How Can I Ripen Kiwi Fruit? What the Trade Stopped Doing

Firm kiwifruit ripens on a kitchen counter at room temperature in two to five days. A paper bag with an apple, banana, pear or orange shortens that to 24 to 48 hours, because those fruits give off ethylene and kiwifruit reacts to as little as 5 to 10 parts per billion of it. The fruit is ready when it yields to gentle pressure in the palm, which corresponds to a flesh firmness of 2 to 3 pounds-force on the scale California packers measure with. Cold slows the process instead of stopping it, so the refrigerator belongs at the end of the sequence, once the fruit has softened.

The rest is a question of which shortcuts survive contact with the published handling data. The trade has spent forty years moving hard fruit to soft on schedule, and much of what it learned took the form of methods it dropped.

The number the whole trade ripens to

"Flesh firmness is the best indicator of kiwifruit ripening and predictor of shelf life," writes Carlos H. Crisosto, the University of California pomologist whose protocols the California kiwifruit industry adopted, in Optimum Procedures for Ripening Kiwifruit, published by UC Agriculture and Natural Resources. His threshold is exact: "Fruit that measures 2-3 pounds-force flesh firmness is ripe and 'ready-to-eat'."

The figure comes off a penetrometer pushed into cut flesh, so no home cook will measure it directly. In the postharvest guidelines Crisosto wrote with the late Adel A. Kader for UC Davis, harvest maturity runs at 14 pounds-force or higher and packers will not ship below 5. Ripe sits at 2 to 3, so hard fruit has most of that distance to travel in a bowl.

Zespri, the New Zealand body behind most of the world's export crop, gives the domestic version of the same test: "Hold the kiwi in the palm of your hand. Gently press the fruit. If it gives a little, it is ripe, ready, and at its best." Its Australian page adds the comparison most people already carry, to a peach or an avocado.

Sweetness arrives with softness. Crisosto and Kader note that starch is high at harvest "but it is converted to soluble sugars during storage and ripening", and they put consumer satisfaction at 12.5 per cent soluble solids at the moment of eating. A hard kiwi tastes sour because the starch has not yet turned. The ceiling is fixed at picking, under a California Kiwifruit Marketing Order minimum of 6.5 per cent, and fruit picked below its potential never catches up in a bowl.

Two to five days, and why the published numbers disagree

Zespri's own regional websites do not agree with each other. Its Singapore, Malaysian, Philippine, Australian and US pages give 2 to 3 days for a firm kiwifruit at room temperature. The New Zealand page says "it normally takes between three-to-five days for a firm Zespri Kiwifruit to ripen". Nobody has reconciled the two.

The softening rates published for the trade favour the longer window. Crisosto puts the flesh softening rate at "about 2.0 pounds per day when exposed to 20 °C", and his shipper guidelines give 2.7 pounds lost per day at 68 °F for fruit that has already had its ethylene treatment. Supermarket fruit sold as firm often sits at 8 to 10 pounds-force, the level at which Crisosto tells warehouses it needs a gassing. Two pounds a day from nine leaves three days of counter time, four if the kitchen is cool. A 16 °C kitchen will not deliver what a 22 °C one does.

The fruit in your bowl was bred to be hard

The green kiwifruit in nearly every shop in the world is one cultivar, 'Hayward', selected by the nurseryman Hayward Wright at Avondale near Auckland and later named for him. Horticultural references date the selection to 1924 or 1928; Te Ara, New Zealand's official encyclopedia, puts it in the 1930s. What they agree on is why it displaced everything else. It keeps. Its storage life outlasts other kiwifruit varieties by a wide margin, and an industry shipping between hemispheres chose accordingly.

Crisosto and Kader instruct that kiwifruit "should be stored as near to 0 °C as possible and under 90 to 95 percent relative humidity", and under those conditions large fruit in ordinary air stays above the 5 pounds-force packing minimum for twelve weeks. The hardness is the cultivar's headline feature working as designed.

Cold storage also changes what the fruit needs on arrival. Crisosto's preconditioning treatment, 100 parts per million of ethylene for 12 hours, "is only effective on freshly harvested kiwifruit or those that have been in cold storage for less than 5 weeks". Fruit held longer ripens on its own once it warms to 15-21 °C, on the ethylene it makes itself. Kiwifruit bought outside the local season has usually been in store far longer than that, and for it warmth is the active ingredient and the apple optional.

The paper bag, and what it is actually doing

The bag is a concentrator. Unripe kiwifruit makes very little ethylene of its own, 0.1 to 0.5 microlitres per kilogram per hour at 20 °C by Crisosto and Kader's measurement, while responding to 5 to 10 parts per billion in the air around it. An apple or a ripening banana supplies far more, and the bag stops it dispersing across the room.

