally. Colour alone is not a ripeness test.

What makes the peel turn orange?

Two things happen in the outer peel, the flavedo. Chlorophyll, the green pigment, breaks down, and carotenoids, the yellow and orange pigments, build up. A review in Reviews in Agricultural Science names low night temperatures, below about 13 °C, with moderate days near 20 °C, as the trigger for this colour break, and says day temperatures above about 25 °C inhibit it.

Can an orange turn green again?

Yes, in some cases. The same review says that Valencia oranges left on the tree into spring or summer can turn from orange back towards green as soil and air warm up. The genes that make chlorophyll switch back on. The fruit inside has not become unripe again.

What is degreening, and is it the same as carbide ripening?

No. Degreening exposes harvested citrus to low levels of ethylene, a hormone that fruit makes naturally, so the green fades faster. A study of Nagpur mandarin found 5 ppm ethylene as effective as higher levels for degreening. FSSAI's guidance note on artificial ripening allows ethylene gas up to 100 ppm, says no ethylene source may touch the fruit, and says calcium carbide and acetylene gas are not permitted.

When is the Nagpur orange season?

A NABARD model profile for citrus describes two main crops of mandarins and sweet oranges. The Ambia bahar crop flowers in January and its fruit comes to market from October to December. The Mrig bahar crop flowers in June and July and is harvested from February to April. It says mature fruit is picked at the colour-break stage.

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