Fruit ripening: climacteric and non-climacteric fruits
What happens as fruit ripens, the climacteric rise and ethylene, how ethylene is made and blocked, ripening rooms and 1-MCP, and a cold store heat sum.
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A green banana picked in Ecuador or the Philippines can travel for weeks and still arrive firm and green, then turn yellow, sweet and soft within days of being exposed to a little ethylene gas in a ripening room. A strawberry or an orange, by contrast, never gets sweeter after picking. The difference lies in the type of ripening each fruit undergoes, and in the plant hormone ethylene.
Fruit ripening links plant physiology to horticulture, post-harvest handling and trade, and the climacteric versus non-climacteric distinction is one of the most reliable exam questions in both subjects.
What changes during ripening
Ripening turns a mature but inedible fruit into an attractive one, encouraging animals to eat it and disperse its seeds. The main changes:
- Colour: chlorophyll breaks down, revealing or producing carotenoids (yellow, orange, red, as in banana, mango and tomato) and anthocyanins (red and purple, as in grape and apple skin).
- Texture: cell wall enzymes such as polygalacturonase, pectin methylesterase, cellulases and expansins loosen the walls, softening the fruit.
- Flavour: starch converts to sugars, organic acids decline, astringent tannins become less soluble, and aroma volatiles such as esters build up.
- Respiration and ethylene: rise sharply in some fruits and not in others.
Climacteric and non-climacteric fruits
In 1925, F. Kidd and C. West, studying stored apples in England, noticed a sharp rise in respiration as the fruit ripened, which they called the climacteric.
| Climacteric fruits | Non-climacteric fruits | |
|---|---|---|
| Respiration at ripening | sharp rise to a peak | steady, slow decline |
| Ethylene | large burst, autocatalytic | low throughout |
| Ripening after harvest | yes, if harvested mature | little or none; harvest ripe |
| Response to applied ethylene | ripening is triggered and synchronised | mainly degreening of the skin, not true ripening |
| Examples | apple, pear, banana, mango, papaya, tomato, avocado, kiwifruit, peach, plum, guava, sapota, passion fruit | citrus, grape, strawberry, cherry, pineapple, pomegranate, litchi, watermelon, cucumber, olive |
Climacteric fruits can be harvested mature but unripe, shipped while firm, and ripened near the market. Non-climacteric fruits must be picked when they are ready to eat, which makes them harder to transport over long distances. Some fruits, such as some melons and peppers, have both climacteric and non-climacteric varieties.
Ethylene, the ripening hormone
Ethylene (C2H4) is a simple gas that acts as a hormone at extremely low concentrations, often below 1 part per million. Its role in ripening was established by S. P. Burg and E. A. Burg in the early 1960s, and its biosynthesis was worked out largely by S. F. Yang.
How it is made
Ethylene is made from the amino acid methionine in three steps:
- Methionine → S-adenosylmethionine (SAM).
- SAM → ACC (1-aminocyclopropane-1-carboxylic acid), by ACC synthase, the main control point.
- ACC → ethylene, by ACC oxidase, which needs oxygen.
The by-product of step 2 is recycled to methionine in the Yang cycle, so a small pool of sulphur can supply continuous ethylene production.
Systems 1 and 2
- System 1: low, self-limiting ethylene production in unripe fruit and vegetative tissues.
- System 2: in climacteric fruit at the onset of ripening, ethylene stimulates its own production (autocatalysis), causing the burst that drives ripening.
How it acts
Ethylene binds to receptors in the endoplasmic reticulum membrane, switching on genes for softening, colour, sugars and aroma. The tomato ripening-inhibitor (rin) mutant, which barely ripens, helped reveal the regulatory genes.
Controlling ripening in practice
Speeding it up
- Ripening rooms: bananas, avocados, mangoes and tomatoes are commonly ripened with about 100 to 150 ppm ethylene for 24 to 48 hours at controlled temperature (for bananas usually about 14 to 18 °C) and high humidity, with ventilation to remove CO2.
- Ethephon, a compound that releases ethylene, is used as a dip or spray for ripening and degreening.