1. Put the firm kiwifruit in a paper bag with one apple, banana, pear or orange, the four Zespri names, and fold the top over loosely so the bag holds gas without sealing.
2. Leave it somewhere warm and out of direct sun, at 18-22 °C.
3. Check by hand after 24 hours, then every 12. Triggered fruit loses 2 to 3 pounds-force a day at room temperature, a difference the hand can feel between checks.
4. Move each fruit to the refrigerator as it starts to yield.

No published measurement exists for how much ethylene one supermarket apple puts into a paper bag. The 24 to 48 hour figure is Zespri's, and it agrees with the commercial softening rates. That is the strongest claim the evidence supports.

Three methods worth dropping

The plastic bag

A plastic bag traps carbon dioxide along with the ethylene, and kiwifruit has a specific documented failure at that combination. Crisosto and Kader list a disorder called hard-core: "This disorder is induced by exposure to ethylene of kiwifruit stored with carbon dioxide levels above 8 percent. The fruit core fails to ripen while the remainder of the fruit is soft and ripe."

Soft outside, hard in the middle, is the texture people complain about after bagging kiwis with a banana overnight. Whether a kitchen bag reaches 8 per cent has not been tested in a published trial, though the arithmetic is not reassuring. The same guidelines put kiwifruit respiration at 14.7 to 19.6 millilitres of carbon dioxide per kilogram per hour at 20 °C. Four kiwifruit weigh close to 400 grams, giving 6 to 8 millilitres an hour into a sealed sandwich bag holding perhaps a litre of air. Twelve hours of that lands in the 8 per cent region. Polyethylene leaks carbon dioxide, so a real bag runs lower than the calculation. Paper removes the question.

The refrigerator

Cold does not stop ripening; it slows it by a factor most people overestimate. Crisosto's shipper table gives the loss for ethylene-treated fruit as 1.2 pounds-force per day at 32 °F and 2.7 at 68 °F, so a domestic refrigerator runs the process at something over a third of counter speed. His ripening table makes the same point from the other side. Treated fruit takes 6.5 to 7.0 days to reach 3 pounds-force at 0 °C, against 3.0 to 4.5 days at 20 °C. Zespri's Singapore page says firm fruit in the fridge "will take 5-7 days to fully ripen", while its US and Australian pages present refrigeration as the way to hold ripening back.

Both are true. The fridge is a slow ripener and a good keeper.

The microwave

Microwaving softens kiwifruit without ripening it, and the distinction is not pedantry. Ripening is enzymatic. Amylases convert stored starch to soluble sugars while cell wall enzymes disassemble pectin and hemicellulose, which is why a ripe fruit is sweeter as well as softer. A microwave supplies heat, which ruptures cells and denatures the enzymes that would have done the work. The result is warm, watery tissue at the sugar level it started with, and the aroma never forms. No study has tested a domestic microwave on a kiwifruit. Nobody in the postharvest literature proposes it.

Kiwifruit, avocado and peach: the same squeeze, three different fridges

Zespri sends readers to the avocado and the peach because the hand test genuinely transfers. All three soften after picking, and the palm reports the same thing in each. The storage schedule does not transfer, and treating the three alike is how good fruit gets ruined.

| | Kiwifruit ('Hayward') | Avocado (mature-green 'Hass') | Peach | |---|---|---|---| | Recommended holding temperature | As near 0 °C as possible | 5-7 °C | Near 0 °C | | Chilling sensitivity | "Not sensitive" | Injury below about 5 °C | Internal breakdown at 2.2-7.6 °C | | A domestic fridge at 4 °C | Ideal; slows softening, no damage | Risks chilling injury while unripe | Sits inside the damage zone | | Ripe by feel | Yields to gentle pressure | Yields to gentle pressure | Yields to gentle pressure |

The peach column is the sharp one. Lurie and Crisosto, writing in Postharvest Biology and Technology in 2005, describe 2.2 to 7.6 °C as the range where flesh browning and mealiness develop fastest, which UC Davis rounds to a 2-8 °C killing range. A domestic refrigerator sits in the middle of it, which makes the same appliance at the same setting the best place available for a kiwifruit and the worst for an unripe peach.

Mould sets the deadline

The limit on any ripening trick is Botrytis cinerea, grey mould, which enters kiwifruit through the cut stem at harvest and through skin wounds, then waits. Crisosto and Kader record that it is "capable of slow growth even at 0 °C" and that in long storage it spreads into healthy fruit, a behaviour packers call nesting. One rotting kiwi takes its neighbours with it.

