Ingredient

Kiwi

A kiwi is bought hard and sour and becomes sweet and fragrant within a few days, which makes it one of the few fruits that genuinely improves on a windowsill. It also contains a protein-digesting enzyme that ruins anything set with gelatine and curdles dairy if left to sit.

Ripening on demand

Kiwis are strongly climacteric and unusually responsive to ethylene. A rock-hard kiwi in a paper bag with a banana or an apple will be ready in twenty-four to forty-eight hours, which is faster than almost any other fruit responds.

This makes them convenient rather than unreliable: buy them hard, keep them in the fridge where they will hold for weeks, and take out what you need three days ahead.

Ripe means giving to gentle pressure all over, like a ripe peach. A hard kiwi is astringent and unpleasantly acidic; an overripe one goes watery and slightly fermented.

Actinidin, and the jelly problem

Kiwi contains actinidin, a protease in the same broad family as the bromelain in pineapple and the papain in papaya. It does the same three things.

It prevents gelatine setting, so a fruit jelly containing fresh kiwi stays liquid. Brief cooking destroys the enzyme and fixes it. It curdles milk and cream over time, so a kiwi pavlova assembled an hour early will weep. And it tenderises meat efficiently, which is why kiwi purée is a standard marinade ingredient in Korean cooking.

It is also why kiwi can make the mouth tingle, particularly when the fruit is not fully ripe.

Green and gold

Green (Hayward)

The standard. Fuzzy brown skin, bright green flesh, markedly acidic, black seeds.

Gold (SunGold)

Smooth hairless bronze skin, yellow flesh, much sweeter and lower in acid, faintly tropical. Softer, and it bruises more easily.

Kiwiberry

Grape-sized, smooth-skinned, eaten whole. A hardy relative, and increasingly grown in Britain.

The skin

Edible on all of them and where much of the fibre sits. On the green fuzzy sort, rub it with a cloth or scrape it lightly first.

Enzyme strength

Highest in green kiwi, lower in gold. Both will stop gelatine setting.

What to do with them

• Eaten with a teaspoon, cut in half, which remains the most efficient method.
• Sliced into a fruit salad at the last minute, not an hour ahead, because the enzyme softens everything around it.
• In a Korean-style marinade for beef — a spoonful of purée, no more than half an hour.
• With chilli and lime in a salsa for fish.
• In a green smoothie, where the acidity does useful work.
• Frozen in slices, which works well for drinks though the texture collapses on thawing.
• Skin on, whole, if you can get past the texture — it is entirely edible and the gold ones are pleasant that way.

Common questions