- Calcium carbide, which releases acetylene, is still used informally in some places but is banned for fruit ripening in many countries, because commercial carbide carries arsenic and phosphorus impurities and gives uneven ripening.
Slowing it down
- Low temperature: the main tool; each 10 °C fall cuts respiration to between a half and a third. Tropical fruits such as banana, mango and papaya suffer chilling injury below about 10 to 13 °C, so they cannot be stored as cold as apples.
- Controlled and modified atmospheres: low O2 and raised CO2 slow respiration and ethylene action.
- Ethylene removal: ventilation, potassium permanganate absorbers and catalytic scrubbers.
- 1-Methylcyclopropene (1-MCP): a gas that binds ethylene receptors and blocks them. Approved for use on apples and other crops from about 2002, it greatly extends the storage life of apples and some other fruit.
- Keeping ethylene producers apart from sensitive produce: ripening apples and bananas can yellow broccoli, sprout potatoes and make carrots bitter.
Heat produced by ripening fruit in a store
Respiring produce releases heat that a cold store must remove. Each milligram of CO2 released corresponds to about 10.7 J of heat (from 2,870 kJ per mole of glucose ÷ 6 moles of CO2 ÷ 44 g per mole).
A ripening room holds 10 tonnes of bananas respiring at 40 mg CO2 per kg per hour.
- CO2 per hour = 40 × 10,000 = 400,000 mg.
- Heat per hour = 400,000 × 10.7 J ≈ 4.3 MJ.
- Heat load = 4.3 MJ ÷ 3,600 s ≈ 1.2 kW, continuously, on top of the heat entering through walls and doors.
At the climacteric peak, or at a higher temperature, respiration and heat output can double or more, which is why ripening rooms need generous refrigeration capacity.
Key points
- Ripening changes colour, texture, sugars, acids and aroma.
- Climacteric fruits (apple, banana, mango, tomato, avocado, papaya) show a respiratory rise and ethylene burst and can ripen after harvest.
- Non-climacteric fruits (citrus, grape, strawberry, cherry, pineapple) must be picked ripe.
- Ethylene: methionine → SAM → ACC (ACC synthase) → ethylene (ACC oxidase); autocatalytic System 2 in climacteric fruit.
- Ripening is promoted by ethylene and ethephon, and slowed by cold, controlled atmospheres, ethylene removal and 1-MCP.
- Respiration releases about 10.7 J of heat per mg CO2.
What the exams ask
- School-leaving and entrance papers: climacteric and non-climacteric fruits with examples, and ethylene as a ripening hormone.
- Degree and research-fellowship entrances: biochemical changes during ripening, ethylene biosynthesis and the Yang cycle, Systems 1 and 2, 1-MCP, and ripening chambers.
- Professional and post-harvest examinations: storage conditions, chilling injury, controlled atmosphere, heat of respiration, and regulations on ripening agents.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why does putting a banana with an avocado make it ripen faster?
The banana gives off ethylene, which triggers the avocado's own ethylene production and ripening. A closed paper bag concentrates the gas.
Are fruits ripened with ethylene safe?
Yes. Ethylene is a natural plant hormone that fruits produce themselves, and it leaves no harmful residue. The concern is with calcium carbide, not ethylene.
Can I ripen strawberries after picking?
They may soften and darken a little, but they will not become sweeter, because they are non-climacteric and do not convert stored starch into sugar after harvest.
Sources
- Kidd, F. and West, C. "The course of respiratory activity throughout the life of an apple." Report of the Food Investigation Board for 1924. HMSO, London, 1925.
- Burg, S. P. and Burg, E. A. "Role of ethylene in fruit ripening." Plant Physiology 37, 179–189 (1962).
- Yang, S. F. and Hoffman, N. E. "Ethylene biosynthesis and its regulation in higher plants." Annual Review of Plant Physiology 35, 155–189 (1984).
- Watkins, C. B. "The use of 1-methylcyclopropene (1-MCP) on fruits and vegetables." Biotechnology Advances 24, 389–409 (2006).
- Kader, A. A. (ed.) Postharvest Technology of Horticultural Crops, 3rd edition. University of California ANR Publication 3311, 2002.
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