Two conditions decide whether ripening turns into rot. The guidelines state that "moist conditions are necessary for infection", the argument against any sealed container that condenses water on the fruit. The second is damage. Dropped or nicked fruit has an entry point intact fruit does not. Sorting the bruised ones out and ripening them first on a plate costs nothing.

The ones that went too far

Overripe kiwifruit is at its sweetest, the starch conversion having finished, so the mushy ones are worth saving. Zespri's advice is the freezer, whole or in chunks, for smoothies. Puréeing first and freezing in a tray works better than freezing fruit whole, which sheds water on thawing.

The one thing overripe kiwi cannot do is set. In 1959 the biochemist A. C. Arcus, writing in Biochimica et Biophysica Acta, isolated the protein-digesting enzyme in Actinidia chinensis and named it actinidin, after noticing that kiwifruit jellies refuse to solidify. It chops gelatin into fragments too short to form a gel, so any panna cotta or mousse holding raw fruit stays liquid. Green 'Hayward' carries far more of it than the gold varieties. Heat destroys the enzyme, which is why cooked and canned kiwifruit set normally.

The same enzyme is the best use for fruit at the other end. Actinidin is a meat tenderiser in the papain family, and trials rate it gentler than papain, which over-tenderises meat surfaces. Mashed underripe kiwifruit rubbed on beef for under an hour loosens connective tissue without turning the outside to paste. Underripe fruit also has the acidity for chutney and pickling.

Holding a larger purchase over weeks

Firm fruit keeps longer than ripe fruit, so a large bag is best bought hard and split. Ripen what will be eaten within a few days on the counter; the rest goes straight into the refrigerator, unwashed. Crisosto tells handlers that kiwifruit "should always be kept at low temperatures below 7.5 °C (45 °F) and enclosed with liners, except if they are going to be consumed within 3 days".

The crisper drawer is the right compartment, since the commercial target is 90 to 95 per cent humidity and the drawer is the only part of a domestic fridge that comes close. Keep the fruit away from apples, pears, bananas and oranges, the four Zespri names as ethylene sources. A fruit that responds to 5 parts per billion does not need much of a neighbour.

How long it lasts there is the figure nobody agrees on. Zespri's Singapore page gives ripe kiwifruit a further 7 to 10 days refrigerated; its US and Australian pages say about two weeks; the New Zealand page stretches to two-to-three weeks. The spread says more about the state of the fruit going in than about refrigeration, and a weekly hand check beats all three numbers.

Frequently asked questions

What can I do with underripe kiwi?

Use it where sourness helps. Underripe kiwifruit still has starch that has not turned to sugar, so it works in chutney, pickles and salsa rather than eaten raw. Its enzyme, actinidin, also tenderises beef when mashed and rubbed on for under an hour, more gently than papain-based products.

Why is my kiwi still hard?

It has been held cold. Commercial kiwifruit is stored as near 0 °C as possible, and the 'Hayward' cultivar was selected for holding its firmness there for months. Nothing much happens until the fruit warms up. At room temperature it loses roughly 2 pounds-force a day and reaches eating condition in two to five days.

Will kiwi fruit ripen on the counter?

Yes, and the counter is the standard method. Zespri's regional guidance gives 2 to 3 days for a firm kiwifruit at room temperature, and its New Zealand page gives three to five. Keep the fruit out of direct sunlight, in a bowl rather than a sealed container, and test it by hand each day.

Can I ripen kiwi fruit overnight?

Not from firm. Published softening rates run at 2 to 3 pounds-force per day at 20 °C, so twelve hours moves a 9 pounds-force kiwi to about 8, well above the 2-3 pounds-force ready-to-eat band. A paper bag with an apple can finish fruit that is already close, in 24 to 48 hours.

Does a refrigerator ripen kiwi fruit?

Slowly, yes. Cold does not stop ripening; it throttles it. Crisosto's tables put softening at 1.2 pounds-force per day at 32 °F against 2.7 at 68 °F, and Zespri's Singapore page says firm fruit in the fridge takes 5 to 7 days to ripen fully. Use the fridge to hold fruit, not to prepare it.

Does a microwave ripen kiwi fruit or merely soften it?

It softens only. Ripening is enzymatic, converting starch to sugar and loosening cell walls over days. Microwave heat ruptures cells and denatures those enzymes, producing warm, watery tissue with the same sugar content it had before. No sweetness or aroma develops, and the damage cannot be reversed.