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How does fruit ripen?

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Quick answer

Fruit ripens when enzymes inside its cells convert starch to sugar, break down acids, soften the cell walls and swap green chlorophyll for coloured pigments. In bananas, mangoes, tomatoes and avocados, the plant hormone ethylene triggers and speeds these changes, which is why such fruits keep ripening after they are picked.

Fruit ripens through a programme of chemical changes, often triggered by the gas ethylene, that soften the flesh, turn starch into sugar, reduce acidity and change colour and aroma. Enzymes inside the fruit's cells carry out each of these changes over days or weeks.

Step by step

  1. Once the seeds mature, hormonal signals switch on the fruit's ripening genes.
  2. In climacteric fruits, ethylene production and respiration jump sharply, and the ethylene spreads through the flesh.
  3. Enzymes break stored starch into sugars such as glucose, fructose and sucrose, raising sweetness.
  4. Pectin-degrading enzymes loosen the glue between cells, so the fruit gradually becomes soft and juicy.
  5. Chlorophyll fades and carotenoids or anthocyanins appear, giving the ripe colour along with new fragrant esters.

Key things to understand

  1. When a fruit's seeds are mature, signals inside the fruit switch on the genes that control ripening.
  2. Climacteric fruits such as bananas and mangoes release a burst of ethylene gas, which speeds up ripening further.
  3. Enzymes convert stored starch into sugars and break down acids, so the fruit tastes sweeter and less sour.
  4. Other enzymes loosen pectin and cell walls, softening the flesh so it becomes easier to bite and digest.
  5. Green chlorophyll breaks down while yellow, red or orange pigments build up, and new aroma compounds form.

Labelled diagram

  1. Ethylene signal

    The fruit releases ethylene gas, which switches on ripening enzymes throughout the flesh.

  2. Starch to sugar

    Amylase enzymes split starch granules into sugars, making the pulp sweet.

  3. Softening cell walls

    Pectin and cellulose links loosen, so firm flesh turns soft.

  4. Colour change

    Chlorophyll breaks down, revealing yellow carotenoid pigments in the peel.

Inside a ripening banana

Real-world examples

  • Mango traders in India ripen raw mangoes in sealed rooms with a controlled trickle of ethylene gas.
  • A hard avocado placed beside a ripe banana softens a day or two sooner because of the banana's ethylene.
  • Bananas are shipped green and ripened in warehouse chambers near the shops that will sell them.
  • Strawberries picked too early stay sour, because they cannot build much more sugar once off the plant.

Common mistake

✗ Myth: All fruit keeps getting sweeter on the counter after it is picked.

✓ Fact: Only climacteric fruits such as bananas, mangoes and tomatoes keep converting starch to sugar after harvest. Grapes, citrus and strawberries may soften or change colour slightly, but they do not become meaningfully sweeter.

People also ask

What makes fruit ripen?
Fruit ripens because enzymes inside its cells change starch into sugar, break down acids, soften the cell walls and replace chlorophyll with other pigments. In many fruits the plant hormone ethylene acts as the trigger, so a small amount of the gas starts a chain reaction that makes the fruit release even more.
Why do bananas ripen other fruit?
Ripening bananas give off a lot of ethylene, the gas that switches on ripening in climacteric fruits. Placing a banana next to an unripe avocado, mango or tomato, especially in a bag, raises the ethylene around it and makes the other fruit soften and sweeten sooner.
How to tell if fruit is climacteric?
Climacteric fruits keep ripening after harvest because they produce their own ethylene and show a burst of respiration. Common examples are bananas, mangoes, apples, tomatoes, papayas, avocados and pears. Grapes, citrus fruits, cherries, strawberries and pineapples are non-climacteric, so they should be picked when already ripe.
Why do some fruits keep ripening after picking?
Climacteric fruits like bananas, mangoes, tomatoes and avocados make their own ethylene, so they continue ripening off the plant. Non-climacteric fruits such as grapes, citrus, strawberries and pineapples do not have this burst, so they barely get sweeter once harvested.
Why does a paper bag ripen fruit faster?
A paper bag traps the ethylene gas that the fruit gives off, so the concentration around it rises and ripening speeds up. Paper still lets some air and moisture through, which reduces the risk of mould compared with a sealed plastic bag.
Why do bananas get brown spots?
As bananas overripen, the yellow peel's chlorophyll is long gone and enzymes start oxidising compounds called phenols, creating brown pigments. The spots signal that most starch has turned into sugar, so the flesh inside is usually at its sweetest.
Is ripening fruit with calcium carbide safe?
No. Calcium carbide reacts with moisture to release acetylene, and industrial grades can carry arsenic and phosphorus impurities. India's food safety regulator bans its use for ripening, while controlled ethylene ripening chambers are permitted as a safer method.

Part of the guide: Food & Kitchen Science →